🚀 JavaScript Interview Questions with Answers — Part 1

1. What is JavaScript?

JavaScript is a high-level, interpreted programming language used to create interactive and dynamic web pages. It runs in web browsers and can also be used for backend development with Node.js.

Common Uses:

Interactive websites

Form validation

Animations

API calls

Web applications

Backend development

Example:

console.log("Hello JavaScript");

2. What are the different data types in JavaScript?

JavaScript has two categories of data types.

Primitive Data Types

String

Number

Boolean

Undefined

Null

BigInt

Symbol

Non-Primitive (Reference) Data Types

Object

Array

Function

Example:

let name = "Deepak";   // String
let age = 25;          // Number
let isActive = true;   // Boolean
let data = null;       // Null
let value;             // Undefined

3. What is the difference between var, let, and const?

Feature | var | let | const

Scope | Function | Block | Block

Reassign | ✅ Yes | ✅ Yes | ❌ No

Redeclare | ✅ Yes | ❌ No | ❌ No

Hoisted | ✅ Yes | ✅ Yes (TDZ) | ✅ Yes (TDZ)

Example:

var a = 10;
let b = 20;
const c = 30;

Interview Tip:

Use const by default, let when the value changes, and avoid var in modern JavaScript.

4. What are primitive and non-primitive data types?

Primitive Data Types

Stored by value.

Examples:

String

Number

Boolean

Null

Undefined

BigInt

Symbol

Non-Primitive Data Types

Stored by reference.

Examples:

Objects

Arrays

Functions

Example:

let a = 10;
let b = a;
b = 20;

console.log(a); // 10

Reference Example:

const obj1 = { name: "John" };
const obj2 = obj1;
obj2.name = "Mike";
console.log(obj1.name); // Mike

5. What is type coercion?

Type coercion is JavaScript's automatic conversion of one data type into another during operations or comparisons.

Example:

console.log("5" + 2); // "52"
console.log("5" - 2); // 3

Types:

Implicit coercion (automatic)

Explicit coercion (manual)

Explicit Example:

Number("10"); // 10
String(100);  // "100"

6. What is the difference between == and ===?

== (Loose Equality)

Compares values only

Performs type conversion

=== (Strict Equality)

Compares both value and data type

No type conversion

Example:

console.log(5 == "5");   // true
console.log(5 === "5");  // false

Best Practice:

Always prefer === to avoid unexpected results.

7. What are truthy and falsy values?

Truthy Values

Values treated as true in a boolean context.

Examples:

Non-empty strings

Non-zero numbers

Objects

Arrays

Falsy Values

JavaScript has these falsy values:

false

0

-0

0n

"" (empty string)

null

undefined

NaN

Example:

if ("Hello") {
    console.log("Truthy");
}
if (0) {
    console.log("Won't execute");
}

8. What is undefined?

undefined means a variable has been declared but has not been assigned a value.

Example:

let value;
console.log(value);

Output:

undefined

Key Point:

undefined is assigned automatically by JavaScript.

9. What is null?

null represents an intentional absence of any object value.

It is assigned manually by the developer.

Example:

let user = null;
console.log(user);

Output:

null

Difference:

undefined → No value assigned.

null → Empty value assigned intentionally.

10. What is NaN?

NaN stands for Not a Number.

It represents an invalid numeric result.

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