JavaScript is a high‑level, interpreted (JIT‑compiled), dynamically & weekly typed, multi‑paradigm, prototype‑based, single‑threaded, event‑driven language for building web, mobile, and server‑side applications.
It supports object‑oriented, functional, and imperative styles, and runs:
- In browsers via engines like V8, SpiderMonkey, JavaScriptCore
- On servers via Node.js (V8)
- 🚀 Ultimate JavaScript Interview Playbook
- Dynamically Typed: Variables are not bound to a specific type.
- Weekly Typed: Type coercion is allowed between incompatible types.
- Prototype-based Inheritance: Objects can inherit directly from other objects.
- Single-threaded with Event Loop: Uses call stack, callback queue/task queue/macrotask queue, and microtask queue to manage concurrency.
- Lexical Scoping & Closures: Functions have access to the scope in which they were defined.
- Garbage Collected: Automatic memory management.
- JIT Compiled: Modern JS engines (V8, SpiderMonkey) optimize execution using Just-in-Time compilation.
- V8 (Chrome, Edge)
- SpiderMonkey (Firefox)
- JavaScriptCore (Safari)
- Node.js (V8)
- APIs: modules, streams, clusters, process, filesystem
- Primitive vs Reference types
- Primitive types: Number, String, Boolean, null, undefined, Symbol, BigInt
- Non-primitive: Object, Array, Function
==vs===, truthy/falsy- Type conversions &
typeof,instanceof,Object.prototype.toString.call()
- Execution context: global, function, eval
- Scope chain and lexical scope
- Function scope vs block scope
- Closures: memory, use cases, leaks
- IIFE (Immediately Invoked Function Expression)
- Variables:
var/let/const - Hoisting & Temporal Dead Zone (TDZ)
- Scopes & Lexical Environment
this,eval(),withstatement- Primitive vs Reference types
- Shallow vs Deep copy
- Strict mode
- The
this,windowkeyword - Closures, IIFE
- call, bind, apply
- Callbacks & Promises
Promise.all,Promise.race,Promise.any- Promises: chaining, error handling
async/await: syntax, error flow, parallel execution- Call Stack & Heap
- Event Loop & Task Queues
- Microtasks vs Macrotasks
- Object creation: literals, constructors,
Object.create - Prototypes vs ES6 Classes
- Prototypes and prototype chaining
__proto__vsprototype- ES6 Classes vs constructor functions
- Inheritance patterns: classical vs prototypal
- DOM traversal & manipulation
- Events: bubbling, capturing,
addEventListenervs inline handlers- LocalStorage, SessionStorage, cookies
- Fetch API, XMLHttpRequest
- Web APIs: Geolocation, Notifications, History API
- Modules: CommonJS vs ES Modules
- Global objects:
process,__dirname,__filename - EventEmitter
EventEmitter, Streams, Buffers- OS, Process, Cluster, File System
- File system, Path module
- Async patterns in Node.js
| Concept | Details |
|---|---|
| Call Stack | Function execution contexts (LIFO) |
| Heap | Dynamic memory allocation |
| Event Loop | Manages callbacks, timers, Promises |
| Micro vs Macro Tasks | Promise callbacks vs setTimeout/I/O |
| Execution Context | Creation (scope, hoisting, this) + execution |
| Hoisting | Declarations moved to top of scope |
| Closures | Inner functions retain access to outer scope |
this |
Determined by call‑site & strict/non‑strict mode |
| Scope Chain | Lexical scope lookup |
| Memory Leaks | Unreleased closures, event listeners, DOM references |
| Shadowing & TDZ | Block scope with let/const before initialization |
- Garbage Collection (Mark‑and‑Sweep)
- Memory leaks: closures, DOM references, timers, Detecting & avoiding memory leaks
- Performance profiling (DevTools)
- Currying, Partial Application
- Debounce & Throttle
- Function Composition
- Memoization
- Module pattern, Revealing Module, Singleton patterns
- Factory vs Constructor pattern
- Pub/Sub and Observer pattern
- Chrome DevTools: breakpoints, performance monitor
debuggerkeyword- Unit testing with Jest, Mocha
- Static analysis: ESLint, Prettier
- Bundlers: Webpack, Vite, Parcel
- Transpilers: Babel
- Block‑scoped
let,const - Arrow functions & lexical
this - Template literals & tagged templates
- Destructuring & spread/rest operators
- Modules (
import/export) - Promises, async/await, top‑level await
- Classes (syntactic over prototypes)
- Optional chaining (
?.), nullish coalescing (??) - BigInt, globalThis, WeakRef, Promise.allSettled, etc.
| Domain | Examples |
|---|---|
| Frontend | React, Vue, Angular |
| Backend | Node.js, Express, Nest.js |
| Mobile | React Native, NativeScript |
| Desktop | Electron, Tauri |
| Automation | Puppeteer, Playwright, Deno |
| AI & ML | TensorFlow.js, ONNX.js |
| Edge Compute | Cloudflare Workers, Vercel Edge Functions |
Practice these key concepts to solidify your understanding:
- Global, Block, Function Scope
- Lexical Scope, Shadowing, TDZ
- Module vs Script scope
- The Modern JavaScript Tutorial - In-depth beginner to advanced guide
- JavaScript Tutorial - Comprehensive beginner guide
- JSkatas - Interactive JavaScript exercises
- TypeOfNaN JavaScript Quiz - Fun quizzes on basics
- JavaScript is Weird - Quirks and edge cases
- Prep - Coding interview prep
- FrontendGeeks - Frontend interview questions
- JavaScript Interview Questions - Curated questions
- JavaScript Questions by Lydia Hallie - In-depth explanations
- CodeChef JavaScript Skill Test - Skill assessment
- Advanced JavaScript Quiz - Advanced concepts quiz
- JavaScript Date Master - Date handling challenges
- Codewars - Kata challenges
- BFE.dev - Big Frontend Dev problems
- Greatfrontend - High-quality frontend questions
- JavaScript Challenge #11 - TestDome challenge
- JavaScript Challenge #13 - TestDome challenge
- JavaScript Challenge #14 - TestDome challenge
- JavaScript Challenge #15 - TestDome challenge
- JavaScript Challenge #17 - TestDome challenge
- JavaScript engines zoo
- risingstars js
- React JS Interview Questions - Top React questions
- Top ReactJS Interview Questions - GitHub repo
- React Interview Questions - Comprehensive list
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Licensed under MIT © Atul Kumar Awasthi.