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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

Ordered so the most useful questions for learning each pattern come first.

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🔒