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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 #3let vs const vs varScope & closuresEasyFree #4== vs ===Types & coercionEasyFree #5Implement debounceBuild it yourselfMediumFree #6How does `this` work?this & prototypesMediumFree #7Promises vs async/awaitAsync & promisesMediumFree #8What is hoisting?Scope & closuresEasyFree #9FizzBuzzArrays & objectsEasyFree #10Implement deepCloneReferences & copyingMediumFree Unlock 90 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🔒#13map, filter and reduceArrays & objectsEasy🔒#14Prototypal inheritancethis & prototypesMedium🔒#15null vs undefinedTypes & coercionEasy🔒#16call, apply and bindthis & prototypesMedium🔒#17Implement Function.prototype.bindthis & prototypesMedium🔒#18Arrow functions vs regular functionsModern syntaxEasy🔒#19Shallow copy vs deep copyReferences & copyingEasy🔒#20Flatten a nested arrayBuild it yourselfMedium🔒#21Implement memoizeBuild it yourselfMedium🔒#22Implement curryBuild it yourselfMedium🔒#23Spread vs rest (`...`)Modern syntaxEasy🔒#24DestructuringModern syntaxEasy🔒#25Truthy and falsy valuesTypes & coercionEasy🔒#26typeof and checking typesTypes & coercionEasy🔒#27Which array methods mutate?Arrays & objectsEasy🔒#28Implement Array.prototype.mapBuild it yourselfEasy🔒#29Implement Array.prototype.reduceBuild it yourselfMedium🔒#30Promise.allSettled, race and anyAsync & promisesMedium🔒#31Write a sleep functionAsync & promisesEasy🔒#32Retry with exponential backoffAsync & promisesMedium🔒#33Implement an EventEmitterBuild it yourselfMedium🔒#34Implement groupByArrays & objectsEasy🔒#35Chunk an arrayArrays & objectsEasy🔒#36Remove duplicates from an arrayArrays & objectsEasy🔒#37const vs Object.freezeReferences & copyingEasy🔒#38Classes in JavaScriptthis & prototypesEasy🔒#39Error handling in async codeAsync & promisesMedium🔒#40Microtasks vs macrotasksEvent loopHard🔒#41What does "use strict" do?Types & coercionEasy🔒#42What is an IIFE?Scope & closuresEasy🔒#43Pure functions and side effectsArrays & objectsEasy🔒#44Higher-order functionsArrays & objectsEasy🔒#45Lexical scope and the scope chainScope & closuresMedium🔒#46Garbage collection and memory leaksReferences & copyingMedium🔒#47WeakMap and WeakSetReferences & copyingMedium🔒#48Map vs plain objectArrays & objectsEasy🔒#49Set intersection, union and differenceArrays & objectsEasy🔒#50What are Symbols?Types & coercionMedium🔒#51Generators and iteratorsModern syntaxMedium🔒#52for…in vs for…ofArrays & objectsEasy🔒#53Optional chaining (?.) and nullish coalescing (??)Modern syntaxEasy🔒#54Capitalize each wordArrays & objectsEasy🔒#55Why is 0.1 + 0.2 !== 0.3?Types & coercionEasy🔒#56NaNTypes & coercionEasy🔒#57Implicit type coercionTypes & coercionMedium🔒#58ES modules vs CommonJSModern syntaxMedium🔒#59Promise chainingAsync & promisesMedium🔒#60Implement a simple PromiseAsync & promisesHard🔒#61Implement deepEqualBuild it yourselfMedium🔒#62Implement onceBuild it yourselfEasy🔒#63Implement pipe and composeBuild it yourselfEasy🔒#64LRU Cache with a MapBuild it yourselfMedium🔒#65Parse a query stringBuild it yourselfEasy🔒#66Safely get a nested value (lodash.get)Build it yourselfEasy🔒#67setTimeout vs setIntervalEvent loopEasy🔒#68Implement promisifyAsync & promisesMedium🔒#69Run async tasks with a concurrency limitAsync & promisesHard🔒#70The Node.js event loop and non-blocking I/OEvent loopMedium🔒#71Web Workers and the main threadEvent loopMedium🔒#72The call stack and stack overflowEvent loopEasy🔒#73The temporal dead zoneScope & closuresMedium🔒#74Function declarations vs expressionsScope & closuresEasy🔒#75JSON.stringify gotchasReferences & copyingMedium🔒#76Why does [10, 1, 2].sort() look wrong?Arrays & objectsEasy🔒#77Array length and holesArrays & objectsMedium🔒#78instanceof vs typeofthis & prototypesEasy🔒#79extends, super and method overridingthis & prototypesMedium🔒#80Custom error classesthis & prototypesMedium🔒#81Debounce vs throttle, when to use whichBuild it yourselfEasy🔒#82Immutable updates of nested stateReferences & copyingMedium🔒#83Composition over inheritancethis & prototypesMedium🔒#84Validate input with a regular expressionArrays & objectsMedium🔒#85JavaScript Date pitfallsTypes & coercionMedium🔒#86Why use TypeScript?Modern syntaxEasy🔒#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🔒#93Implement deepFreezeReferences & copyingEasy🔒#94Implement instanceofthis & prototypesMedium🔒#95Most frequent characterArrays & objectsEasy🔒#96Reverse a string and check palindromesArrays & objectsEasy🔒#97Object.keys, values, entries and fromEntriesArrays & objectsEasy🔒#98Sort objects by multiple keysArrays & objectsMedium🔒#99Polyfills vs transpilingModern syntaxEasy🔒#100Making JavaScript fasterEvent loopMedium🔒