Technical interview Q&A plus 100+ career & HR questions—notice period, salary negotiation, resume, LinkedIn, freelancing, AI careers, and behavioral interviews with detailed, real-world answers.
JavaScript JavaScript Tutorial · JavaScript
Answer: CSS stands for Cascading Style Sheets. It is used to style and layout HTML elements — controlling colors, fonts, spacing, and responsiveness. Key Takeaway: CSS separates design from content, improving flexibility and maintainability.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
HTML5 is the modern evolution of HTML4, introducing better structure, multimedia support,
nd APIs.
Follow me on LinkedIn:
HTML4 mainly focused on document markup, while HTML5 focuses on building interactive
web applications.
Key differences:
Feature HTML4 HTML5
Doctype Long and complex Simple <!DOCTYPE html>
Multimedia Needs Flash Native <audio> and <video>
Semantics Limited New tags: <header>, <footer>,
<article>, <nav>
Storage Cookies localStorage, sessionStorage
Forms Basic New input types: email, date, numberExample:
<!DOCTYPE html>
<html>
<body>
<header>
<h1>HTML5 Example</h1>
</header>
</body>
</html>
Key Takeaway:
HTML5 = Simpler, smarter, and built for modern web apps.
JavaScript JavaScript Tutorial · JavaScript
Answer: Follow me on LinkedIn: Bootstrap is a popular front-end framework for building responsive, mobile-first web pages using HTML, CSS, and JavaScript components.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
A closure is created when a function remembers variables from its outer scope, even
fter that outer function has finished executing.
Example:
function makeCounter() {
let count = 0;
return function() {
return ++count;};
}
const counter = makeCounter();console.log(counter()); // 1
console.log(counter()); // 2
✅ Use cases: data privacy, memoization, and function factories.
JavaScript JavaScript Tutorial · JavaScript
HTML stands for HyperText Markup Language. It is the standard language used to create
nd structure content on web pages.
Example:
simple HTML document:
<!DOCTYPE html>
<html>
<head>
Follow me on LinkedIn:
<title>My First Page</title>
</head>
<body>
<h1>Hello, World!</h1>
</body>
</html>
Key Takeaway:
HTML defines the structure; CSS styles it; JavaScript makes it interactive.
JavaScript JavaScript Tutorial · JavaScript
When a browser loads a webpage, it goes through these main steps:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: JavaScript is a high-level, interpreted, dynamic programming language used to make web pages interactive and dynamic. It runs in browsers and also on servers (using Node.js).
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: The browser reads HTML line-by-line and builds a DOM (Document Object Model) tree.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
<style> p { color: blue; } </style>
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
.highlight { background: yellow; }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
height, margin, and padding.
Example:
.block {
display: block;
width: 100px;
}.inline-block {
display: inline-block;
width: 100px;
}Key Takeaway:
inline-block combines the flow of inline with the flexibility of block.
Follow me on LinkedIn:
JavaScript JavaScript Tutorial · JavaScript
A higher-order function is a function that takes another function as an argument or
returns a function.They are key to functional programming in JavaScript.
Example:
function greet(name) {
return `Hello, ${name}`;
}Follow me on LinkedIn:
function processUser(callback) {
return callback("Alice");
}console.log(processUser(greet)); // Hello, Alice
Functions like map(), filter(), and reduce() are higher-order functions.
JavaScript JavaScript Tutorial · JavaScript
String, Number, Boolean, Undefined, Null, BigInt, Symbol
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
They are part of the browser’s rendering process when the DOM or styles change.
changing size, position, or adding elements).
→ Expensive operation since it recalculates layout.
Follow me on LinkedIn:
without affecting layout.
Example:
div.style.width = "200px"; /* triggers reflow + repaint */
div.style.background = "red"; /* triggers repaint only */Key Takeaway:
Minimize layout changes; batch DOM updates to improve performance.
JavaScript JavaScript Tutorial · JavaScript
✅ Best Practices:
Key Takeaway:
Efficient CSS = fewer reflows, fewer repaints, and lightweight selectors.
JavaScript JavaScript Tutorial · JavaScript
The event loop manages asynchronous operations in JavaScript.
It continuously checks the call stack and callback queue — executing queued tasks once
the stack is empty.
Example:
console.log("A");
setTimeout(() => console.log("B"), 0);console.log("C");
// Output: A, C, B
Even though setTimeout is 0ms, it runs after the main thread finishes due to the event
loop.
JavaScript JavaScript Tutorial · JavaScript
Answer: ✅ Responsive design ✅ Pre-styled components ✅ Cross-browser compatibility ✅ Customizable via variables and themes ✅ Speeds up development
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
V8 is Google’s open-source JavaScript engine used in Chrome and Node.js. It:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Object (includes Arrays, Functions, Dates, etc.)
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
The <!DOCTYPE html> declaration tells the browser which version of HTML the page
uses.
In modern websites, it triggers HTML5 standards mode, ensuring consistent rendering
cross browsers.
Example:
<!DOCTYPE html>
<html>
...
</html>
Key Takeaway:
lways include <!DOCTYPE html> at the top of every HTML file.
JavaScript JavaScript Tutorial · JavaScript
These are semantic elements that help structure a webpage meaningfully:
Follow me on LinkedIn:
Example:
<section>
<article>
<h2>HTML Interview Guide</h2>
<p>Learn important HTML concepts easily.</p>
</article>
<aside>
<p>Check out our CSS guide next!</p>
</aside>
</section>
Key Takeaway:
Use these for clear content hierarchy and SEO-friendly structure.
JavaScript JavaScript Tutorial · JavaScript
#main { width: 80%; }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Flexbox (Flexible Box Layout) is a one-dimensional layout system for aligning items
horizontally or vertically.
Example:
.container {
display: flex;
justify-content: center;
lign-items: center;
}Why use it:
Key Takeaway:
Flexbox simplifies alignment and space distribution in one direction.
JavaScript JavaScript Tutorial · JavaScript
Answer: CSS is parsed into a CSSOM (CSS Object Model). JavaScript may modify the DOM or halt parsing (especially with synchronous scripts).
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
ARIA (Accessible Rich Internet Applications) roles make web content more accessible
for users relying on assistive technologies (like screen readers).Example:
<button role="switch" aria-checked="false">Dark Mode</button>
Follow me on LinkedIn:
Key Takeaway:
RIA makes dynamic interfaces accessible by adding semantic meaning beyond native
HTML.
JavaScript JavaScript Tutorial · JavaScript
Answer: JavaScript is interpreted, runs mainly in browsers. Java is compiled, runs on JVM. Syntax differs, and Java is strongly typed, JS is dynamically typed.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: There are three ways to include CSS: Inline CSS: <p style="color: blue;">Hello!</p>
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
<link rel="stylesheet" href="styles.css">
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Follow me on LinkedIn:
Example:
<img src="logo.png" alt="Company Logo">
Here, <img> is a tag, and src & alt are attributes.
Key Takeaway:
Tags = structure; Attributes = extra information.JavaScript JavaScript Tutorial · JavaScript
Use classes for reusable styles, IDs for unique elements.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Element selector: Targets HTML tags. p { color: green; }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Example:
<user-card></user-card>
<template id="user-template">
<p>Hello, <span id="name"></span></p>
</template>
<script>
class UserCard extends HTMLElement {connectedCallback() {
const tmpl = document.getElementById("user-template");
const content = tmpl.content.cloneNode(true);
content.querySelector("#name").textContent = "Sandeep";this.appendChild(content);
}
}customElements.define("user-card", UserCard);
</script>
Key Takeaway:
Web Components = reusable, encapsulated building blocks for modern web apps.
JavaScript JavaScript Tutorial · JavaScript
The Shadow DOM is a web component feature that encapsulates a section of the DOM —
meaning styles and markup inside it don’t leak out or get affected by the rest of the page.
Example:
<custom-card></custom-card>
<script>
class CustomCard extends HTMLElement {constructor() {
super();
const shadow = this.attachShadow({ mode: 'open' });
shadow.innerHTML = `<style>p { color: blue; }</style><p>HelloShadow!</p>`;
}
}customElements.define('custom-card', CustomCard);
</script>
Key Takeaway:
Shadow DOM = style and structure isolation for reusable, predictable components.
JavaScript JavaScript Tutorial · JavaScript
DOM + CSSOM = Render Tree (a visual structure of elements and styles).
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
The <canvas> element is used to draw graphics, animations, charts, or games using
JavaScript.
Example:
<canvas id="myCanvas" width="200" height="100"></canvas>
<script>
const canvas = document.getElementById("myCanvas");
const ctx = canvas.getContext("2d");
ctx.fillStyle = "blue";ctx.fillRect(10, 10, 180, 80);
</script>
Follow me on LinkedIn:
Key Takeaway:
<canvas> gives you a pixel-based drawing area inside HTML — perfect for dynamic
graphics.
JavaScript JavaScript Tutorial · JavaScript
Feature Flexbox Grid
Layout One-dimensional (row or column) Two-dimensional (rows and columns)
Use Case Aligning items in a line Building full-page layouts
Alignment Easier for single direction Easier for full layout structure
Example:
/* Flexbox */
display: flex;
/* Grid */
Follow me on LinkedIn:
display: grid;
grid-template-columns: repeat(3, 1fr);
Key Takeaway:
Use Flexbox for alignment; Grid for complete layouts.
JavaScript JavaScript Tutorial · JavaScript
Use the prefers-color-scheme media query or toggle themes via classes.
Example (automatic via system theme):
Follow me on LinkedIn:
@media (prefers-color-scheme: dark) {
body {
background: #121212;
color: #fff;
}
}Example (manual toggle):
body.dark-mode {
background: #000;
color: white;
}Key Takeaway:
Dark mode can adapt automatically or via user preference toggles.
JavaScript JavaScript Tutorial · JavaScript
lignment Easier for single direction Easier for full layout structure
Example:
/* Flexbox */
display: flex;
/* Grid */
Follow me on LinkedIn:
display: grid;
grid-template-columns: repeat(3, 1fr);
Key Takeaway:
Use Flexbox for alignment; Grid for complete layouts.
JavaScript JavaScript Tutorial · JavaScript
Follow me on LinkedIn:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Garbage collection automatically frees memory that’s no longer referenced.
The V8 engine uses the mark-and-sweep algorithm:
Example:
let user = { name: "John" };
user = null; // eligible for garbage collectionJavaScript JavaScript Tutorial · JavaScript
Follow me on LinkedIn:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Feature Bootstrap 4 Bootstrap 5 jQuery Required Removed Grid 5 breakpoints 6 (added xxl) Forms Legacy Redesigned Icons None Separate Bootstrap Icons Utility API Limited Expanded and customizable
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Primitive types: string, number, boolean, null, undefined, symbol, bigint Reference types: object, array, function Example: let name = "Sandeep"; let obj = { age: 30 };
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: The browser calculates positions (layout) and then paints pixels on the screen. Key Takeaway: The DOM is built first, then styles are applied, and finally pixels are rendered — optimizing this pipeline improves page performance.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Both store data in the browser, but they differ in duration and scope.
Feature localStorage sessionStorage
Lifetime Until manually cleared Until the tab is closed
Scope Shared across tabs Specific to one tab
Capacity ~5–10MB ~5MB
Example:
localStorage.setItem("username", "Sandeep");
sessionStorage.setItem("theme", "dark");
Key Takeaway:
Use localStorage for long-term data; sessionStorage for temporary, tab-specific data.
JavaScript JavaScript Tutorial · JavaScript
Preprocessors extend CSS with variables, nesting, mixins, and functions, compiled into
plain CSS.
Example (SASS):
$main-color: #3498db;
.button {
background: $main-color;
&:hover { background: darken($main-color, 10%); }
}Key Takeaway:
Preprocessors make CSS more modular, maintainable, and reusable.
JavaScript JavaScript Tutorial · JavaScript
A service worker is a background script that runs independently of the web page.
It enables offline caching, push notifications, and background sync.
Example:
navigator.serviceWorker.register('/sw.js');
✅ Used in Progressive Web Apps (PWAs).
JavaScript JavaScript Tutorial · JavaScript
Answer: Example: div { width: 100px; padding: 10px; border: 5px solid black; margin: 20px; Key Takeaway: Total width = content + padding + border + margin.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
✅ Result: lightning-fast execution of JS.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Metho
Purpose Arguments Invokes
Immediately?
call(
Call a function with a specific this Comma-separat
✅ Yes
apply
Same as call(), but takes array Array ✅ Yes
bind(
Returns a new function with fixed
this
Comma-separat
❌ No
Example:
function greet(greeting) {
console.log(`${greeting}, ${this.name}`);
const user = { name: "John" };
greet.call(user, "Hi");
greet.apply(user, ["Hello"]);
const bound = greet.bind(user, "Hey");
bound();
JavaScript JavaScript Tutorial · JavaScript
Answer: Use fractional units (fr) and media queries. Example: .container { display: grid; grid-template-columns: repeat(auto-fit, minmax(250px, 1fr)); gap: 10px; } Key Takeaway: uto-fit and minmax() automatically adapt the grid to screen size.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Variables are containers to store data. Example: let age = 25;
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Type Scope Example Priority
Inline Single element style="color:re
d;"
Highest
Internal One page <style> inside<head>
Medium
External Multiple pages Linked .css file Lowest
Key Takeaway:
Use external CSS for maintainable, large-scale projects.
Follow me on LinkedIn:
JavaScript JavaScript Tutorial · JavaScript
SEO optimization starts with clean, structured, and semantic HTML.
Best practices:
Follow me on LinkedIn:
(<h1> → <h6>).
Example:
<meta name="description" content="Learn web development step-by-step
with examples.">
<h1>HTML Interview Questions</h1>
Key Takeaway:
SEO-friendly HTML = clear structure + accurate metadata + accessible content.
JavaScript JavaScript Tutorial · JavaScript
Answer: container wraps page content and provides horizontal padding. <div class="container">Content</div> <div class="container-fluid">Full width</div>
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
pply
()
Same as call(), but takes array Array ✅ Yes
bind(
Returns a new function with fixedthis
Comma-separat
ed
❌ No
Example:
function greet(greeting) {
console.log(`${greeting}, ${this.name}`);
}
const user = { name: "John" };greet.call(user, "Hi");
greet.apply(user, ["Hello"]);
const bound = greet.bind(user, "Hey");bound();
JavaScript JavaScript Tutorial · JavaScript
Example:
<div>Block Section</div>
<p>This is a <span style="color: red;">red word</span> in asentence.</p>
Key Takeaway:
Use <div> for layout; <span> for inline styling.
JavaScript JavaScript Tutorial · JavaScript
Makes an element toggle between relative and fixed based on scroll position.
Example:
header {
position: sticky;
top: 0;
background: white;
}Follow me on LinkedIn:
Key Takeaway:
sticky elements stay visible within their parent container while scrolling.
JavaScript JavaScript Tutorial · JavaScript
Follow me on LinkedIn:
Aspect @import <link>
Loaded Inside CSS Inside HTML
<head>
Performance Slower (sequential) Faster (parallel)
Media Queries Can include Can include
Recommended
❌ Avoid ✅ Preferred
Key Takeaway:
Use <link> — it loads faster and supports preloading and caching better.
JavaScript JavaScript Tutorial · JavaScript
A Promise represents a value that may be available now, later, or never.
It helps handle asynchronous operations in a cleaner way.
Follow me on LinkedIn:
Example:
fetch('/data')
.then(res => res.json())
.then(data => console.log(data))
.catch(err => console.error(err));✅ async/await simplifies promise syntax.
JavaScript JavaScript Tutorial · JavaScript
In JavaScript, every object can inherit properties from another object called its prototype.
This enables object reusability.
Example:
const animal = { eats: true };
const dog = Object.create(animal);Follow me on LinkedIn:
dog.barks = true;console.log(dog.eats); // true (inherited)
JavaScript JavaScript Tutorial · JavaScript
Use the <a> tag with the href attribute.
Example:
<a href="
target="_blank">Visit Google</a>
Explanation:
Follow me on LinkedIn:
Key Takeaway:
lways add descriptive link text for accessibility.
JavaScript JavaScript Tutorial · JavaScript
They let you embed audio and video files directly in HTML without plugins like Flash.
Example:
<audio controls>
<source src="music.mp3" type="audio/mpeg">
</audio>
<video controls width="320">
<source src="movie.mp4" type="video/mp4">
</video>
Follow me on LinkedIn:
Key Takeaway:
HTML5 makes multimedia playback native, fast, and accessible.
JavaScript JavaScript Tutorial · JavaScript
Every HTML element is a box made of:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Example: p { color: black; } /* low */ #intro { color: blue; } /* higher */ <p id="intro" style="color:red;">text</p> <!-- highest --> Key Takeaway: Use balanced specificity; avoid overuse of !important.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Keyword Scope Re-declar
Re-assig
Hoisted?
var Functio✅ Yes ✅ Yes ✅ Yes (undefined)
let Block ❌ No ✅ Yes 🚫 Temporarily dead
zone
const Block ❌ No ❌ No 🚫 Temporarily dead
zone
Example:
Follow me on LinkedIn:
let x = 10; const y = 20; var z = 30;JavaScript JavaScript Tutorial · JavaScript
Answer: Functions are blocks of reusable code that perform a specific task. Example: function greet(name) { return `Hello, ${name}`; }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Bootstrap uses a 12-column grid based on flexbox.
You divide your layout using .row and .col-* classes.
Follow me on LinkedIn:
<div class="row">
<div class="col-6">Half</div>
<div class="col-6">Half</div>
</div>
JavaScript JavaScript Tutorial · JavaScript
Lazy loading defers loading of non-critical images or iframes until they are visible in the
viewport — improving page speed.
Example:
<img src="photo.jpg" loading="lazy" alt="Nature view">
Key Takeaway:
loading="lazy" helps reduce initial load time and saves bandwidth.
JavaScript JavaScript Tutorial · JavaScript
Add the draggable="true" attribute and use drag events in JavaScript.
Example:
<p id="drag1" draggable="true" ondragstart="drag(event)">Drag
me!</p>
<script>
function drag(e) {
e.dataTransfer.setData("text", e.target.id);
}</script>
Key Takeaway:
Use draggable="true" plus ondragstart and ondrop to enable drag-and-drop
functionality.
JavaScript JavaScript Tutorial · JavaScript
Answer: Hoisting is JavaScript’s behavior of moving variable and function declarations to the top of their scope during compilation. Example: console.log(a); // undefined var a = 5; Here, var a is hoisted but not initialized.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Pseudo-class: targets an element’s state.
hover { color: red; }
p::first-line { font-weight: bold; }
Key Takeaway:
:hover changes behavior; ::before and ::after insert content.
JavaScript JavaScript Tutorial · JavaScript
Answer: ES6 introduced modules for code reusability and organization. They use export and import keywords. Example: // math.js export function add(a, b) { return a + b; } // main.js import { add } from './math.js';
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Property Description min-wid th Element won’t shrink below this width max-wid th Element won’t grow beyond this width Example: div { min-width: 200px; max-width: 600px; } Key Takeaway: Use both for responsive, constrained resizing.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
They help browsers pick the best image for the user’s device and screen size.
Follow me on LinkedIn:
Example:
<img
src="small.jpg"
srcset="medium.jpg 768w, large.jpg 1200w"
sizes="(max-width: 768px) 100vw, 50vw"
lt="Landscape">
Key Takeaway:
srcset + sizes = responsive images that load efficiently across devices.
JavaScript JavaScript Tutorial · JavaScript
Semantic elements clearly describe their purpose in the document structure.
Examples include: <header>, <footer>, <article>, <nav>, <section>.
Example:
<article>
<h2>Breaking News</h2>
<p>New tech trends are emerging every day.</p>
</article>
Key Takeaway:
Semantic tags improve SEO and accessibility.
JavaScript JavaScript Tutorial · JavaScript
Answer: Breakpoints define responsive screen widths: Breakpoint Prefix Size Extra small none <576px Small sm ≥576px Medium md ≥768px Large lg ≥992px Extra large xl ≥1200px XXL xxl ≥1400px
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
✅ Common techniques:
Use feature queries:
@supports (display: grid) {
.container { display: grid; }
}background: #000;
background: linear-gradient(to right, #000, #333);
Key Takeaway:
Graceful degradation and progressive enhancement keep CSS cross-browser safe.
Follow me on LinkedIn:
JavaScript JavaScript Tutorial · JavaScript
Answer: Web Workers allow JavaScript to run code in background threads, keeping the UI responsive. Example: // main.js const worker = new Worker('worker.js'); worker.postMessage('Start'); // worker.js onmessage = e => postMessage('Task done');
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
The <head> tag contains metadata — information about the document that isn’t displayed
on the page (like title, styles, or scripts).
Example:
<head>
<title>Portfolio</title>
<meta charset="UTF-8">
<link rel="stylesheet" href="style.css">
</head>
Follow me on LinkedIn:
Key Takeaway:
Think of <head> as your page’s control room.
JavaScript JavaScript Tutorial · JavaScript
Answer: Use the grid system and responsive classes: <div class="col-12 col-md-6 col-lg-4">Responsive column</div>
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Example:
<script>
document.write("JavaScript is enabled!");
Follow me on LinkedIn:
</script>
<noscript>
<p>Please enable JavaScript to view this content.</p>
</noscript>
Key Takeaway:
<noscript> provides graceful fallback content.
JavaScript JavaScript Tutorial · JavaScript
Semantic HTML elements (like <nav>, <main>, <form>, <button>) provide meaning to
content — making it easier for assistive technologies to navigate.
Example:
<nav aria-label="Main navigation">
<a href="#home">Home</a>
<a href="#about">About</a>
</nav>
dvantages:
Key Takeaway:
Semantics = communication between developers, browsers, and accessibility tools.
Follow me on LinkedIn:
JavaScript JavaScript Tutorial · JavaScript
BEM is a naming convention for writing scalable and reusable CSS.
Structure:
.block__element--modifier
Example:
<div class="card card--featured">
<h2 class="card__title">Profile</h2>
</div>
Key Takeaway:
BEM enforces clarity:
JavaScript JavaScript Tutorial · JavaScript
Answer: Using Flexbox: .parent { display: flex; justify-content: center; /* horizontal */ lign-items: center; /* vertical */ height: 100vh; } Key Takeaway: flexbox is the easiest and modern way to center elements.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Shallow equality: Compares only top-level properties. Deep equality: Compares nested values recursively. Example: {a:1} === {a:1} // false (different refs) Use libraries like Lodash’s _.isEqual() for deep comparison. Follow me on LinkedIn:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Template literals allow embedded expressions and multi-line strings using backticks (`). Example: let name = "John"; console.log(`Hello, ${name}!`);
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Use @keyframes and the animation property.
Example:
@keyframes fadeIn {
from { opacity: 0; }
to { opacity: 1; }
}.box {
nimation: fadeIn 2s ease-in-out;
}Follow me on LinkedIn:
Key Takeaway:
@keyframes define the animation steps; animation applies them.
JavaScript JavaScript Tutorial · JavaScript
Answer: Primitive types: stored in stack, immutable (number, string, boolean). Reference types: stored in heap, mutable (object, array, function).
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Transitions animate property changes smoothly. Example: button { background: blue; transition: background 0.3s ease; } button:hover { background: red; } Key Takeaway: Transitions are great for hover effects and simple UI feedback.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Use the <img> tag with the src and alt attributes. Example: <img src="images/photo.jpg" alt="A sunset at the beach"> Key Takeaway: The alt attribute improves accessibility and helps SEO.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Web Components are reusable custom elements built with:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Predefined classes that handle spacing, display, text, alignment, etc. Example: <p class="text-center m-3 p-2">Centered text</p>
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: The rest operator ... collects multiple arguments into an array. Example: function sum(...nums) { return nums.reduce((a, b) => a + b); Follow me on LinkedIn: } console.log(sum(1, 2, 3)); // 6
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: bsolut Nearest positioned ncestor ✅ Yes Fixed Viewport ❌ No Example: div { position: absolute; top: 10px; left: 20px; } Key Takeaway: bsolute is relative to parent; fixed stays on screen. Follow me on LinkedIn:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Position Based On Moves with Page
Scroll?
Relative Its normal position ✅ Yes
Absolut
Nearest positioned
ancestor
✅ Yes
Fixed Viewport ❌ No
Example:
div { position: absolute; top: 10px; left: 20px; }
Key Takeaway:
Absolute is relative to parent; fixed stays on screen.
Follow me on LinkedIn:
JavaScript JavaScript Tutorial · JavaScript
Answer: Values that evaluate to false in Boolean context: false, 0, "", null, undefined, NaN Example: if (!0) console.log("Falsy"); // prints "Falsy"
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Specificity determines which rule wins when multiple styles target the same element. Priority order (lowest → highest):
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
JS manages memory via automatic garbage collection.
Memory allocation happens when objects are created, and deallocation occurs when they’re
no longer reachable.
Example:
Creating large arrays without clearing references may cause memory leaks.
JavaScript JavaScript Tutorial · JavaScript
Use CSS or the HTML5 srcset and sizes attributes.
HTML Example:
<img
src="small.jpg"
srcset="medium.jpg 768w, large.jpg 1200w"
sizes="(max-width: 768px) 100vw, 50vw"
lt="Mountain view">
CSS Example:
img {
max-width: 100%;
height: auto;
}Key Takeaway:
Responsive images adjust to different screen sizes for faster load and better UX.
JavaScript JavaScript Tutorial · JavaScript
Answer: Returns the type of a variable. Example: typeof "hello"; // "string" typeof 42; // "number"
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Arrow functions are a shorter syntax for writing functions. They don’t have their own this. Example: const sum = (a, b) => a + b; Follow me on LinkedIn:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Example:
<ol>
<li>HTML</li>
<li>CSS</li>
</ol>
<ul>
<li>Apple</li>
<li>Banana</li>
</ul>
Follow me on LinkedIn:
<dl>
<dt>HTML</dt>
<dd>HyperText Markup Language</dd>
</dl>
Key Takeaway:
Choose list type based on how you want to present data.
JavaScript JavaScript Tutorial · JavaScript
Answer: Method Returns Chainable Use Case forEach () undefin ed ❌ No Iteration map() New array ✅ Yes Transformation Example: [1,2,3].map(x => x*2); // [2,4,6]
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Follow me on LinkedIn: .row → Groups columns and manages horizontal alignment .col → Defines how many columns an element spans
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Proxy allows you to intercept and redefine fundamental operations on objects.
Example:
const user = { name: "Alice" };const proxy = new Proxy(user, {
get: (target, prop) => `${prop} -> ${target[prop]}`
});
console.log(proxy.name); // name -> Alice
✅ Used in data validation, reactive frameworks (like Vue.js).
JavaScript JavaScript Tutorial · JavaScript
It hints the browser that an element will soon change, so the browser can optimize
rendering ahead of time.
Example:
.box {
will-change: transform, opacity;
}Key Takeaway:
Improves animation performance but use carefully — too many can waste memory.
JavaScript JavaScript Tutorial · JavaScript
Answer: calc() performs dynamic calculations for CSS values. Example: div { width: calc(100% - 50px); padding: calc(1em + 5px); } Follow me on LinkedIn: Key Takeaway: calc() mixes units and adjusts layouts dynamically.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
It links human-readable text with a machine-readable value — useful for analytics or scripts.
Follow me on LinkedIn:
Example:
<p>Price: <data value="499">₹499</data></p>
Key Takeaway:
<data> helps embed structured data inside readable content.
JavaScript JavaScript Tutorial · JavaScript
Answer: z-index controls stacking order of overlapping elements. Example: .box1 { z-index: 1; } .box2 { z-index: 10; } Higher values appear on top. Key Takeaway: Only works on positioned elements (position ≠ static).
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
contain tells the browser which aspects of an element’s rendering are independent
from the rest of the document — reducing reflows and repaints.
Example:
.card {
Follow me on LinkedIn:
contain: layout paint;
}Key Takeaway:
Containment isolates elements for performance optimization.
JavaScript JavaScript Tutorial · JavaScript
Answer: Operator Description Example == Compares values after type conversion '5' == 5 → true === Compares values and types '5' === 5 → false Follow me on LinkedIn:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Set equal height and width and use border-radius: 50%. Example: .circle { width: 100px; height: 100px; background: teal; border-radius: 50%; } Key Takeaway: Equal dimensions + border-radius: 50% = perfect circle.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Generators are special functions that can pause and resume execution using the function* syntax and yield. Example: function* counter() { yield 1; yield 2; } const gen = counter(); console.log(gen.next().value); // 1
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Objects store data in key-value pairs. Example: const user = { name: "Alice", age: 25 }; console.log(user.name); // Alice
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: It completely hides the element — it’s not visible and doesn’t take up space in the layout. Example: .hidden { display: none; } Key Takeaway: display: none removes the element from the document flow.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Via CDN: <link href=" p.min.css" rel="stylesheet"> Or via NPM: npm install bootstrap Intermediate
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Example:
<div id="main-header"></div>
<div class="card"></div>
<div class="card"></div>
Key Takeaway:
Use id for specific targeting; class for grouping and styling.
JavaScript JavaScript Tutorial · JavaScript
Instead of attaching listeners to every element, attach one listener to a parent — it
captures events from its children using bubbling.
Example:
document.querySelector('#list').addEventListener('click', e => {
if (e.target.tagName === 'LI') console.log(e.target.textContent);});
Follow me on LinkedIn:
✅ Improves performance and memory usage.
JavaScript JavaScript Tutorial · JavaScript
The <meta> tag provides metadata — like page description, author, and viewport settings.
Example:
<meta name="description" content="Learn web development with real
examples.">
<meta name="viewport" content="width=device-width,
initial-scale=1.0">
Key Takeaway:
Meta tags are essential for SEO and responsive design.
JavaScript JavaScript Tutorial · JavaScript
Use the W3C HTML Validator to check your markup.
To fix errors:
Example:
✅ Correct:
<img src="logo.png" alt="Company Logo">
❌ Incorrect:
<img src="logo.png" alt=Company Logo>
Follow me on LinkedIn:
Key Takeaway:
Clean, valid HTML ensures better rendering, SEO, and accessibility.
JavaScript JavaScript Tutorial · JavaScript
Use <table>, <tr> (row), <th> (header cell), and <td> (data cell).
Example:
<table border="1">
<tr>
<th>Name</th>
<th>Role</th>
</tr>
<tr>
<td>Sandeep</td>
Follow me on LinkedIn:
<td>Developer</td>
</tr>
</table>
Key Takeaway:
Tables are for data — not for layout.
JavaScript JavaScript Tutorial · JavaScript
Logical properties adapt layouts for different writing directions (LTR, RTL, vertical text).
Physical Logical
margin-le
ft
margin-inline-s
tart
padding-t
op
padding-block-s
tart
Example:
p {
padding-inline-start: 10px; /* Works for both LTR and RTL */
}Key Takeaway:
Logical properties make designs multilingual and direction-aware.
JavaScript JavaScript Tutorial · JavaScript
Answer: .container: Fixed width, adjusts at each breakpoint. .container-fluid: Always spans 100% width.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Currying transforms a function that takes multiple arguments into a sequence of functions
that take one argument each.
Example:
function add(a) {
return b => a + b;
}console.log(add(5)(3)); // 8
✅ Useful for function reusability and functional composition.
JavaScript JavaScript Tutorial · JavaScript
const arr1 = [1, 2, 3]; const arr2 = new Array(1, 2, 3);
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: function is a block of reusable code that performs a task or returns a value. Example: function greet(name) { return `Hello, ${name}!`; } console.log(greet("Sandeep"));
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
The <template> tag defines reusable HTML that isn’t rendered until you activate it via
JavaScript.
Example:
<template id="card-template">
<div class="card">
<h3></h3>
<p></p>
</div>
</template>
<script>
const tmpl = document.getElementById("card-template");
const clone = tmpl.content.cloneNode(true);
clone.querySelector("h3").textContent = "HTML Tips";
clone.querySelector("p").textContent = "Use semantic tags for SEO.";document.body.appendChild(clone);
</script>
Key Takeaway:
<template> = invisible HTML blueprint ready to be cloned dynamically.
JavaScript JavaScript Tutorial · JavaScript
<iframe> embeds another HTML page inside the current page.
Example:
<iframe src="
width="500" height="300"></iframe>
Key Takeaway:
Use iframes to display external content, but avoid excessive use for performance and SEO
reasons.
Follow me on LinkedIn:
JavaScript JavaScript Tutorial · JavaScript
Answer: Use visibility: hidden; Example: .invisible { visibility: hidden; } Follow me on LinkedIn: Key Takeaway: Hidden = invisible but occupies space. display: none = gone completely.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
@keyframes define the steps of an animation over time.
Example:
@keyframes moveBox {
0% { left: 0; }
100% { left: 200px; }
}.box {
position: relative;
nimation: moveBox 2s linear infinite;
}Follow me on LinkedIn:
Key Takeaway:
Keyframes control how elements move, rotate, or fade during animation.
JavaScript JavaScript Tutorial · JavaScript
Answer: async/await makes asynchronous code look synchronous. It works with Promises. Follow me on LinkedIn: Example: sync function fetchData() { const data = await fetch("/api"); return data.json(); }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
GPU acceleration is triggered when you use transform, opacity, or will-change.
It moves rendering to the GPU, making animations smoother.
Example:
.box {
transform: translateZ(0);
Follow me on LinkedIn:
}Key Takeaway:
Use GPU-friendly properties for smoother transitions; avoid triggering layout changes.
JavaScript JavaScript Tutorial · JavaScript
Browsers are forgiving — they use an HTML parser that tries to fix mistakes automatically.
For example, missing closing tags are inferred.Example:
<p>This is valid
Follow me on LinkedIn:
<p>This is auto-closed by browser</p>
Key Takeaway:
Browsers recover from errors, but writing valid HTML ensures consistent rendering.
JavaScript JavaScript Tutorial · JavaScript
Answer: Both create objects, but: {} is simpler and faster. new Object() is less common and can be overridden. Example: const a = {}; // Preferred const b = new Object(); // Not preferred
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: It defines a shape that clips (hides) parts of an element. Example: .image { clip-path: circle(50% at 50% 50%); } Key Takeaway: clip-path creates creative shapes like circles, polygons, or custom SVG paths.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
It collects user input and sends it to a server or script for processing.
Example:
<form action="/submit" method="POST">
<input type="text" name="username">
<button type="submit">Submit</button>
</form>
Key Takeaway:
Forms are the foundation of user interaction on the web.JavaScript JavaScript Tutorial · JavaScript
Answer: Controls what happens when content exceeds the container’s size. Values: visible (default) hidden scroll auto Example: div { overflow: auto; } Key Takeaway: overflow: auto adds scrollbars only when needed.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: YouTube provides an embed link using <iframe>. Example: <iframe width="560" height="315" src=" llowfullscreen> </iframe> Key Takeaway: lways use the /embed/ URL format for proper YouTube embedding.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Cards are flexible content containers for images, text, and links. <div class="card" style="width:18rem;"> <img src="..." class="card-img-top"> <div class="card-body">...</div> </div> Follow me on LinkedIn:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Variables declared inside a function are accessible only within that function. Example: function demo() { let x = 10; console.log(x); // 10 } console.log(x); // ReferenceError
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: An event listener waits for user actions (like clicks or keypresses) and runs a function when the event occurs. Example: button.addEventListener("click", () => alert("Clicked!"));
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Functions that execute immediately after being defined. Example: (function() { console.log("Runs instantly!"); })(); Used to create private scopes before let and const.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: A polyfill is a piece of code that adds a feature missing in older browsers. Follow me on LinkedIn: Example: if (!Array.prototype.includes) { rray.prototype.includes = function(item) { return this.indexOf(item) !== -1; }; }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
CSS Houdini is a set of APIs that let developers extend CSS by writing code that hooks
directly into the CSS engine.
Examples:
Example (Paint API):
registerPaint('dots', class {
paint(ctx, size) {
ctx.fillStyle = 'red';ctx.arc(50, 50, 10, 0, 2 * Math.PI);
ctx.fill();
}});
Key Takeaway:
Houdini gives developers low-level control over how CSS works under the hood.
JavaScript JavaScript Tutorial · JavaScript
ssigned Follow me on LinkedIn:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Modals are created using the .modal class and toggled with JS or data attributes.
<button data-bs-toggle="modal"
data-bs-target="#myModal">Open</button>
<div class="modal fade" id="myModal">
<div class="modal-dialog"><div
class="modal-content">...</div></div></div>
JavaScript JavaScript Tutorial · JavaScript
Answer: Type Meaning null Intentional absence of value undefin Variable declared but not assigned Follow me on LinkedIn:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Use the following combination: p { width: 200px; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; } Key Takeaway: Truncates long text with “…” when it exceeds container width.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Functions that: Return the same output for the same input. Have no side effects. Example: function add(a, b) { return a + b; // pure }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
<picture> allows serving different image versions depending on device, screen size, or
media type.
Example:
<picture>
<source srcset="photo-large.jpg" media="(min-width: 800px)">
<source srcset="photo-small.jpg" media="(max-width: 799px)">
<img src="photo-default.jpg" alt="Beautiful landscape">
</picture>
Key Takeaway:
Use <picture> for art direction — showing different images per device or context.
JavaScript JavaScript Tutorial · JavaScript
Entities display reserved characters (like <, >, &) or symbols not on the keyboard.
Example:
<p>Use <div> for containers and & for ampersands.</p>
Key Takeaway:
Entities prevent HTML from confusing symbols with code.
JavaScript JavaScript Tutorial · JavaScript
Example:
<head>
<title>My Portfolio</title>
</head>
<header>
<h1>Welcome to My Portfolio</h1>
</header>
Follow me on LinkedIn:
Key Takeaway:
<head> = document info; <header> = visible header area.JavaScript JavaScript Tutorial · JavaScript
Answer: Property Location Affects Background? Padding Inside border ✅ Yes Margin Outside border ❌ No Example: div { Follow me on LinkedIn: margin: 10px; padding: 20px; } Key Takeaway: Padding = inner spacing, Margin = outer spacing.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Variables declared with let or const inside curly braces {} are accessible only within that block. Example: if (true) { let y = 20; const z = 30; } // console.log(y, z); // ReferenceError
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Add the lang attribute in the <html> tag. Example: <html lang="en"> Key Takeaway: Helps screen readers and search engines understand the page’s language.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Reusable UI blocks like buttons, modals, navbars, forms, alerts, etc., that Bootstrap’s design standards.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Example:
Follow me on LinkedIn:
<script src="main.js" defer></script>
<img src="hero.jpg" loading="lazy" alt="Hero image">
Key Takeaway:
Speed = smaller files, fewer requests, smarter loading.
JavaScript JavaScript Tutorial · JavaScript
Answer: Defines how images or videos resize within their container. Follow me on LinkedIn: Example: img { width: 100%; height: 300px; object-fit: cover; } Key Takeaway: cover fills the box; contain fits the whole image inside.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
✅ Use CSS to improve readability and focus visibility:
Follow me on LinkedIn:
Maintain sufficient color contrast:
body { color: #111; background: #fff; }
button:focus { outline: 2px solid #005fcc; }
Respect user preferences:
@media (prefers-reduced-motion: reduce) {
* { animation: none; }
}ccessible CSS ensures inclusivity for all users — especially keyboard and screen-reader
users.
JavaScript JavaScript Tutorial · JavaScript
Combinators define relationships between selectors.
Combinator Description Example
Descendant ( ) Any nested element div p
Child (>) Direct child only div >
djacent sibling (+) Immediately next element h1 + p
General sibling (~) All next siblings h1 ~ p
Key Takeaway:
Combinators refine selector targeting.
JavaScript JavaScript Tutorial · JavaScript
Answer: rrow functions are a shorter syntax for writing functions and do not have their own this. Example: const add = (a, b) => a + b; console.log(add(2, 3)); // 5
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Static typing: Variable types are known at compile-time (e.g., TypeScript, Java). Dynamic typing: Types are determined at runtime (JavaScript). Example: let x = 10; // number x = "text"; // allowed
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: NaN stands for “Not-a-Number”, representing an invalid numeric operation. Example: console.log("hello" / 2); // NaN
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Targets elements with a specific class attribute.
Example:
.card { background-color: lightgray; }
<div class="card">Profile</div>
Follow me on LinkedIn:
Key Takeaway:
Classes are reusable; use them for consistent styling across elements.JavaScript JavaScript Tutorial · JavaScript
Performance:
increasing page load time.
SEO:
Best Practice:
Example:
<iframe src="
title="Location
Map"></iframe>
Key Takeaway:
Iframes isolate content — great for embedding, but can slow pages and weaken SEO.Follow me on LinkedIn:
CSS Interview Questions
JavaScript JavaScript Tutorial · JavaScript
They allow you to store extra data on HTML elements — useful for scripts or dynamic
behavior.
Example:
<button data-user-id="101" data-role="admin">View Profile</button>
<script>
const btn = document.querySelector("button");console.log(btn.dataset.userId); // 101
console.log(btn.dataset.role); // admin
</script>
Key Takeaway:
data-* attributes are perfect for embedding small pieces of custom data without affecting
layout.
Follow me on LinkedIn:
dvanced
JavaScript JavaScript Tutorial · JavaScript
It applies visual effects (like blur, brightness, or contrast) to the area behind an element.
Example:
.blur-bg {
backdrop-filter: blur(10px);
background: rgba(255, 255, 255, 0.3);
}Key Takeaway:
Used for modern UI effects (like frosted glass) — supported in modern browsers only.
Follow me on LinkedIn:
JavaScript Interview Questions
JavaScript JavaScript Tutorial · JavaScript
Example:
console.log("1");
setTimeout(() => console.log("2"), 0);console.log("3");
// Output: 1, 3, 2
Follow me on LinkedIn:
JavaScript JavaScript Tutorial · JavaScript
Answer: Used to find the type of a variable. typeof "Hello"; // "string" typeof 10; // "number" typeof null; // "object" (bug)
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Feature Regular Function Arrow Function this Dynamic Lexical (inherits from parent) Syntax function f(){} (a,b)=>a+b Can be used as constructor ✅ ❌
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Selects elements based on their order within a parent.
Examples:
li:nth-child(2) { color: red; } /* 2nd item */
li:nth-child(odd) { background: #eee; } /* odd items */
li:nth-child(3n) { font-weight: bold; } /* every 3rd item */
Key Takeaway:
nth-child() gives you precise control over repeating patterns in lists or grids.
dvanced
JavaScript JavaScript Tutorial · JavaScript
Answer: Images that automatically scale with the parent container using .img-fluid: <img src="..." class="img-fluid" alt="Responsive image">
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
<section>).
Example:
<div>Block</div>
<span>Inline</span>
Key Takeaway:
Block = structure; Inline = styling or small content.JavaScript JavaScript Tutorial · JavaScript
The alt attribute provides alternative text when an image fails to load and is read by
screen readers.
Example:
Follow me on LinkedIn:
<img src="team.jpg" alt="Our development team">
Key Takeaway:
lways include meaningful alt text — it’s good for accessibility and SEO.
JavaScript JavaScript Tutorial · JavaScript
Answer: Promises handle asynchronous operations. They represent a value that may be vailable now, later, or never. Example: let promise = new Promise((resolve, reject) => { resolve("Success!"); }); promise.then(res => console.log(res));
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Type Description
Shallow
Copy
Copies top-level properties only
Deep Copy Copies all nested objects too
Follow me on LinkedIn:
Example:
const obj = { a: { b: 1 } };
const shallow = { ...obj }; // same reference
const deep = JSON.parse(JSON.stringify(obj)); // new copyJavaScript JavaScript Tutorial · JavaScript
Answer: function that accepts another function as a parameter or returns a function. Example: function multiplyBy(factor) { return x => x * factor; } const double = multiplyBy(2); console.log(double(5)); // 10
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Override variables using SCSS. Use Bootstrap’s Theme Customizer or utility API. Or override styles in a custom CSS file.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: A style where functions are pure, stateless, and composable. Example: const double = x => x * 2; const square = x => x * x; const result = square(double(3)); // 36
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Unit Relative To Example
em Parent’s font size 2em = 2 × parent
font-size
rem Root (<html>) font
size
2rem = 2 × root font-size
Example:
html { font-size: 16px; }
p { font-size: 2rem; } /* = 32px */Key Takeaway:
Use rem for consistent sizing across the document.
JavaScript JavaScript Tutorial · JavaScript
Example:
<b>Warning:</b> Incorrect password.<br>
<strong>Warning:</strong> Incorrect password.
Key Takeaway:
Use <strong> for emphasis that affects meaning.
JavaScript JavaScript Tutorial · JavaScript
Answer: Promise chaining allows multiple async tasks to run sequentially. Example: fetch('/data') .then(res => res.json()) .then(data => console.log(data)) .catch(err => console.error(err));
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: function passed as an argument to another function to be executed later. Example: function greet(name, callback) { console.log(`Hello, ${name}`); callback(); } greet("Sandeep", () => console.log("Callback executed"));
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Decorators modify classes or methods at runtime. Common in TypeScript and frameworks like Angular. Example (TypeScript): function log(target, key) { console.log(`${key} was called`); } class Example { @log test() {} }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Define a variable with --name and access it with var().
Example:
:root {
}button {
background: var(--main-color);
padding: var(--padding);
}Follow me on LinkedIn:
Key Takeaway:
CSS variables make styles dynamic and reusable.
JavaScript JavaScript Tutorial · JavaScript
Answer: DOM (Document Object Model) represents the structure of an HTML document as a tree of objects, allowing JavaScript to access and manipulate elements dynamically. Follow me on LinkedIn:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Use .navbar and .navbar-expand-* classes: Follow me on LinkedIn: <nav class="navbar navbar-expand-lg navbar-light bg-light"> <a class="navbar-brand" href="#">Brand</a> </nav>
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Example:
<i>Book titles</i> are italicized.
Please <em>do not</em> touch that.
Follow me on LinkedIn:
Key Takeaway:
<em> adds importance — <i> adds style.
JavaScript JavaScript Tutorial · JavaScript
Answer: Media queries apply styles based on device conditions (width, orientation, etc.). Example: @media (max-width: 768px) { body { background-color: lightgray; } } Key Takeaway: Media queries enable responsive design across devices.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: They are collections that hold weak references to objects — allowing garbage collection if no other reference exists. Example: let obj = {}; let wm = new WeakMap(); wm.set(obj, "value"); obj = null; // entry removed automatically
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: JSON (JavaScript Object Notation) is a lightweight format for storing and transferring data. Example: let obj = { name: "Bob" }; let json = JSON.stringify(obj); // Convert to JSON string
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Using prototypes or classes. Example (ES6): class Animal { eat() { console.log("Eating"); } } class Dog extends Animal { bark() { console.log("Bark!"); } Follow me on LinkedIn: }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: function that: Always returns the same output for the same input Has no side effects (doesn’t modify external variables) Example: function sum(a, b) { return a + b; }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Provides consistent button styling: <button class="btn btn-primary">Click</button>
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Async iterators allow looping over asynchronous data sources. Example: sync function* fetchItems() { yield await fetch('/item1'); yield await fetch('/item2'); } for await (let item of fetchItems()) { console.log(item); }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: A favicon is the small icon shown in the browser tab. Example: <link rel="icon" type="image/png" href="favicon.png"> Key Takeaway: Favicons help brand your site in the browser.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Styled inputs, selects, and textareas: <input type="text" class="form-control" placeholder="Enter name">
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Method Mutates Original? Purpose slice(start, end) ❌ No Extracts portion Follow me on LinkedIn: splice(start, count, ...items) ✅ Yes Adds/removes items
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: float moves elements to the left or right — allowing text and inline elements to wrap round. Example: img { float: right; margin: 10px; } Key Takeaway: Used for text wrapping, but Flexbox/Grid is better for layout today.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: localStorage stores key-value pairs in the browser permanently (until cleared). Example: localStorage.setItem("user", "Alice"); console.log(localStorage.getItem("user")); // Alice
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: function that calls itself until a base condition is met. Example: function factorial(n) { if (n === 0) return 1; return n * factorial(n - 1); } console.log(factorial(5)); // 120
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Mutable: Can be changed after creation (e.g., arrays, objects). Immutable: Cannot be changed once created (e.g., strings, numbers).
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
dvanced
What is the difference between order and offset classes?
<div class="col-md-4 order-2 offset-md-1"></div>
Follow me on LinkedIn:
JavaScript JavaScript Tutorial · JavaScript
Type Example Priority
Microtask Promise.then,
MutationObserver
Higher
Macrotask setTimeout, setInterval Lower
Example:
setTimeout(() => console.log("Macro"), 0);
Promise.resolve().then(() => console.log("Micro"));// Output: Micro → Macro
JavaScript JavaScript Tutorial · JavaScript
Use Flexbox utilities:
<div class="d-flex align-items-center" style="height:200px;">
<p>Vertically centered</p>
</div>
Advanced
What is the difference between order and offset classes?
<div class="col-md-4 order-2 offset-md-1"></div>
Follow me on LinkedIn:
JavaScript JavaScript Tutorial · JavaScript
Example:
function greet(name) { // name = parameter
console.log(`Hello ${name}`);
}
greet("Sandeep"); // "Sandeep" = argumentJavaScript JavaScript Tutorial · JavaScript
Answer: It defines the page title shown in the browser tab and search results. Example: <title>My Portfolio | </title> Key Takeaway: Make your titles descriptive for better SEO and UX. Intermediate
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Prefixes ensure CSS works across browsers before full support.
Example:
Follow me on LinkedIn:
.box {
border-radius: 10px; /* Standard */
}Key Takeaway:
Vendor prefixes provide cross-browser compatibility for experimental features.
IntermediateJavaScript JavaScript Tutorial · JavaScript
Answer: It allows unpacking values from arrays or objects. Example: const [a, b] = [1, 2]; const { name, age } = { name: "John", age: 30 };
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
A closure is when a function remembers variables from its outer scope even after the
outer function has finished executing.
Example:
function counter() {
let count = 0;
return function() {count++;
return count;};
}
const add = counter();console.log(add()); // 1
JavaScript JavaScript Tutorial · JavaScript
Answer: An optimization technique to cache function results for repeated inputs. Example: function memoize(fn) { const cache = {}; return x => cache[x] || (cache[x] = fn(x)); }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Type Description Example Synchronous Executes line by line for loop Follow me on LinkedIn: Asynchronou Doesn’t block — runs later via callbacks, promises setTimeout, fetch()
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: They provide default values when no argument is passed. Example: function greet(name = "Guest") { return `Hello, ${name}`; }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: The period between variable declaration and initialization where it cannot be accessed. Example: console.log(x); // ReferenceError let x = 10;
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Use a custom CSS file loaded after Bootstrap. Or customize variables via SCSS before compilation.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: llowing access to outer function variables even after the outer function finishes.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Closures work because functions remember the scope in which they were created, allowing access to outer function variables even after the outer function finishes.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Modules are separate files that export code and import it elsewhere, ensuring encapsulation and reusability. Example: // export.js export const PI = 3.14; // import.js import { PI } from './export.js'; Follow me on LinkedIn:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Example – private counter:
function createCounter() {
let count = 0; // private variable
return {increment: () => ++count,
decrement: () => --count
};
}
const counter = createCounter();console.log(counter.increment()); // 1
console.log(counter.decrement()); // 0
JavaScript JavaScript Tutorial · JavaScript
Answer: rrays/iterable Values for... in Objects Keys Example: for (let i in obj) console.log(i); // keys for (let v of arr) console.log(v); // values
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: No own this No arguments object Cannot be used as constructors No super or new.target
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Bootstrap 5 uses Flexbox by default for its grid system and utilities (.d-flex, .justify-content-*, .align-items-*).
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Method Add Props Modify Delete preventExtensio ns() ❌ No ✅ Yes ✅ Yes seal() ❌ No ✅ Yes ❌ No freeze() ❌ No ❌ No ❌ No
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Loop Used For Iterates Over for Traditional loop Index/count for... Arrays/iterable Values for... Objects Keys Example: for (let i in obj) console.log(i); // keys for (let v of arr) console.log(v); // values
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Arrow functions are shorter, and they don’t have their own this, arguments, or
prototype.
Follow me on LinkedIn:
Example:
const obj = {
val: 10,
normal() { console.log(this.val); }, // Works
rrow: () => console.log(this.val), // Undefined (no own this)
};
JavaScript JavaScript Tutorial · JavaScript
Answer: Yes. Variables inside the outer function cannot be accessed directly, only via inner functions. Example: See createCounter() above.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Use margin (m-*) and padding (p-*) utilities: <div class="p-3 m-2 text-center"></div>
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Context this Refers To
Follow me on LinkedIn:
Global window or global
Method The object owning the method
Constructor The new instance
Arrow function Lexical (surrounding) scope
Example:
const obj = {
value: 10,
show: function() { console.log(this.value); }
obj.show(); // 10
JavaScript JavaScript Tutorial · JavaScript
Answer: A safe way to access nested properties without throwing an error if something is undefined or null. Example: console.log(user?.address?.city);
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Using try...catch...finally or Promise .catch(). Example: try { throw new Error("Something went wrong!"); } catch (err) { console.error(err.message); } finally { console.log("Cleanup code"); } Intermediate
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Use try...catch inside async functions or .catch() for Promises.
Example:
sync function loadData() {
try {
const res = await fetch('/data');
return await res.json();} catch (err) {
console.error("Error:", err);
}Follow me on LinkedIn:
}dvanced
JavaScript JavaScript Tutorial · JavaScript
Lexical scoping means that the scope of a variable is determined by its position in the
source code. Inner functions have access to variables declared in their outer scope.
Example:
function outer() {
let name = "Sandeep";function inner() {
console.log(name); // accesses outer variable
}inner();
}outer(); // Sandeep
JavaScript JavaScript Tutorial · JavaScript
Answer: Install React-Bootstrap (npm install react-bootstrap bootstrap). Import components: import { Button } from 'react-bootstrap'; <Button variant="primary">Click</Button>
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Both execute dynamic code, but:
Example:
eval("var a = 5");
const b = new Function("return 5;");⚠ Both are discouraged due to security and performance issues.
Bootstrap Interview Questions
JavaScript JavaScript Tutorial · JavaScript
Answer: n object is a collection of key-value pairs used to store related data and functions. Example: let person = { name: "Sandeep", age: 30 };
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Reboot is a modernized CSS reset that standardizes browser styles for consistent rendering. Follow me on LinkedIn:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Using object literal: {} Using new Object() constructor Example: let car = { brand: "Tesla", model: "Model 3" }; let car2 = new Object(); car2.brand = "BMW";
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Reusable SCSS functions that generate CSS dynamically. Example: @include border-radius(10px);
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Functions defined inside objects are called methods. Example: let person = { name: "Sandeep", greet() { console.log(`Hello, ${this.name}`); } }; person.greet(); // Hello, Sandeep
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Wrap the table in .table-responsive: <div class="table-responsive"> <table class="table">...</table> </div>
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
this refers to the context in which a function is called.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Import only needed components via SCSS. Use tools like PurgeCSS to remove unused classes. Use custom builds from Bootstrap’s build system.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: rrow: () => console.log(this.name), // undefined in global regular() { console.log(this.name); } // Sandeep }; obj.regular(); obj.arrow();
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Load Bootstrap first, then your custom CSS to override styles. Use namespacing or BEM methodology to prevent conflicts. Combine selectively via SCSS imports. Follow me on LinkedIn:
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Example:
const obj = {
name: "Sandeep",
arrow: () => console.log(this.name), // undefined in global
regular() { console.log(this.name); } // Sandeep
obj.regular();
obj.arrow();
JavaScript JavaScript Tutorial · JavaScript
Answer: Extract values from objects into variables. Example: const person = { name: "Sandeep", age: 30 }; const { name, age } = person; console.log(name, age); // Sandeep 30
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
rrays are ordered collections of values. Example: let numbers = [1, 2, 3, 4];
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
for loop for...of forEach Example: numbers.forEach(n => console.log(n));
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Extract elements from arrays into variables. Example: let [first, second] = [10, 20]; console.log(first, second); // 10 20
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Used to expand arrays or objects into individual elements. Example: const arr = [1, 2, 3]; const newArr = [...arr, 4, 5]; // [1, 2, 3, 4, 5]
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
JavaScript objects inherit properties and methods from other objects through
prototypes. This allows reuse of methods and shared behavior.
Example:
const parent = { greet() { console.log("Hello"); } };
const child = Object.create(parent);child.greet(); // Hello
JavaScript JavaScript Tutorial · JavaScript
The prototype chain is the chain of objects that JavaScript follows to look up properties
or methods.
continues up the chain.
Example:
console.log(child.toString()); // from Object.prototype
JavaScript JavaScript Tutorial · JavaScript
or classes (ES6).
Example using constructor:
function Person(name) { this.name = name; }Person.prototype.greet = function() { console.log(`Hello
${this.name}`); };
const p = new Person("Sandeep");p.greet(); // Hello Sandeep
JavaScript JavaScript Tutorial · JavaScript
Answer: Creates a new object with the specified prototype object. Example: const parent = { greet() { console.log("Hello"); } }; const child = Object.create(parent); child.greet(); // Hello
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
property of functions, used when creating objects with new.
__proto
__
property of objects, points to the object’s prototype (used in the prototype
chain).
Example:
function Person() {}
console.log(Person.prototype); // prototype object
const p = new Person();console.log(p.__proto__); // same as Person.prototype
JavaScript JavaScript Tutorial · JavaScript
Property Description
prototy
A property of functions, used when creating objects with new.
__proto
A property of objects, points to the object’s prototype (used in the prototype
chain).
Example:
function Person() {}
console.log(Person.prototype); // prototype object
const p = new Person();
console.log(p.__proto__); // same as Person.prototype
🔹 6. Classes (ES6+) – Q&A
JavaScript JavaScript Tutorial · JavaScript
Classes are blueprints for creating objects. They provide syntactic sugar over thetraditional prototype-based inheritance.
Example:
class Person {constructor(name, age) {
this.name = name;
this.age = age;
}
}
const sandeep = new Person("Sandeep", 30);console.log(sandeep.name); // Sandeep
JavaScript JavaScript Tutorial · JavaScript
Answer: The constructor is a special method for initializing new objects created from a class. Example: class Car { constructor(brand, model) { this.brand = brand; this.model = model; } }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Feature Function Constructor Class Syntax function Person(){} class Person{} Hoisting Yes, function declarations are hoisted No, classes are not hoisted new required Recommended Required
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Classes support extends to create a subclass that inherits properties and methods from aparent class.
Example:
class Animal {speak() { console.log("Animal speaks"); }
}
class Dog extends Animal {speak() { console.log("Dog barks"); }
}
const dog = new Dog();dog.speak(); // Dog barks
JavaScript JavaScript Tutorial · JavaScript
super() calls the parent class constructor. Must be called before using this in
subclass.
Example:
class Animal {
constructor(name) { this.name = name; }
}
class Dog extends Animal {constructor(name, breed) {
super(name);
this.breed = breed;
}
}JavaScript JavaScript Tutorial · JavaScript
Answer: Static methods are called on the class itself, not on instances. Example: class MathUtils { static square(x) { return x * x; } } console.log(MathUtils.square(5)); // 25
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Yes, getters and setters control access to object properties.
Example:
class Person {
constructor(name) { this._name = name; }get name() { return this._name; }
set name(value) { this._name = value; }
}
const p = new Person("Sandeep");console.log(p.name); // Sandeep
p.name = "Ravi";console.log(p.name); // Ravi
JavaScript JavaScript Tutorial · JavaScript
Answer: llows non-blocking execution, letting other code run while waiting for tasks (e.g., network requests).
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
A callback is a function passed as an argument to another function to be executed after
some operation completes.
Example:
function fetchData(callback) {
setTimeout(() => {
callback("Data received!");
}, 1000);
Follow me on LinkedIn:
}fetchData(console.log);
JavaScript JavaScript Tutorial · JavaScript
Answer: Nested callbacks causing hard-to-read code. Solution: Use Promises or async/await.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
n object representing future completion or failure of an async task.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
pending fulfilled rejected
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: fetch('/api/data') .then(res => res.json()) .then(data => console.log(data)) .catch(err => console.error(err));
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: sync function fetchData() { try { const res = await fetch('/api/data'); const data = await res.json(); console.log(data); } catch (err) { console.error(err); } }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Promise.all – waits for all promises to resolve Promise.race – resolves/rejects as soon as one completes
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Mechanism that manages async callbacks and executes them after the call stack is empty.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Call stack: executes synchronous code Microtask queue: handles Promises Macrotask queue: handles setTimeout, I/O, etc.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
n action like click, load, input, keypress.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
button.addEventListener('click', () => console.log('Clicked!'));
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
When an event occurs in a nested element:
Example:
element.addEventListener('click', handler, true); // capturing
element.addEventListener('click', handler, false); // bubbling
JavaScript JavaScript Tutorial · JavaScript
ttach a listener to a parent to handle events on child elements.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: preventDefault() – stops default browser action stopPropagation() – stops event bubbling
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Catching and managing runtime errors to prevent app crashes.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: Blocks to handle exceptions. try { JSON.parse("invalid"); } catch(e) { console.error(e); }
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Runs always, whether error occurs or not.
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
Answer: class ValidationError extends Error { } throw new ValidationError("Invalid input");
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
ReferenceError, TypeError, SyntaxError, RangeError, EvalError, URIError
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
`Hello ${name}`
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
const [a,b]=[1,2]; const {x,y}={x:10,y:20};
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
let arr2 = [...arr1,4,5]; function sum(...nums){}
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
function greet(name="Guest"){console.log(name);}
In a production JavaScript application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.
Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.
JavaScript JavaScript Tutorial · JavaScript
?, ?. (ES11)
🔹 76. includes() method for arrays (ES7)
Checks if an array contains a specific element and returns true or
false.
const fruits = ["apple", "banana", "mango"];
console.log(fruits.includes("banana")); // true
console.log(fruits.includes("grapes")); // false
🔹 77. Exponentiation operator ** (ES7)
Raises the left operand to the power of the right operand.
console.log(2 ** 3); // 8
console.log(5 ** 2); // 25
🔹 78. async/await (ES8)
Definition: Syntactic sugar over Promises to write asynchronous code
like synchronous code.
Example:
async function fetchData() {
try {
const response = await fetch("
const data = await response.json();
console.log(data);
} catch (err) {
console.error(err);
fetchData();
Notes:
Promise.
rejects.
🔹 79. Object.entries() and Object.values() (ES8)
Object.entries(obj) – Returns an array of [key, value] pairs.
const user = {name: "Sandeep", age: 30};
console.log(Object.entries(user));
// [["name", "Sandeep"], ["age", 30]]
Object.values(obj) – Returns an array of values.
console.log(Object.values(user));
// ["Sandeep", 30]
🔹 80. String.padStart() and String.padEnd() (ES8)
padStart(targetLength, padString) – Pads the start of a string to
reach a desired length.
const str = "5";
console.log(str.padStart(3, "0")); // "005"
padEnd(targetLength, padString) – Pads the end of a string.
console.log(str.padEnd(3, "*")); // "5**"
🔹 81. Rest/Spread for objects (ES9)
Rest operator {...rest} collects remaining properties into a new object.
Spread operator {...obj} copies properties from one object to another.
Example – Rest:
const {a, b, ...rest} = {a:1, b:2, c:3, d:4};
console.log(rest); // {c: 3, d: 4}
Example – Spread:
const obj1 = {x:1, y:2};
const obj2 = {...obj1, z:3};
console.log(obj2); // {x:1, y:2, z:3}
🔹 82. Promise.finally() (ES9)
Runs a callback after a Promise is settled, regardless of success or failure.
Example:
fetch('/api/data')
.then(res => res.json())
.catch(err => console.error(err))
.finally(() => console.log('Fetch attempt finished'));
🔹 83. Asynchronous iterators (ES9)
Allow iterating over asynchronous data streams using for await...of.
Example:
async function* asyncGenerator() {
yield await Promise.resolve(1);
yield await Promise.resolve(2);
yield await Promise.resolve(3);
(async () => {
for await (const num of asyncGenerator()) {
console.log(num);
})();
// Output: 1, 2, 3
🔹 84. Array.flat() and Array.flatMap() (ES10)
Array.flat(depth) – Flattens nested arrays up to the specified depth.
Example:
const arr = [1, [2, [3, 4]]];
console.log(arr.flat(1)); // [1, 2, [3, 4]]
console.log(arr.flat(2)); // [1, 2, 3, 4]
Array.flatMap() – Maps each element and flattens the result by one level.
Example:
const arr = [1, 2, 3];
const result = arr.flatMap(x => [x, x*2]);
console.log(result); // [1, 2, 2, 4, 3, 6]
🔹 85. Object.fromEntries() (ES10)
Converts an array of key-value pairs into an object.
Example:
const entries = [['name', 'Sandeep'], ['age', 30]];
const obj = Object.fromEntries(entries);
console.log(obj); // {name: "Sandeep", age: 30}
🔹 86. Optional catch binding (ES10)
Allows omitting the error parameter in catch if it’s not needed.
Example:
try {
throw new Error("Oops");
} catch {
console.log("Error handled without using the error object");
🔹 87. BigInt (ES11 / 2020)
Represents integers larger than Number.MAX_SAFE_INTEGER.
Example:
const bigNumber = 123456789012345678901234567890n;
console.log(bigNumber + 1n); // 123456789012345678901234567891n
Notes:
🔹 88. Nullish coalescing operator ?? (ES11 / 2020)
Returns the right-hand side value only if the left-hand side is null or undefined.
Example:
const name = null;
console.log(name ?? "Default Name"); // "Default Name"
const age = 0;
console.log(age ?? 18); // 0 (not null or undefined)
🔹 89. Optional chaining ?. (ES11 / 2020)
Safely accesses nested object properties without throwing an error if a property is null or
undefined.
Example:
const user = { profile: { name: "Sandeep" } };
console.log(user.profile?.name); // "Sandeep"
console.log(user.address?.city); // undefined (no error)
🔹 90. Promise.allSettled() (ES11 / 2020)
Waits for all promises to settle (fulfilled or rejected) and returns an array of their results.
Example:
const promises = [Promise.resolve(1), Promise.reject("Error")];
Promise.allSettled(promises).then(results => console.log(results));
// [
// { status: "fulfilled", value: 1 },
// { status: "rejected", reason: "Error" }
// ]
🔹 91. replaceAll() for strings (ES12 / 2021)
Replaces all occurrences of a substring in a string.
Example:
const text = "JavaScript is fun. JavaScript is powerful.";
console.log(text.replaceAll("JavaScript", "JS"));
// "JS is fun. JS is powerful."
Note: Unlike replace(), replaceAll() replaces every match, not just the first.
🔹 92. Promise.any() (ES12 / 2021)
Returns the first fulfilled promise. Rejects only if all promises are rejected.
Example:
const promises = [
Promise.reject("Error1"),
Promise.resolve(42),
Promise.resolve(100)
Promise.any(promises).then(result => console.log(result)); // 42
🔹 93. Logical assignment operators (ES12 / 2021)
Combine logical operations with assignment: &&=, ||=, ??=.
Examples:
let a = true;
let b = false;
// AND assignment
a &&= false;
console.log(a); // false
// OR assignment
b ||= true;
console.log(b); // true
// Nullish assignment
let c = null;
c ??= 10;
console.log(c); // 10
🔹 94. at() method for arrays and strings (ES13 / 2022)
Returns the element at a specific index, supporting negative indexing.
Example – Array:
const arr = [10, 20, 30, 40];
console.log(arr.at(1)); // 20
console.log(arr.at(-1)); // 40 (last element)
Example – String:
const str = "JavaScript";
console.log(str.at(0)); // "J"
console.log(str.at(-3)); // "i"
🔹 95. Top-level await (ES13 / 2022)
Allows using await outside async functions in modules.
Example:
// In a module (ESM)
const data = await fetch("
.then(res => res.json());
console.log(data);
Notes:
🔹 96. Object.hasOwn() (ES13 / 2022)
Checks whether an object has a specific own property (safer than hasOwnProperty).
Example:
const obj = { name: "Sandeep" };
console.log(Object.hasOwn(obj, "name")); // true
console.log(Object.hasOwn(obj, "age")); // false
Advantage:
🔹 97. Array.findLast() and Array.findLastIndex() (ES14+ /
2023+)
testing function.
Example – findLast():
const arr = [1, 2, 3, 4, 5, 6];
const lastEven = arr.findLast(x => x % 2 === 0);
console.log(lastEven); // 6
Example – findLastIndex():
const arr = [1, 2, 3, 4, 5, 6];
const lastEvenIndex = arr.findLastIndex(x => x % 2 === 0);
console.log(lastEvenIndex); // 5
🔹 98. New built-in methods introduced recently (ES14+ / 2023+)
Some useful new methods in modern JavaScript:
Array.toSorted() – Returns a sorted copy of the array without mutating the original.
const arr = [3, 1, 2];
const sortedArr = arr.toSorted();
console.log(sortedArr); // [1, 2, 3]
console.log(arr); // [3, 1, 2]
or replaced.
Note: These methods preserve immutability, unlike their older counterparts like sort()
and reverse().
🔹 99. What is the DOM?
The Document Object Model (DOM) is a programmatic representation of a webpage,
allowing JavaScript to read, manipulate, and modify HTML and CSS.
Example:
console.log(document.body); // Accesses the <body> element
🔹 100. Difference between innerHTML, innerText, and textContent
Example:
const div = document.querySelector("#myDiv");
div.innerHTML = "<p>Hello</p>"; // Inserts <p>Hello</p>
div.innerText = "<p>Hello</p>"; // Displays "<p>Hello</p>"
div.textContent = "<p>Hello</p>"; // Displays "<p>Hello</p>"
🔹 101. How do you select DOM elements?
Use selectors like:
document.getElementById("id")
document.getElementsByClassName("className")
document.getElementsByTagName("tagName")
document.querySelector(".className")
document.querySelectorAll("div p")
🔹 102. Difference between getElementById and querySelector
etc.).
Example:
const el1 = document.getElementById("myDiv");
const el2 = document.querySelector("#myDiv"); // Same result
🔹 103. How do you create and append DOM elements?
const div = document.createElement("div");
div.textContent = "Hello World";
document.body.appendChild(div); // Adds the div to body
🔹 104. How do you update styles using JavaScript?
const div = document.querySelector("#myDiv");
div.style.backgroundColor = "lightblue";
div.style.fontSize = "20px";
🔹 105. What is addEventListener?
Attaches event listeners to DOM elements without overwriting existing ones.
Example:
const button = document.querySelector("#myButton");
button.addEventListener("click", () => {
alert("Button clicked!");
});
🔹 106. What is localStorage?
localStorage allows you to store key-value pairs in the browser that persist even
after the browser is closed.
Example:
localStorage.setItem("username", "Sandeep");
console.log(localStorage.getItem("username")); // "Sandeep"
🔹 107. What is sessionStorage?
sessionStorage stores key-value pairs for the duration of a page session. Data is
cleared when the tab or browser is closed.
Example:
sessionStorage.setItem("token", "abc123");
console.log(sessionStorage.getItem("token")); // "abc123"
🔹 108. Difference between localStorage and sessionStorage
Feature localStorage sessionStorage
Lifetime Persist after browser closes Cleared on tab/browser close
Storage limit ~5-10 MB ~5 MB
Scope Shared across tabs of the same
origin
Only available in the current tab
🔹 109. How do cookies work in JavaScript?
Cookies are small pieces of data stored in the browser and sent to the server with each
request.
Example:
// Set a cookie
document.cookie = "username=Sandeep; expires=Fri, 31 Dec 2025
23:59:59 GMT; path=/";
// Read cookies
console.log(document.cookie);
🔹 110. Difference between cookies and localStorage
Feature Cookies localStorage
Sent to
server?
Yes, with every HTTP request No, only client-side
Storage size ~4 KB ~5-10 MB
Expiry Can have expiration date Persistent until explicitly removed
Accessibility Both client & server Client only
🔹 111. What is the BOM (Browser Object Model)?
The Browser Object Model represents the browser’s window and environment, allowing
JavaScript to interact with the browser itself (not the page content).
Example:
console.log(window.innerWidth); // Width of browser window
console.log(window.navigator.userAgent); // Browser info
🔹 112. Difference between window and document
timers, and document.
Example:
console.log(window.location.href); // Current URL
console.log(document.title); // Page title
🔹 113. How do you use navigator and location objects?
console.log(navigator.userAgent);
console.log(navigator.language);
console.log(location.href); // Full URL
location.href = "
// Redirects page
🔹 114. What is setTimeout() and setInterval()?
setTimeout(() => console.log("Hello after 2s"), 2000);
setInterval(() => console.log("Repeating every 1s"), 1000);
🔹 115. How do you detect browser features?
Feature detection ensures code runs only if a feature is supported.
if ('geolocation' in navigator) {
console.log("Geolocation is supported!");
} else {
console.log("Geolocation not supported");
🔹 116. What is a JavaScript module?
A module is a file that encapsulates code (variables, functions, classes) and exports it so
it can be imported into other files, promoting modularity and reusability.
Example (module.js):
export const pi = 3.14;
export function area(radius) {
return pi * radius * radius;
Usage (main.js):
import { pi, area } from './module.js';
console.log(area(5)); // 78.5
🔹 117. What are named vs default exports?
export const x = 10;
export const y = 20;
// Import
import { x, y } from './module.js';
export default function greet() { console.log("Hello!"); }
// Import
import greetFunc from './module.js';
greetFunc(); // Hello!
🔹 118. How do ES6 modules differ from CommonJS?
Feature ES6 Modules CommonJS
Syntax import / export require / module.exports
Loading Static (compile-time) Dynamic (runtime)
Browser support Native in modern browsers Needs bundler (Webpack,
Node.js)
Tree shaking Supported Not supported
🔹 119. What is tree shaking?
Tree shaking is a technique to remove unused code from a module during bundling,
reducing file size.
Example:
// utils.js
export function usedFunc() { return 1; }
export function unusedFunc() { return 2; }
// Only import usedFunc
import { usedFunc } from './utils.js';
Bundler removes unusedFunc from the final bundle.
🔹 120. What is a memory leak in JavaScript?
A memory leak occurs when memory that is no longer needed is not released, leading
to increased memory usage and potential slowdowns.
Example:
let arr = [];
function addToArray() {
arr.push(new Array(1000000).fill('*')); // Keeps adding large
arrays
Memory keeps growing because references are not cleared.
🔹 121. How can you avoid memory leaks?
🔹 122. What is debouncing and throttling?
call. Useful for input events.
function debounce(func, delay) {
let timer;
return function(...args) {
clearTimeout(timer);
timer = setTimeout(() => func.apply(this, args), delay);
for scroll or resize events.
function throttle(func, limit) {
let lastCall = 0;
return function(...args) {
const now = Date.now();
if (now - lastCall >= limit) {
lastCall = now;
func.apply(this, args);
🔹 123. What is garbage collection?
Garbage collection is the automatic process of freeing memory that is no longer
referenced, handled by the JavaScript engine (e.g., V8).
Example:
let obj = { name: "Sandeep" };
obj = null; // Previous object is now eligible for garbage
collection
🔹 124. What is event loop starvation?
Event loop starvation occurs when long-running synchronous code blocks the event
loop, preventing other tasks (like UI updates or async callbacks) from executing.
Example:
while(true) {} // Infinite loop blocks everything
🔹 125. What are best practices for writing efficient JavaScript?
🔹 126. What is functional programming?
Functional programming (FP) is a programming paradigm where functions are treated as
first-class citizens and focus on pure functions, immutability, and avoiding side effects.
Example:
const add = (a, b) => a + b; // Pure function
🔹 127. What is immutability?
Immutability means data cannot be changed after it is created. Instead, new objects or
arrays are returned when updates are needed.
Example:
const arr = [1, 2, 3];
const newArr = [...arr, 4]; // Original arr remains unchanged
🔹 128. What are map(), filter(), and reduce()?
[1,2,3].map(x => x*2); // [2,4,6]
[1,2,3].filter(x => x>1); // [2,3]
[1,2,3].reduce((sum, x) => sum+x, 0); // 6
🔹 129. What is function composition?
Function composition combines multiple functions into a single function, passing the
output of one as input to the next.
Example:
const double = x => x*2;
const increment = x => x+1;
const compose = (f, g) => x => f(g(x));
compose(double, increment)(3); // (3+1)*2 = 8
🔹 130. What is currying?
Currying transforms a function that takes multiple arguments into a sequence of functions
each taking a single argument.
Example:
const add = a => b => a + b;
add(2)(3); // 5
🔹 131. What is partial application?
Partial application pre-fills some arguments of a function to create a new function.
Example:
const multiply = (a, b) => a * b;
const double = multiply.bind(null, 2);
double(5); // 10
🔹 132. What is a pure function?
A pure function always returns the same output for the same input and has no side
effects.
Example:
const square = x => x*x; // Pure
🔹 133. What is referential transparency?
Referential transparency means an expression can be replaced by its value without
changing the program behavior.
Example:
const x = 10;
console.log(x + 5); // Can replace x with 10 directly
🔹 134. What is the difference between null and undefined?
let a; // undefined
let b = null; // explicitly no value
🔹 135. What is NaN?
NaN stands for Not-a-Number, representing an invalid numeric operation.
console.log("abc" * 2); // NaN
🔹 136. What is eval() and why is it dangerous?
eval() executes a string as JavaScript code. It is dangerous because it can execute
malicious code and cause security vulnerabilities.
eval("2 + 3"); // 5
🔹 137. What is strict mode ("use strict")?
Strict mode enables stricter parsing and error handling in JavaScript, preventing unsafe
actions like creating global variables accidentally.
"use strict";
x = 5; // Error: x is not defined
🔹 138. What is a Symbol?
Symbols are unique and immutable primitive values, often used as object property
keys.
const sym = Symbol('id');
const obj = { [sym]: 123 };
🔹 139. What is a generator function?
Generator functions use function* and can yield multiple values over time, allowing
lazy evaluation.
function* gen() {
yield 1;
yield 2;
const g = gen();
console.log(g.next().value); // 1
🔹 140. What is a WeakMap and WeakSet?
let wm = new WeakMap();
wm.set({}, 'value'); // Key can be garbage collected
🔹 141. What is tail call optimization?
Tail call optimization allows recursive functions to reuse stack frames if the recursive
call is the last operation.
function factorial(n, acc = 1) {
if (n === 0) return acc;
return factorial(n - 1, n * acc); // Tail call
🔹 142. Difference between deep copy and shallow copy?
referenced.
const obj = { a: 1, b: { c: 2 } };
const shallow = { ...obj };
const deep = JSON.parse(JSON.stringify(obj));
🔹 143. What is JSON and how do you parse/serialize it?
const obj = { name: "John" };
const str = JSON.stringify(obj); // serialize
const parsed = JSON.parse(str); // parse
🔹 144. What are template literals?
Template literals allow embedded expressions using backticks `.
const name = "John";
console.log(`Hello ${name}`); // Hello John
🔹 145. Difference between synchronous and asynchronous code?
🔹 146. What are modules and why are they useful?
Modules allow splitting code into reusable files and encapsulation, improving
maintainability.
// math.js
export const add = (a,b) => a+b;
// main.js
import { add } from './math.js';
🔹 147. What is transpilation (Babel, TypeScript)?
Transpilation converts modern JS/TypeScript code into older JavaScript compatible with
older browsers.
🔹 148. What is Type Coercion in JavaScript?
Type coercion is the automatic conversion between types.
console.log('5' - 2); // 3 (string -> number)
🔹 149. How do you detect if a variable is an array?
Array.isArray([1,2,3]); // true
🔹 150. What are polyfills?
Polyfills implement features in older browsers that don’t support them.
if (!Array.prototype.includes) {
Array.prototype.includes = function(el) {
return this.indexOf(el) !== -1;
🔹 151. How do you prevent global namespace pollution?
(function(){
const x = 10;
})();
🔹 152. What are IIFEs (Immediately Invoked Function Expressions)?
IIFE – A function that runs immediately after definition, often used for scoping.
(function() {
console.log("IIFE executed");
})();