Interview Q&A

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]

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.

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

The prototype chain is the chain of objects that JavaScript follows to look up properties

or methods.

• If a property isn’t found on the object itself, JS checks the object's prototype, and

continues up the chain.

Example:

console.log(child.toString()); // from Object.prototype

• Objects inherit from other objects via Object.create(), constructor functions,

or classes (ES6).

• Methods can be shared via prototypes.

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

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

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.

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

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

Classes are blueprints for creating objects. They provide syntactic sugar over the

traditional 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

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; } }

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: Feature Function Constructor Class Syntax function Person(){} class Person{} Hoisting Yes, function declarations are hoisted No, classes are not hoisted new required Recommended Required

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.

Classes support extends to create a subclass that inherits properties and methods from a

parent 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

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;
}
}

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

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.

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

Answer: llows non-blocking execution, letting other code run while waiting for tasks (e.g., network requests).

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.

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);

Answer: Nested callbacks causing hard-to-read code. Solution: Use Promises or async/await.

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.

n object representing future completion or failure of an async task.

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.

pending fulfilled rejected

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: fetch('/api/data') .then(res => res.json()) .then(data => console.log(data)) .catch(err => console.error(err));

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: sync function fetchData() { try { const res = await fetch('/api/data'); const data = await res.json(); console.log(data); } catch (err) { console.error(err); } }

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: Promise.all – waits for all promises to resolve Promise.race – resolves/rejects as soon as one completes

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: Mechanism that manages async callbacks and executes them after the call stack is empty.

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: Call stack: executes synchronous code Microtask queue: handles Promises Macrotask queue: handles setTimeout, I/O, etc.

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.

n action like click, load, input, keypress.

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.