JavaScript Interview Questions and Answers

JavaScript interview questions for .NET developers — closures, the event loop, prototypes, this binding, promises and async/await, and var vs let vs const.

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JavaScript

31Questions

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Basic to AdvancedDifficulty

Fresher to 5+ YrsExperience

All JavaScript Questions (31) Sort by

01 What is the difference between debouncing and throttling? Debounce — wait until events stop for X ms, then fire once (search-as-you-type: run the query only after the user… Performance Intermediate 1–2 Yrs
02 What are first-class functions and higher-order functions? First-class functions means functions are values — you can store them in variables, put them in arrays/objects, pass… Functions & Scope Basic Fresher
03 What is the difference between == and === in JavaScript? === (strict equality) compares value and type with no conversion. == (loose equality) coerces the operands to a common… Fundamentals Basic Fresher
04 What is the difference between null and undefined in JavaScript? undefined means a variable has been declared but not assigned, or a property/return value doesn't exist — it's the… Fundamentals Basic Fresher
05 What is a Promise, and how does it compare to callbacks and async/await? A Promise is an object representing a value that will be available later — it's pending, then fulfilled or rejected.… Async Intermediate 1–2 Yrs
06 What is hoisting in JavaScript? Hoisting is the engine processing declarations before executing code. var declarations are hoisted and initialised to… Functions & Scope Basic Fresher
07 What is the difference between map, filter and reduce? map transforms each element and returns a new array of the same length. filter keeps elements that pass a test and… Arrays Basic Fresher
08 What is event bubbling and capturing in the DOM? When an event fires on an element, it propagates in three phases: capture (document → target, listeners with {capture:… DOM Intermediate 1–2 Yrs
09 What is the difference between an arrow function and a regular function? Arrow functions have no this of their own — they inherit it from the enclosing scope (lexical this). They also have no… Functions & Scope Basic Fresher
10 What is prototypal inheritance in JavaScript? Every JavaScript object has an internal link to another object — its prototype. When you access a property that isn't… Objects Intermediate 2–3 Yrs
11 What is the difference between the spread and rest operators? Both use .... Spread expands an iterable into individual elements — [...a, ...b], fn(...args), {...obj, extra: 1}.… ES6+ Basic Fresher
12 What is destructuring in JavaScript? Destructuring pulls values out of arrays or objects into variables in one statement: const { name, age } = user; or… ES6+ Basic Fresher
13 What are ES modules, and how do they differ from CommonJS? ES modules (import/export) are the standard JavaScript module system — statically analysable, so bundlers can… ES6+ Intermediate 2–3 Yrs
14 What is the difference between debounce and throttle? Debounce — wait until events stop for N ms, then fire once. Good for a search box: run the query after the user pauses… Performance Intermediate 1–2 Yrs
15 What is the difference between localStorage, sessionStorage and cookies? localStorage — key/value, ~5MB, persists until cleared, per origin, not sent to the server. sessionStorage — same but… Browser Basic Fresher
16 What is the DOM, and roughly how does the browser render a page? The DOM is an in-memory tree representation of the HTML that JavaScript can read and modify. Rendering roughly: parse… Browser Intermediate 1–2 Yrs
17 How does fetch work, and how do you handle errors with it? fetch(url) returns a promise that resolves to a Response. The catch: it only rejects on a network failure — a 404 or… Async Intermediate 1–2 Yrs
18 What is the temporal dead zone? The temporal dead zone (TDZ) is the region from the start of a block until a let/const declaration is reached. The… Functions & Scope Intermediate 2–3 Yrs
19 What is currying and partial application? Currying transforms a function of N arguments into N functions of one argument each: add(a)(b)(c). Partial application… Functions & Scope Advanced 3–5 Yrs
20 What is 'callback hell', and how do promises and async/await solve it? Callback hell (“the pyramid of doom”) is deeply nested callbacks — each async step nested inside the previous one's… Async Basic Fresher
21 What is the difference between typeof and instanceof? typeof returns a string for the primitive type ("string", "number", "boolean", "undefined", "function", "object",… Fundamentals Basic Fresher
22 What is memoization, and where is it useful in JavaScript? Memoization caches a function's result keyed by its arguments, so a repeated call with the same inputs returns the… Performance Intermediate 2–3 Yrs
23 What is Cross-Site Scripting (XSS), and how do you prevent it? XSS is injecting attacker-controlled script into a page so it runs in other users' browsers — stealing cookies/tokens,… Security Intermediate 2–3 Yrs
24 Is JavaScript single-threaded or multi-threaded? JavaScript execution is single-threaded — one call stack, one thing at a time. Concurrency comes from the environment:… Async Basic Fresher
25 What is the difference between slice and splice? slice(start, end) returns a shallow copy of a portion of the array and doesn't change the original. splice(start,… Arrays Basic Fresher
26 What are optional chaining (?.) and nullish coalescing (??)? a?.b?.c short-circuits to undefined if any link is null/undefined, instead of throwing. a ?? b returns b only when a… ES6+ Basic Fresher
27 What are Set and Map in JavaScript, and when do you use them over objects and arrays? Set holds unique values — great for de-duplication ([...new Set(arr)]) and fast membership checks. Map is a key/value… ES6+ Basic Fresher
28 What is a closure in JavaScript? A closure is a function bundled with references to the variables from the scope where it was defined. Even after that… Functions & Scope Intermediate 1–2 Yrs
29 How does the JavaScript event loop work? JavaScript runs on a single thread with a call stack. Async work (timers, I/O, promises) is handed to the host, and… Async Advanced 3–5 Yrs
30 What is the difference between var, let and const? var is function-scoped, hoisted and initialised to undefined, and can be redeclared. let and const are block-scoped… Functions & Scope Basic Fresher
31 How is `this` determined in JavaScript? this is set by how a function is called, not where it's defined. Rules in priority order: new → the new object;… Functions & Scope Intermediate 2–3 Yrs

About these JavaScript questions

Full-stack .NET roles almost always include a JavaScript round. The recurring themes are closures, the single-threaded event loop and the task/microtask queues, prototypal inheritance, how this is determined at call time, and the move from callbacks to promises to async/await.

JavaScript Interview Questions and Answers

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About these JavaScript questions