Top JavaScript Interview Topics for Frontend Developers
The most important JS topics to study for frontend interviews at top companies.
Top JavaScript Interview Topics for Frontend Developers
Here are the most important JavaScript topics to study for frontend interviews.
1. Closures
Functions that remember their lexical scope. Used in: data privacy, currying, debounce, throttle, memoize.
2. The Event Loop
How JS handles async. Microtasks (promises) vs macrotasks (setTimeout). The call stack, Web APIs, and queues.
3. The this Keyword
How this is determined: default, implicit, explicit (call/apply/bind), new, and arrow functions (lexical).
4. call, apply, bind
Setting this explicitly. call (comma args), apply (array args), bind (returns a new function).
5. Promises and async/await
Async handling. Promise.all, allSettled, race, any. try/catch with await. Parallel vs sequential.
6. Prototypes and Prototypal Inheritance
How objects inherit. Prototype chain. Object.create, constructor functions, ES6 classes.
7. Debounce and Throttle
Optimizing event handlers. Debounce: delay until activity stops. Throttle: limit to once per interval.
8. Currying
Transforming f(a, b, c) into f(a)(b)(c). Used in: function composition, partial application.
9. Event Bubbling and Delegation
Event propagation in the DOM. Bubbling (bottom-up), delegation (one listener on parent).
10. Polyfills
Implementing built-in methods from scratch: map, filter, reduce, bind, Promise.all.
11. CORS
Cross-Origin Resource Sharing. Preflight requests (OPTIONS). Headers.
12. async and defer
Script loading strategies. async: download in parallel, execute when ready. defer: download in parallel, execute after parse.
The Takeaway
Top JS interview topics: closures, event loop, this keyword, call/apply/bind, promises/async-await, prototypes, debounce/throttle, currying, event bubbling/delegation, polyfills, CORS, and async/defer. Study each deeply and be able to implement from scratch.
Closures, event loop, this keyword, call/apply/bind, promises/async-await, prototypes, debounce/throttle, currying, event bubbling/delegation, polyfills, CORS, and async/defer. Study each deeply and implement from scratch.
Closures are the foundation of many interview questions: data privacy, currying, debounce, throttle, memoize, and the loop closure bug. Understanding closures is essential for almost every JS interview.
Implementing debounce or throttle from scratch (Flipkart, Walmart), polyfills for map/filter/reduce/bind, the event loop output question (sync, microtask, macrotask order), and currying (sum(1)(2)(3) - Amazon).
Yes. Be able to implement map, filter, reduce, bind, and Promise.all from scratch. Interviewers often ask for polyfills to test your understanding of how these methods work internally.
async: download in parallel, execute as soon as ready (pauses parsing). defer: download in parallel, execute after HTML parsing completes (in order). Use defer for most scripts; async for independent scripts like analytics.
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