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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

#3let vs const vs varScope & closuresEasyFree #4== vs ===Types & coercionEasyFree #8What is hoisting?Scope & closuresEasyFree #9FizzBuzzArrays & objectsEasyFree Unlock 43 more JavaScript questions · all 1,200 questions for $39/year (about 11¢ a day)See full access #13map, filter and reduceArrays & objectsEasy🔒#15null vs undefinedTypes & coercionEasy🔒#18Arrow functions vs regular functionsModern syntaxEasy🔒#19Shallow copy vs deep copyReferences & copyingEasy🔒#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🔒#31Write a sleep functionAsync & promisesEasy🔒#34Implement groupByArrays & objectsEasy🔒#35Chunk an arrayArrays & objectsEasy🔒#36Remove duplicates from an arrayArrays & objectsEasy🔒#37const vs Object.freezeReferences & copyingEasy🔒#38Classes in JavaScriptthis & prototypesEasy🔒#41What does "use strict" do?Types & coercionEasy🔒#42What is an IIFE?Scope & closuresEasy🔒#43Pure functions and side effectsArrays & objectsEasy🔒#44Higher-order functionsArrays & objectsEasy🔒#48Map vs plain objectArrays & objectsEasy🔒#49Set intersection, union and differenceArrays & objectsEasy🔒#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🔒#62Implement onceBuild it yourselfEasy🔒#63Implement pipe and composeBuild it yourselfEasy🔒#65Parse a query stringBuild it yourselfEasy🔒#66Safely get a nested value (lodash.get)Build it yourselfEasy🔒#67setTimeout vs setIntervalEvent loopEasy🔒#72The call stack and stack overflowEvent loopEasy🔒#74Function declarations vs expressionsScope & closuresEasy🔒#76Why does [10, 1, 2].sort() look wrong?Arrays & objectsEasy🔒#78instanceof vs typeofthis & prototypesEasy🔒#81Debounce vs throttle, when to use whichBuild it yourselfEasy🔒#86Why use TypeScript?Modern syntaxEasy🔒#93Implement deepFreezeReferences & copyingEasy🔒#95Most frequent characterArrays & objectsEasy🔒#96Reverse a string and check palindromesArrays & objectsEasy🔒#97Object.keys, values, entries and fromEntriesArrays & objectsEasy🔒#99Polyfills vs transpilingModern syntaxEasy🔒