What JavaScript concepts are tested in front-end interviews?

Front-end JavaScript interview topics: event loop and call stack, closures and lexical scope, prototype chain and inheritance, async/await vs Promises vs callbacks, this binding rules (call/apply/bind), ES6+ features (destructuring, spread, optional chaining), Array methods (map, filter, reduce), and common design patterns.

What is the JavaScript event loop?

The event loop processes the call stack first, then the microtask queue (Promises, queueMicrotask), then the macrotask queue (setTimeout, setInterval, I/O). Understanding this order explains why Promise callbacks run before setTimeout callbacks even with a 0ms delay. This is a frequent interview question at front-end roles.

Is JavaScript asked in FAANG software engineer interviews?

JavaScript is used for front-end SWE roles at all FAANG companies and for full-stack roles. For general SWE coding rounds, most candidates use Python or Java. JavaScript-specific knowledge (DOM, event loop, closures) is tested in front-end or Node.js-specific interviews at companies like Meta, Google, and LinkedIn.

What is a JavaScript closure?

A closure is a function that retains access to its outer scope's variables even after the outer function has returned. Closures are the mechanism behind: module patterns, factory functions, memoization, and partial application. Interview questions often test closures via loop variable capture bugs and counter implementations.

JavaScript Interview Questions 2026

JavaScript interviews at FAANG test deep language knowledge, not just framework familiarity. Closures, the event loop, and async patterns are tested directly.

Quick Answer

JavaScript front-end interviews test: the event loop (call stack, microtask queue, macrotask queue), closures and lexical scope, prototype chain, async/await and Promise internals, and this binding rules. For general FAANG coding rounds, most candidates use Python or Java, but JavaScript is required for front-end SWE and full-stack roles.

Closures and Scope

A closure is a function that retains access to its outer scope even after the outer function has returned. Classic interview question: what does the following log? for(var i=0;i<3;i++){setTimeout(()=>console.log(i),0)}. Answer: 3, 3, 3. Fix with let or an IIFE.

Common closure question: implement a counter() function that returns an object with increment, decrement, and value methods. The counter value should be private. This tests whether you understand closure-based encapsulation.

The Event Loop

JavaScript is single-threaded. The event loop processes the call stack, then the microtask queue (Promises, queueMicrotask), then the macrotask queue (setTimeout, setInterval, I/O). Microtasks always drain before the next macrotask.

Interview question: what order do these execute? setTimeout(()=>console.log('macro'),0) then Promise.resolve().then(()=>console.log('micro')). Answer: micro first, then macro. Understanding this order is fundamental to async JavaScript debugging.

Promises and async/await

Promise.all runs in parallel and rejects if any promise rejects. Promise.allSettled waits for all to settle regardless of rejection. Promise.race resolves/rejects with the first settled promise. Promise.any resolves with the first fulfilled promise.

Implement a function that retries a promise-returning function up to N times with delay. This is a direct interview prompt. Know how to chain .then() in a loop vs using async/await with a for loop.

Prototype Chain and this

Every object has a [[Prototype]] internal slot pointing to its prototype. Property lookup traverses the chain until it finds the property or reaches null. Object.create(proto) creates an object with a specific prototype.

this binding: in a regular function, this is the caller. In an arrow function, this is lexically bound to the enclosing context. Common mistake: using a regular function as a callback in a class method loses this. Fix with arrow function or .bind(this).

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