What is the output of the following code? ```javascript console.log([] == ![]); console.log([] == []); ```

Output: ``` true false ``` **Explanation:** - `[] == ![]`: The unary `!` has higher precedence than `==`. Since `[]` is an object, it is truthy, so `![]` becomes `false`. Now we have `[] == false`. When comparing an object to a boolean, both convert to numbers: `false` becomes `0`, and `[]` calls `[].toString()` -> `""` -> `0`. Since `0 == 0`, it e...

What is React and how does the Virtual DOM work?

React is a JS library for building UIs with reusable components. The Virtual DOM is an in-memory representation of the real DOM. On state change, React creates a new Virtual DOM tree, diffs it against the previous one (reconciliation), computes minimal updates, and batches them to the real DOM. This avoids expensive direct DOM manipulation and enab...

What is Next.js and how does it differ from plain React?

Next.js is a React framework providing server-side rendering (SSR), static site generation (SSG), file-based routing, API routes, image optimization, and more. Plain React is a client-side library — you need to configure bundlers (Vite/Webpack), routing (React Router), data fetching, and SEO manually. Next.js gives these out-of-the-box with convent...

What is Angular and how does it differ from AngularJS?

Angular (2+) is a complete rewrite of AngularJS (1.x). Key differences: Angular uses TypeScript, component-based architecture (vs MVC in AngularJS), supports mobile, has a new change detection system (Zone.js), uses dependency injection with hierarchical injectors, and has a CLI. AngularJS uses JavaScript, controllers/$scope, and is not mobile-read...

What is Node.js and how does it differ from browser JavaScript?

Node.js is a JS runtime built on V8 engine for server-side development. Differences: no DOM/BOM APIs, has server modules (fs, http, path), supports file system, process management, CommonJS/ESM modules, global objects (process, global, Buffer, __dirname). Browser JS has DOM APIs, window, fetch built-in, HTML APIs. Node adds server capabilities; bro...

What is Python and what are its key features?

Python is a high-level, interpreted, dynamically-typed programming language. Key features: readable syntax (indentation-based), dynamically typed (but supports type hints), garbage collected, large standard library, extensive third-party packages (PyPI), cross-platform, supports multiple paradigms (OOP, functional, procedural), interpreted executio...

What is the difference between supervised, unsupervised, and reinforcement learning?

Supervised: labeled data (input→output mapping). Regression and classification tasks. Unsupervised: no labels — find patterns in data (clustering, dimensionality reduction). Reinforcement: agent learns via rewards/punishments by interacting with an environment (game playing, robotics). Key difference: label availability. Supervised needs labels; un...

What is a Large Language Model (LLM) and how does it generate text autoregressively?

An LLM is a deep neural network (typically a Transformer decoder) trained on massive corpora to model the conditional probability distribution of sequences of tokens $P(w_1, w_2, ..., w_n) = \prod_{i=1}^n P(w_i | w_1, ..., w_{i-1})$. Autoregressive generation means the model predicts one token at a time: given an input prompt, it computes logits fo...

What is Retrieval-Augmented Generation (RAG) and what core problems does it solve?

RAG is an architectural pattern that enhances LLM generation by retrieving authoritative, up-to-date external documents from a knowledge base (such as vector databases, enterprise docs, or web search) and injecting them into the LLM's prompt context. It solves three critical LLM limitations: 1. **Hallucinations:** Grounds generation in factual refe...

3145 Questions · 18 Tech Stacks

🎯 Tech Interview Prep & Mastery Studio

Master 3145+ high-frequency interview questions across 18 technologies with instant progress tracking, detailed explanations, and flashcard practice mode.

🎨 Visualizer Studio

Questions Mastered

3145

Total Question Deck

Overall Mastery

Tech Topics

JavaScript

0/200

Tricky JS

0/164

React

0/200

Next.js

0/200

Angular

0/200

Node.js

0/200

Python

0/200

AI / ML

0/233

LLM

0/75

RAG

0/58

CSS

0/200

HTML

0/200

DSA

0/200

MongoDB

0/200

MySQL

0/200

Redis

0/81

Docker

0/200

OOP

0/134

⌘K

JavaScript Interview Questions & Answers200 Questions

0 of 200 Mastered · (0% Completion)

Category Score

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

Easy

Explain hoisting in JavaScript.

Medium

What are closures and how do they work?

Hard

What is the event loop in JavaScript?

Hard

Explain the concept of prototypal inheritance.

Hard

What is the difference between == and ===

Easy

What are Promises and how do they work?

Medium

Explain async/await and how it relates to Promises.

Medium

What is the difference between null and undefined?

Easy

What is a higher-order function? Give examples.

Medium

Explain the difference between call, apply, and bind.

Hard

What is the 'this' keyword and how does it behave?

Hard

What is event delegation and why is it useful?

Medium

What are arrow functions and how are they different from regular functions?

Medium

Explain the concept of scope (global, function, block).

Medium

What is the Temporal Dead Zone (TDZ)?

Hard

What are generators and iterators?

Hard

What is a WeakMap and WeakSet? When would you use them?

Hard

What is memoization? Implement a simple memoize function.

Hard

What are Symbol types used for?

Hard

Explain debouncing and throttling with examples.

Hard

What is the difference between deep copy and shallow copy?

Medium

How does JavaScript handle asynchronous operations?

Medium

What is the difference between setTimeout and setInterval?

Easy

Explain how the prototype chain works.

Hard

What is currying in JavaScript?

Hard

What are pure functions and side effects?

Medium

Explain destructuring assignment in ES6.

Medium

What are template literals and tagged templates?

Medium

What is the spread and rest operator?

Medium

What is optional chaining (?.) and nullish coalescing (??)

Medium

Explain Map vs Object in JavaScript.

Medium

What is Set and how is it different from Array?

Easy

What is the Proxy object in JavaScript?

Hard

How does JavaScript's garbage collection work?

Hard

What is an IIFE and when would you use it?

Medium

What is the difference between synchronous and asynchronous code?

Easy

Explain the concept of coercion in JavaScript.

Medium

What is the module system in JavaScript (CommonJS vs ES Modules)?

Medium

What is tree shaking?

Medium

Explain the difference between Array.map, Array.filter, and Array.reduce.

Medium

What is Array.flat() and Array.flatMap()?

Medium

How does Object.freeze() differ from const?

Medium

What is the difference between Object.keys, Object.values, Object.entries?

Easy

What is a Proxy and Reflect in JavaScript?

Hard

What is function composition?

Hard

What is the event bubbling and capturing?

Medium

What is stopPropagation vs preventDefault?

Medium

What are Web Workers?

Hard

What is Service Worker and how does it differ from Web Worker?

Hard

What is IndexedDB?

Medium

Explain localStorage vs sessionStorage vs cookies.

Medium

What is CORS and how do you handle it?

Medium

Explain XSS and CSRF attacks and prevention.

Hard

What is JSON.stringify and JSON.parse? Gotchas?

Medium

What is the difference between for...in and for...of?

Medium

What are Symbol.iterator and custom iterables?

Hard

What is Promise.all vs Promise.race vs Promise.allSettled vs Promise.any?

Hard

What is microtask queue vs macrotask queue?

Hard

Explain the concept of function overloading in JS.

Medium

What are getters and setters in JavaScript?

Medium

What is the difference between class and function-based constructors?

Medium

What are static methods in a class?

Medium

What is the instanceof operator?

Easy

What is typeof and what are its quirks?

Medium

How does Array.sort work internally?

Hard

What is the difference between slice and splice?

Easy

What are truthy and falsy values in JavaScript?

Easy

What is short-circuit evaluation?

Easy

What is tail call optimization?

Hard

Explain Object.assign vs spread operator for objects.

Medium

What is chaining in JavaScript?

Medium

What is lazy evaluation?

Hard

What are design patterns? Name 3 common ones in JS.

Hard

What is the Observer pattern?

Hard

What is the Singleton pattern?

Medium

What is the Module pattern?

Medium

How do you handle errors in async/await?

Medium

What is try/catch/finally?

Easy

What are custom errors in JavaScript?

Medium

What is the difference between throw and reject?

Medium

What is function memoization and implement it.

Hard

What is the difference between composition and inheritance?

Hard

What is Array.from() used for?

Medium

Explain the concept of lazy loading.

Medium

What are bitwise operators in JavaScript?

Hard

What is NaN and how to check for it?

Medium

What is Infinity in JavaScript?

Easy

What are property descriptors in JavaScript?

Hard

What is Object.create()?

Hard

What is the difference between arguments object and rest params?

Medium

What are tagged template literals?

Hard

What is the globalThis object?

Medium

What is the AbortController API?

Hard

What is the Intersection Observer API?

Medium

What is ResizeObserver?

Medium

How do you implement a polyfill?

Hard

What is the difference between declaration and expression functions?

Medium

What is an async generator?

Hard

100

What is BigInt in JavaScript?

Medium

101

What is the difference between Object.seal() and Object.freeze()?

Hard

102

How would you optimize the loading performance of a large JavaScript bundle?

Hard

103

What are the Core Web Vitals (LCP, INP, CLS) and how do you improve them?

Hard

104

What is hydration in modern frameworks and what is the hydration mismatch problem?

Hard

105

Explain server-side rendering (SSR) vs static site generation (SSG) vs client-side rendering.

Hard

106

What are React Server Components and how do they change data fetching?

Hard

107

How do you prevent prototype pollution attacks in JavaScript?

Hard

108

What is Content Security Policy (CSP) and how does it mitigate XSS?

Hard

109

How would you implement rate limiting on the client side for API calls?

Medium

110

What is the difference between HTTP polling, long polling, SSE, and WebSockets?

Hard

111

How do you handle authentication tokens securely (where to store JWT)?

Hard

112

What is the difference between cookies with HttpOnly, Secure, and SameSite attributes?

Medium

113

How does the JavaScript engine optimize code (JIT, hidden classes, inline caching)?

Hard

114

What are top-level await and dynamic import() and when are they useful?

Medium

115

What is the difference between structuredClone() and JSON deep copy?

Medium

116

What are the new Array methods like at(), findLast(), and toSorted()?

Medium

117

What is the Temporal API and what problems does it solve over Date?

Hard

118

How would you debug a memory leak in a long-running single-page application?

Hard

119

What is the difference between throttling network and CPU in performance profiling?

Medium

120

How do you implement optimistic UI updates with rollback on failure?

Hard

121

What strategies do you use for client-side caching of API responses?

Medium

122

What is code splitting and how do you implement route-based splitting?

Medium

123

How does HTTP/2 multiplexing change front-end bundling strategies?

Hard

124

What is the difference between defer and async attributes on script tags?

Medium

125

How would you architect a feature flag system on the front end?

Medium

126

How do you measure and reduce time-to-interactive in a web app?

Hard

127

What are the security implications of using eval() and the Function constructor?

Medium

128

How would you implement retry with exponential backoff for failed requests?

Medium

129

What is the difference between idempotent and non-idempotent API operations?

Medium

130

How do you handle race conditions when multiple async requests update the same state?

Hard

131

What is the PRPL pattern in web performance?

Hard

132

How does the browser's preload, prefetch, and preconnect resource hints work?

Medium

133

What is the difference between a monorepo and polyrepo for JS projects?

Medium

134

How would you design a plugin architecture in a JavaScript application?

Hard

135

What is the difference between ESBuild, SWC, Webpack, and Vite as build tools?

Medium

136

Implement a debounce function with leading and trailing options.

Hard

137

Implement a throttle function from scratch.

Hard

138

Write a function that deep clones an object without using structuredClone.

Hard

139

Implement a curry function that supports partial application.

Hard

140

Write a function to flatten a deeply nested array without using Array.flat.

Medium

141

Implement Promise.all from scratch.

Hard

142

Implement Promise.race from scratch.

Hard

143

Implement a custom memoize function with a configurable cache key.

Hard

144

Write a function that retries a promise N times before rejecting.

Hard

145

Implement an event emitter (on, off, emit) class.

Medium

146

Write a function to deep compare two objects for equality.

Hard

147

Implement a function that limits concurrency of async tasks (promise pool).

Hard

148

Write a polyfill for Array.prototype.map.

Medium

149

Write a polyfill for Function.prototype.bind.

Hard

150

Implement a function to chunk an array into groups of size n.

Easy

151

Write a function that groups an array of objects by a given key.

Medium

152

Implement a simple LRU cache.

Hard

153

Write a function to convert a callback-based API into a promise-based one.

Medium

154

Implement a function that pipes multiple functions left to right (pipe).

Medium

155

Implement compose (right to left function composition).

Medium

156

Write a function to remove duplicates from an array of objects by a property.

Medium

157

Implement a recursive function to compute the nth Fibonacci with memoization.

Medium

158

Write a function to deeply freeze an object.

Medium

159

Implement a function that parses a query string into an object.

Medium

160

Write a function that serializes an object into a query string.

Medium

161

Implement a simple Pub/Sub system.

Medium

162

Write a function to detect if a string is a valid palindrome ignoring punctuation.

Easy

163

Implement a function to find the intersection of two arrays.

Easy

164

Write a custom implementation of JSON.stringify for simple objects.

Hard

165

Implement a function that runs async tasks in series and collects results.

Medium

166

Write a function that throttles using requestAnimationFrame.

Hard

167

Implement a function to convert a flat array with parentId into a tree.

Hard

168

Write a function that capitalizes the first letter of each word.

Easy

169

Implement a once function that ensures a function runs only one time.

Medium

170

Write a function that schedules a task to run after the current call stack clears.

Medium

171

What is the output of: console.log(typeof null + " " + typeof undefined)?

Medium

172

What is the output of: console.log(0.1 + 0.2 === 0.3)?

Easy

173

What is the output of: console.log([] + [])?

Medium

174

What is the output of: console.log([] + {})?

Medium

175

What is the output of: console.log(1 < 2 < 3); console.log(3 > 2 > 1)?

Hard

176

What is the output of: console.log([1,2,3] + [4,5,6])?

Medium

177

What is the output of: console.log(typeof NaN)?

Easy

178

What is the output of: console.log(0.1 + 0.2)?

Medium

179

What is the output of: var a = 1; function f(){ console.log(a); var a = 2; } f()?

Hard

180

What is the output of: for(var i=0;i<3;i++){ setTimeout(()=>console.log(i),0); }?

Hard

181

What is the output of: for(let i=0;i<3;i++){ setTimeout(()=>console.log(i),0); }?

Hard

182

What is the output of: console.log(1 + "2" + 3)?

Easy

183

What is the output of: console.log("5" - 2 + "1")?

Medium

184

What is the output of: console.log([1,2,3].map(parseInt))?

Hard

185

What is the output of: console.log(!!"false" == !!"true")?

Medium

186

What is the output of: const o = {}; console.log(o.toString())?

Medium

187

What is the output of: console.log(null == undefined, null === undefined)?

Medium

188

What is the output of: console.log(NaN === NaN)?

Easy

189

What is the output of: async function f(){ return 1; } console.log(f())?

Medium

190

What is the output of: console.log("a","b"); console.log(Promise.resolve(1))?

Medium

191

What is the output order of: console.log(1); setTimeout(()=>console.log(2)); Promise.resolve().then(()=>console.log(3)); console.log(4)?

Hard

192

What is the output of: const obj = { a: 1, b: this.a }; console.log(obj.b)?

Hard

193

What is the output of: function f(){ return; { ok: true } } console.log(f())?

Hard

194

What is the output of: console.log(0 == "")?

Medium

195

What is the output of: console.log([] == ![])?

Hard

196

What is the output of: let x = (1,2,3); console.log(x)?

Medium

197

What is the output of: console.log(typeof typeof 1)?

Medium

198

What is the output of: console.log((function(){ return typeof arguments; })())?

Medium

199

What is the output of: const a = [1,2,3]; a.length = 0; console.log(a[0])?

Easy

200

What is the output of: console.log(parseInt("0x10"), parseInt("10", 2))?

Hard