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⚡ Interview Prep

JavaScript

Closures, this, promises, the event loop, and the utility functions front-end and full-stack interviews ask you to build by hand.

100 questions, most asked first

Ranked by how often each question comes up in real interviews. #1 is the most common.

Any difficulty Easy Medium Hard

All patterns Scope & closures Event loop Types & coercion Build it yourself this & prototypes Async & promises Arrays & objects References & copying Modern syntax

#1What is a closure?Scope & closuresMediumFree #2Explain the event loopEvent loopMediumFree #5Implement debounceBuild it yourselfMediumFree #6How does `this` work?this & prototypesMediumFree #7Promises vs async/awaitAsync & promisesMediumFree #10Implement deepCloneReferences & copyingMediumFree Unlock 44 more JavaScript questions · all 1,200 questions for $39/year (about 11¢ a day)See full access #11Implement throttleBuild it yourselfMedium🔒#12Implement Promise.allAsync & promisesMedium🔒#14Prototypal inheritancethis & prototypesMedium🔒#16call, apply and bindthis & prototypesMedium🔒#17Implement Function.prototype.bindthis & prototypesMedium🔒#20Flatten a nested arrayBuild it yourselfMedium🔒#21Implement memoizeBuild it yourselfMedium🔒#22Implement curryBuild it yourselfMedium🔒#29Implement Array.prototype.reduceBuild it yourselfMedium🔒#30Promise.allSettled, race and anyAsync & promisesMedium🔒#32Retry with exponential backoffAsync & promisesMedium🔒#33Implement an EventEmitterBuild it yourselfMedium🔒#39Error handling in async codeAsync & promisesMedium🔒#45Lexical scope and the scope chainScope & closuresMedium🔒#46Garbage collection and memory leaksReferences & copyingMedium🔒#47WeakMap and WeakSetReferences & copyingMedium🔒#50What are Symbols?Types & coercionMedium🔒#51Generators and iteratorsModern syntaxMedium🔒#57Implicit type coercionTypes & coercionMedium🔒#58ES modules vs CommonJSModern syntaxMedium🔒#59Promise chainingAsync & promisesMedium🔒#61Implement deepEqualBuild it yourselfMedium🔒#64LRU Cache with a MapBuild it yourselfMedium🔒#68Implement promisifyAsync & promisesMedium🔒#70The Node.js event loop and non-blocking I/OEvent loopMedium🔒#71Web Workers and the main threadEvent loopMedium🔒#73The temporal dead zoneScope & closuresMedium🔒#75JSON.stringify gotchasReferences & copyingMedium🔒#77Array length and holesArrays & objectsMedium🔒#79extends, super and method overridingthis & prototypesMedium🔒#80Custom error classesthis & prototypesMedium🔒#82Immutable updates of nested stateReferences & copyingMedium🔒#83Composition over inheritancethis & prototypesMedium🔒#84Validate input with a regular expressionArrays & objectsMedium🔒#85JavaScript Date pitfallsTypes & coercionMedium🔒#87TypeScript interface vs typeModern syntaxMedium🔒#88TypeScript genericsModern syntaxMedium🔒#89any vs unknown vs neverTypes & coercionMedium🔒#90Bundlers, tree shaking and code splittingModern syntaxMedium🔒#91Add a timeout to any promiseAsync & promisesMedium🔒#92Deduplicate in-flight requestsAsync & promisesMedium🔒#94Implement instanceofthis & prototypesMedium🔒#98Sort objects by multiple keysArrays & objectsMedium🔒#100Making JavaScript fasterEvent loopMedium🔒