Interview Q&A

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.

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JavaScript Interview Questions

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

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.

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dvanced

• Block-level elements start on a new line and take full width (e.g., <div>, <p>,

<section>).

• Inline elements stay within a line (e.g., <span>, <a>, <strong>).

Example:

<div>Block</div>

<span>Inline</span>

Key Takeaway:

Block = structure; Inline = styling or small content.

Answer: Override variables using SCSS. Use Bootstrap’s Theme Customizer or utility API. Or override styles in a custom CSS file.

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Promises handle asynchronous operations. They represent a value that may be vailable now, later, or never. Example: let promise = new Promise((resolve, reject) =&gt; { resolve("Success!"); }); promise.then(res =&gt; console.log(res));

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Use .navbar and .navbar-expand-* classes: Follow me on LinkedIn: &lt;nav class="navbar navbar-expand-lg navbar-light bg-light"&gt; &lt;a class="navbar-brand" href="#"&gt;Brand&lt;/a&gt; &lt;/nav&gt;

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

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() {} }

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Define a variable with --name and access it with var().

Example:

:root {

• -main-color: #007bff;
• -padding: 10px;

}

button {

background: var(--main-color);

padding: var(--padding);

}

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Key Takeaway:

CSS variables make styles dynamic and reusable.

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: }

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

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

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

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.

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Styled inputs, selects, and textareas: &lt;input type="text" class="form-control" placeholder="Enter name"&gt;

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

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); }

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

dvanced

What is the difference between order and offset classes?

• .order-*: Changes element order in flex containers.
• .offset-*: Adds left margin space in grids.

<div class="col-md-4 order-2 offset-md-1"></div>

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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?

• .order-*: Changes element order in flex containers.
• .offset-*: Adds left margin space in grids.

<div class="col-md-4 order-2 offset-md-1"></div>

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Prefixes ensure CSS works across browsers before full support.

Example:

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.box {

• webkit-border-radius: 10px; /* Chrome, Safari */
• moz-border-radius: 10px; /* Firefox */

border-radius: 10px; /* Standard */

}

Key Takeaway:

Vendor prefixes provide cross-browser compatibility for experimental features.

Intermediate

Answer: llowing access to outer function variables even after the outer function finishes.

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Use a custom CSS file loaded after Bootstrap. Or customize variables via SCSS before compilation.

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

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:

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: They provide default values when no argument is passed. Example: function greet(name = "Guest") { return `Hello, ${name}`; }

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

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.

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Bootstrap 5 uses Flexbox by default for its grid system and utilities (.d-flex, .justify-content-*, .align-items-*).

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: No own this No arguments object Cannot be used as constructors No super or new.target

What interviewers expect

• A clear definition tied to JavaScript in JavaScript projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

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.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in JavaScript architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

• Data privacy / encapsulation
• Callbacks and event handlers
• Memoization / caching
• Module pattern for structuring code

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