JavaScript Interview Questions: Basic to Advanced

1What is Single-threaded execution model, and what problem does it solve?Basicconceptual30 minIn writing
2Walk me through how call stack and execution contexts actually works under the hood.Basicconceptual35 minIn writing
3What's the practical difference that trips people up about interpreted, then jit-compiled?Basicconceptual30 minIn writing
4How would you use engines: v8 vs spidermonkey in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
5What's the tradeoff with global execution context, and when would you choose differently?Intermediatepractical35 minIn writing
6What's a subtle bug or edge case people hit with strict mode's effect on semantics?Intermediateconceptual30 minIn writing
7In production, an issue traces back to single-threaded execution model. How do you diagnose and fix it?Scenarioscenario45 minIn writing
8Quiz: check your grasp of the javascript runtime and execution model.Basicconceptual20 minIn writing

9What is var vs let vs const scoping, and what problem does it solve?Basicconceptual30 minIn writing
10Walk me through how hoisting and the temporal dead zone actually works under the hood.Basicconceptual35 minIn writing
11What's the practical difference that trips people up about function scope vs block scope?Basicconceptual30 minIn writing
12How would you use shadowing variables in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
13What's the tradeoff with the global object and implicit globals, and when would you choose differently?Intermediatepractical35 minIn writing
14What's a subtle bug or edge case people hit with iifes and why they were needed?Intermediateconceptual30 minIn writing
15In production, an issue traces back to var vs let vs const scoping. How do you diagnose and fix it?Scenarioscenario45 minIn writing
16Quiz: check your grasp of variables, scope, and hoisting.Basicconceptual20 minIn writing

17What is What a closure actually captures, and what problem does it solve?Basicconceptual30 minIn writing
18Walk me through how lexical vs dynamic scoping actually works under the hood.Basicconceptual35 minIn writing
19What's the practical difference that trips people up about closures in loops (var vs let pitfall)?Basicconceptual30 minIn writing
20How would you use module pattern via closures in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
21What's the tradeoff with memory implications of closures, and when would you choose differently?Intermediatepractical35 minIn writing
22What's a subtle bug or edge case people hit with practical uses: memoization, private state?Intermediateconceptual30 minIn writing
23In production, an issue traces back to what a closure actually captures. How do you diagnose and fix it?Scenarioscenario45 minIn writing
24Quiz: check your grasp of closures and lexical scope.Basicconceptual20 minIn writing

25What is Default, implicit, explicit, and new binding, and what problem does it solve?Basicconceptual30 minIn writing
26Walk me through how call/apply/bind actually works under the hood.Basicconceptual35 minIn writing
27What's the practical difference that trips people up about arrow functions and lexical this?Basicconceptual30 minIn writing
28How would you use this inside a class method passed as a callback in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
29What's the tradeoff with losing this with destructured methods, and when would you choose differently?Intermediatepractical35 minIn writing
30What's a subtle bug or edge case people hit with fixing this without bind (arrow wrapper)?Intermediateconceptual30 minIn writing
31In production, an issue traces back to default, implicit, explicit, and new binding. How do you diagnose and fix it?Scenarioscenario45 minIn writing
32Quiz: check your grasp of the `this` keyword and binding.Basicconceptual20 minIn writing

33What is The prototype chain, and what problem does it solve?Basicconceptual30 minIn writing
34Walk me through how object.create vs constructor functions actually works under the hood.Basicconceptual35 minIn writing
35What's the practical difference that trips people up about class syntax as prototype sugar?Basicconceptual30 minIn writing
36How would you use hasownproperty vs inherited properties in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
37What's the tradeoff with object.getprototypeof and setprototypeof, and when would you choose differently?Intermediatepractical35 minIn writing
38What's a subtle bug or edge case people hit with prototype pollution as a security issue?Intermediateconceptual30 minIn writing
39In production, an issue traces back to the prototype chain. How do you diagnose and fix it?Scenarioscenario45 minIn writing
40Quiz: check your grasp of prototypes and prototypal inheritance.Basicconceptual20 minIn writing

41What is Object property descriptors, and what problem does it solve?Basicconceptual30 minIn writing
42Walk me through how array methods: map/filter/reduce vs mutation actually works under the hood.Basicconceptual35 minIn writing
43What's the practical difference that trips people up about spread and rest syntax?Basicconceptual30 minIn writing
44How would you use destructuring with defaults in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
45What's the tradeoff with iterables and the iterator protocol, and when would you choose differently?Intermediatepractical35 minIn writing
46What's a subtle bug or edge case people hit with sparse arrays and holes?Intermediateconceptual30 minIn writing
47In production, an issue traces back to object property descriptors. How do you diagnose and fix it?Scenarioscenario45 minIn writing
48Quiz: check your grasp of objects, arrays, and iteration.Basicconceptual20 minIn writing

49What is Call stack, task queue, microtask queue, and what problem does it solve?Basicconceptual30 minIn writing
50Walk me through how where promises queue relative to settimeout actually works under the hood.Basicconceptual35 minIn writing
51What's the practical difference that trips people up about requestanimationframe vs settimeout(0)?Basicconceptual30 minIn writing
52How would you use starvation from a microtask loop in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
53What's the tradeoff with node's phases vs the browser's loop, and when would you choose differently?Intermediatepractical35 minIn writing
54What's a subtle bug or edge case people hit with why sync code always blocks the loop?Intermediateconceptual30 minIn writing
55In production, an issue traces back to call stack, task queue, microtask queue. How do you diagnose and fix it?Scenarioscenario45 minIn writing
56Quiz: check your grasp of the event loop, microtasks, and macrotasks.Basicconceptual20 minIn writing

57What is Promise states and settling once, and what problem does it solve?Basicconceptual30 minIn writing
58Walk me through how async/await desugars to .then chains actually works under the hood.Basicconceptual35 minIn writing
59What's the practical difference that trips people up about error handling: try/catch vs .catch?Basicconceptual30 minIn writing
60How would you use promise.all vs allsettled vs race vs any in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
61What's the tradeoff with sequential vs parallel awaits, and when would you choose differently?Intermediatepractical35 minIn writing
62What's a subtle bug or edge case people hit with unhandled promise rejections?Intermediateconceptual30 minIn writing
63In production, an issue traces back to promise states and settling once. How do you diagnose and fix it?Scenarioscenario45 minIn writing
64Quiz: check your grasp of promises and async/await.Basicconceptual20 minIn writing

65What is require vs import/export, and what problem does it solve?Basicconceptual30 minIn writing
66Walk me through how synchronous vs static module resolution actually works under the hood.Basicconceptual35 minIn writing
67What's the practical difference that trips people up about live bindings in esm vs copied values in cjs?Basicconceptual30 minIn writing
68How would you use circular dependency behavior differs by system in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
69What's the tradeoff with dynamic import(), and when would you choose differently?Intermediatepractical35 minIn writing
70What's a subtle bug or edge case people hit with interop pain points (dual packages)?Intermediateconceptual30 minIn writing
71In production, an issue traces back to require vs import/export. How do you diagnose and fix it?Scenarioscenario45 minIn writing
72Quiz: check your grasp of modules: commonjs vs esm.Basicconceptual20 minIn writing

73What is class fields and private (#) fields, and what problem does it solve?Basicconceptual30 minIn writing
74Walk me through how static methods and properties actually works under the hood.Basicconceptual35 minIn writing
75What's the practical difference that trips people up about super and constructor chaining?Basicconceptual30 minIn writing
76How would you use mixins since js has no multiple inheritance in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
77What's the tradeoff with getters and setters, and when would you choose differently?Intermediatepractical35 minIn writing
78What's a subtle bug or edge case people hit with class syntax vs factory functions?Intermediateconceptual30 minIn writing
79In production, an issue traces back to class fields and private (#) fields. How do you diagnose and fix it?Scenarioscenario45 minIn writing
80Quiz: check your grasp of classes and modern oop in js.Basicconceptual20 minIn writing

81What is Error vs custom Error subclasses, and what problem does it solve?Basicconceptual30 minIn writing
82Walk me through how try/catch/finally semantics actually works under the hood.Basicconceptual35 minIn writing
83What's the practical difference that trips people up about stack traces and source maps?Basicconceptual30 minIn writing
84How would you use error boundaries in async code in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
85What's the tradeoff with console methods beyond log, and when would you choose differently?Intermediatepractical35 minIn writing
86What's a subtle bug or edge case people hit with debugging with breakpoints vs debugger statement?Intermediateconceptual30 minIn writing
87In production, an issue traces back to error vs custom error subclasses. How do you diagnose and fix it?Scenarioscenario45 minIn writing
88Quiz: check your grasp of error handling and debugging.Basicconceptual20 minIn writing

89What is Higher-order functions, and what problem does it solve?Basicconceptual30 minIn writing
90Walk me through how currying and partial application actually works under the hood.Basicconceptual35 minIn writing
91What's the practical difference that trips people up about function composition?Basicconceptual30 minIn writing
92How would you use pure functions and side effects in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
93What's the tradeoff with immutability conventions in js, and when would you choose differently?Intermediatepractical35 minIn writing
94What's a subtle bug or edge case people hit with recursion vs iteration tradeoffs?Intermediateconceptual30 minIn writing
95In production, an issue traces back to higher-order functions. How do you diagnose and fix it?Scenarioscenario45 minIn writing
96Quiz: check your grasp of functional patterns (higher-order functions, currying).Basicconceptual20 minIn writing

97What is Mark-and-sweep basics, and what problem does it solve?Basicconceptual30 minIn writing
98Walk me through how common leak sources: detached dom nodes, timers, closures actually works under the hood.Basicconceptual35 minIn writing
99What's the practical difference that trips people up about weakmap and weakset for avoiding leaks?Basicconceptual30 minIn writing
100How would you use heap snapshots for finding a leak in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
101What's the tradeoff with garbage collection pauses and their cost, and when would you choose differently?Intermediatepractical35 minIn writing
102What's a subtle bug or edge case people hit with object pooling tradeoffs?Intermediateconceptual30 minIn writing
103In production, an issue traces back to mark-and-sweep basics. How do you diagnose and fix it?Scenarioscenario45 minIn writing
104Quiz: check your grasp of memory management and garbage collection.Basicconceptual20 minIn writing

105What is Event bubbling, capturing, and delegation, and what problem does it solve?Basicconceptual30 minIn writing
106Walk me through how addeventlistener options (once, passive, capture) actually works under the hood.Basicconceptual35 minIn writing
107What's the practical difference that trips people up about reflow and repaint costs?Basicconceptual30 minIn writing
108How would you use mutationobserver and intersectionobserver in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
109What's the tradeoff with web storage vs cookies vs indexeddb, and when would you choose differently?Intermediatepractical35 minIn writing
110What's a subtle bug or edge case people hit with same-origin policy basics?Intermediateconceptual30 minIn writing
111In production, an issue traces back to event bubbling, capturing, and delegation. How do you diagnose and fix it?Scenarioscenario45 minIn writing
112Quiz: check your grasp of browser apis and the dom.Basicconceptual20 minIn writing

113What is Debounce vs throttle, and what problem does it solve?Basicconceptual30 minIn writing
114Walk me through how requestidlecallback and scheduling work actually works under the hood.Basicconceptual35 minIn writing
115What's the practical difference that trips people up about avoiding layout thrashing?Basicconceptual30 minIn writing
116How would you use profiling with the performance panel in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
117What's the tradeoff with memoization for expensive computation, and when would you choose differently?Intermediatepractical35 minIn writing
118What's a subtle bug or edge case people hit with lazy loading and code splitting?Intermediateconceptual30 minIn writing
119In production, an issue traces back to debounce vs throttle. How do you diagnose and fix it?Scenarioscenario45 minIn writing
120Quiz: check your grasp of performance: debouncing, throttling, and profiling.Basicconceptual20 minIn writing

121What is Babel and transpilation targets, and what problem does it solve?Basicconceptual30 minIn writing
122Walk me through how bundlers: tree-shaking and code splitting actually works under the hood.Basicconceptual35 minIn writing
123What's the practical difference that trips people up about source maps for debugged production code?Basicconceptual30 minIn writing
124How would you use unit testing async code in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
125What's the tradeoff with mocking timers and fetch in tests, and when would you choose differently?Intermediatepractical35 minIn writing
126What's a subtle bug or edge case people hit with eslint/prettier's role in a js toolchain?Intermediateconceptual30 minIn writing
127In production, an issue traces back to babel and transpilation targets. How do you diagnose and fix it?Scenarioscenario45 minIn writing
128Quiz: check your grasp of testing and tooling (bundlers, transpilers).Basicconceptual20 minIn writing