JavaScript

The language, deeply. These are the questions that get asked in every frontend loop — and where machine-coding rounds start.

⭐ Flagship: Promise polyfills + debounce/throttle deep-dive · Output-based questions with answers

Core language #

Topic	Difficulty	Time	Tags	Best Resources
var / let / const, scope, hoisting, TDZ	🟢	45m	#basics	javascript.info: variables ⭐
Data types & coercion (== vs ===)	🟢	45m	#basics	javascript.info: type conversions ⭐
Closures	🟡	1h	#closures	javascript.info: closures ⭐
this binding (4 rules)	🟡	45m	#this	javascript.info: this ⭐
call / apply / bind (+ polyfills)	🟡	45m	#this #polyfill	MDN: bind ⭐
Prototypes & prototype chain	🟡	1h	#prototype #oop	javascript.info: prototypes ⭐
Classes & inheritance	🟡	45m	#oop	javascript.info: classes ⭐
Objects, descriptors, getters/setters	🟡	45m	#objects	javascript.info: property flags ⭐
Destructuring, spread, rest	🟢	30m	#basics	javascript.info: destructuring ⭐
Optional chaining & nullish coalescing	🟢	20m	#basics	MDN: optional chaining ⭐
Symbol & well-known symbols	🟡	30m	#advanced	javascript.info: symbols ⭐
Map / Set vs objects/arrays	🟢	30m	#basics	javascript.info: Map/Set ⭐

Async #

Topic	Difficulty	Time	Tags	Best Resources
Event loop, micro/macrotasks	🟡	1h	#async #internals	Jake Archibald: in the loop ⭐
Callbacks & callback hell	🟢	30m	#async	javascript.info: callbacks ⭐
Promises (states, chaining)	🟡	1h	#async #promises	javascript.info: promises ⭐
Promise.all/allSettled/race/any	🟡	45m	#async #polyfill	Flagship ⭐
async / await + error handling	🟡	45m	#async	javascript.info: async/await ⭐
Debounce & throttle	🟡	45m	#patterns #performance	Flagship ⭐
AbortController & cancellation	🟡	30m	#async	MDN: AbortController ⭐
Async iterators & for await	🔴	45m	#async #iterators	javascript.info: async iteration ⭐
Concurrency control (promise pool)	🔴	45m	#async #patterns	BFE.dev ⭐

Functions & FP #

Topic	Difficulty	Time	Tags	Best Resources
Higher-order functions	🟢	30m	#functional	javascript.info ⭐
Currying & partial application	🟡	45m	#functional	javascript.info: currying ⭐
Composition (pipe/compose)	🟡	30m	#functional	BFE.dev ⭐
Pure functions & immutability	🟡	45m	#functional #state	Immer docs ⭐
Memoization	🟡	30m	#functional #performance	BFE.dev ⭐
Generators	🔴	1h	#generators	javascript.info: generators ⭐
Iterators & iterables	🟡	45m	#iterators	javascript.info: iterables ⭐

Modules, workers & advanced #

Topic	Difficulty	Time	Tags	Best Resources
Modules (ESM vs CJS)	🟡	45m	#modules	javascript.info: modules ⭐
Web Workers	🟡	1h	#workers #performance	MDN: web workers ⭐
Service Workers	🔴	1.5h	#workers #offline	MDN: service workers ⭐
Shared Workers	🔴	45m	#workers	MDN: SharedWorker ⭐
Streams API	🔴	1h	#streams	MDN: streams ⭐
Proxy & Reflect	🔴	1h	#metaprogramming	javascript.info: Proxy ⭐
WeakMap & WeakSet	🟡	45m	#memory	javascript.info: WeakMap ⭐
WeakRef & FinalizationRegistry	🔴	45m	#memory #advanced	MDN: WeakRef ⭐
Memory leaks in JS	🔴	1h	#memory	web.dev: memory ⭐
Regular expressions	🟡	1h	#regex	javascript.info: regexp ⭐
structuredClone & deep clone	🟡	30m	#patterns	MDN: structuredClone ⭐
Polyfills (map/filter/reduce/bind/Promise)	🔴	2h	#polyfill	Flagship ⭐
Numbers, BigInt, floating point	🟡	30m	#basics	javascript.info: numbers ⭐
Date & time handling	🟢	30m	#basics	MDN: Date ⭐

❓ Rapid-fire JavaScript interview questions #

The exact questions asked in real JavaScript interviews. Answer out loud, then verify against the resources above.

1. What is a closure? Give a real-world use case.
2. Explain the event loop — how do microtasks and macrotasks differ?
3. What's the difference between == and ===?
4. What is hoisting? How do var, let, and const differ?
5. How does this work? What do call, apply, and bind do?
6. What is the prototype chain and prototypal inheritance?
7. Difference between null and undefined?
8. What is event delegation and why is it useful?
9. How do you deep clone an object? Shallow vs deep copy?
10. What are debounce and throttle — and when do you use each?
11. Explain Promise.all vs allSettled vs race vs any.
12. What is async/await and how do you handle errors in it?
13. What is the temporal dead zone (TDZ)?
14. How do ES Modules differ from CommonJS?
15. What is currying and partial application?
16. map vs forEach vs reduce — when to use which?
17. What is a generator function?
18. What causes memory leaks in JavaScript?
19. What are WeakMap/WeakSet and when are they useful?
20. What is a Proxy and what can it do?
21. What's the difference between synchronous and asynchronous code?
22. How does garbage collection work in JS engines?
23. What is a pure function and why does immutability matter?
24. What do optional chaining (?.) and nullish coalescing (??) do?
25. How do you cancel a fetch request (AbortController)?
26. slice vs splice? What does Object.freeze do?
27. setTimeout vs setInterval vs requestAnimationFrame?
28. What is the difference between call by value and call by reference?
29. How does Array.prototype.reduce work — implement it.
30. What is the output? (event-loop / closure / hoisting trick questions)

🧩 Output-based & "predict the output" questions #

The trick questions that separate people who know JS from people who use JS.

📖 Want the answers + explanations? See the full worked set → output-based-questions.md

1. What logs? console.log(1); setTimeout(()=>console.log(2)); Promise.resolve().then(()=>console.log(3)); console.log(4);
2. for (var i=0;i<3;i++) setTimeout(()=>console.log(i)) — output? Fix with let.
3. What does [1,2,3].map(parseInt) return and why?
4. Why is 0.1 + 0.2 === 0.3 false?
5. What is typeof null, typeof NaN, typeof function(){}?
6. [] == ![] — true or false? Walk through the coercion.
7. Arrow vs regular function: what is this in each here?
8. Predict the output with hoisting (function vs var declarations).
9. console.log("b" + "a" + +"a" + "a") — what prints?
10. What's the order of async/await + .then + setTimeout logs?
11. {} + [] vs [] + {} — explain.
12. Does modifying a copied object mutate the original? (reference vs value)

💻 "Implement this" coding challenges #

Classic implement-from-scratch prompts — see the flagship and machine-coding utilities.

1. Implement debounce and throttle.
2. Implement Promise.all, allSettled, race, any.
3. Implement Array.prototype.map / filter / reduce.
4. Implement Function.prototype.bind.
5. Implement deepClone (handle cycles).
6. Implement curry, pipe, compose.
7. Implement memoize.
8. Implement an EventEmitter (on/off/emit/once).
9. Implement flatten for a nested array.
10. Implement retry with exponential backoff.
11. Implement once (call a function at most once).
12. Implement a promise-based sleep(ms).
13. Implement groupBy / chunk / uniqBy.
14. Implement an LRU cache.
15. Implement a promise concurrency limiter (pool).

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Related: 02-browser · 04-typescript · 18-design-patterns · 16-machine-coding

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