JavaScript Interview Questions

Question 1

What are the different data types in JavaScript?

Accepted Answer

JavaScript has 8 data types: String, Number, BigInt, Boolean, Undefined, Null, Symbol, and Object. Primitive types (String, Number, BigInt, Boolean, Undefined, Null, Symbol) are immutable and stored by value. Object is reference type and includes arrays, functions, dates, and regular expressions. typeof operator can check types, though typeof null returns 'object' (a known bug). ES6 added Symbol for unique identifiers and BigInt for arbitrarily large integers.

Question 2

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

Accepted Answer

var is function-scoped and hoisted; let is block-scoped and not hoisted; const is block-scoped and cannot be reassigned. var declarations are hoisted to the top of their function scope and initialized as undefined. let and const are block-scoped (within {}) and exist in a 'temporal dead zone' until declared. const requires initialization at declaration and prevents reassignment, but object/array contents can still be mutated. Best practice: use const by default, let when reassignment is needed, avoid var.

Question 3

What is hoisting in JavaScript?

Accepted Answer

Hoisting is JavaScript's behavior of moving declarations to the top of their scope before code execution. During compilation, JavaScript moves variable and function declarations to the top of their containing scope. Function declarations are fully hoisted (can be called before declaration). var variables are hoisted but initialized as undefined. let/const are hoisted but not initialized, creating a temporal dead zone. Class declarations are not hoisted. This is why you can call a function before its declaration in code.

Question 4

Explain the difference between == and ===.

Accepted Answer

== performs type coercion before comparison; === compares both value and type without coercion. The == operator converts operands to the same type before comparing (e.g., '5' == 5 is true). The === operator checks both value and type strictly (e.g., '5' === 5 is false). Always prefer === to avoid unexpected coercion bugs. Notable: null == undefined is true, NaN !== NaN, and objects are compared by reference, not value.

Question 5

What are closures in JavaScript?

Accepted Answer

A closure is a function that retains access to its outer scope's variables even after the outer function has returned. When a function is created inside another function, it forms a closure over the outer function's variables. The inner function 'closes over' and remembers the environment in which it was created. This enables data privacy (module pattern), factory functions, and callback patterns. Example: function counter() { let count = 0; return () => ++count; } — the returned function remembers count.

Question 6

What is the difference between null and undefined?

Accepted Answer

undefined means a variable has been declared but not assigned; null is an intentional assignment representing 'no value'. undefined is the default value of uninitialized variables, missing function parameters, and non-existent object properties. null is explicitly assigned to indicate 'no value' or 'empty'. typeof undefined is 'undefined'; typeof null is 'object' (a legacy bug). null == undefined is true (loose), but null === undefined is false (strict). Use null for intentional absence of value.

Question 7

Explain the event loop in JavaScript.

Accepted Answer

The event loop is the mechanism that handles asynchronous callbacks by monitoring the call stack and task queues. JavaScript is single-threaded. The event loop continuously checks: (1) Is the call stack empty? (2) If yes, take the first callback from the microtask queue (Promises, queueMicrotask). (3) If microtask queue is empty, take from the macrotask queue (setTimeout, setInterval, I/O). Microtasks always execute before macrotasks. This is why Promise.then() runs before setTimeout(fn, 0). The browser also handles rendering between macrotasks.

Question 8

What is the difference between call, apply, and bind?

Accepted Answer

call invokes with arguments listed; apply invokes with arguments as array; bind returns a new function with bound this. func.call(thisArg, arg1, arg2) — immediately invokes with given this and individual arguments. func.apply(thisArg, [args]) — same but accepts arguments as an array. func.bind(thisArg, arg1) — returns a new function with this permanently bound (does not invoke immediately). Use call/apply for borrowing methods; bind for event handlers and callbacks where you need to preserve context.

Question 9

What are Promises and how do they work?

Accepted Answer

Promises represent a value that may be available now, later, or never. They have three states: pending, fulfilled, or rejected. A Promise is created with new Promise((resolve, reject) => {}). It starts as 'pending', then transitions to 'fulfilled' (resolve called) or 'rejected' (reject called). Chain .then() for success, .catch() for errors, .finally() for cleanup. Promise.all() waits for all to resolve; Promise.race() resolves with the first settled; Promise.allSettled() waits for all regardless of outcome. Async/await is syntactic sugar over Promises.

Question 10

Explain prototypal inheritance in JavaScript.

Accepted Answer

Objects can inherit properties from other objects through the prototype chain, where each object has an internal [[Prototype]] link. Every JavaScript object has a hidden [[Prototype]] property linking to another object. When accessing a property, JavaScript first checks the object itself, then walks up the prototype chain. Object.create(proto) creates an object with proto as its prototype. Constructor functions set prototypes via .prototype property. ES6 classes are syntactic sugar over prototypal inheritance. The chain ends at Object.prototype, whose prototype is null.

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JavaScript Fundamentals & Getting Started

Control Flow & Loops

Functions, Scope & Execution Context

Arrays & Data Handling

Objects & OOP in JavaScript

DOM Manipulation & Events

Browser APIs & Web Features

Asynchronous JavaScript

Modern JavaScript (ES6+)

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JavaScript Career & Real-World Development

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JavaScript Fundamentals & Getting Started

Control Flow & Loops

Functions, Scope & Execution Context

Arrays & Data Handling

Objects & OOP in JavaScript

DOM Manipulation & Events

Browser APIs & Web Features

Asynchronous JavaScript

Modern JavaScript (ES6+)

Error Handling & Debugging

Performance & Optimization

JavaScript Career & Real-World Development

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