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JavaScript interview questions

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Closures, the event loop, prototypes, promises and the quirks that trip people up in front-end loops.

80 questions · 40 theory · 40 coding

Practice this track

Subtopics

Types, coercion & equality7
Scope, hoisting & closures7
this, call, apply & bind7
Prototypes & classes7
Promises & async/await7
The event loop & microtasks7
Arrays & iteration methods7
Objects, destructuring & spread7
Modules (ESM vs CommonJS)6
Error handling6
Maps, Sets & WeakMaps6
Regular expressions & strings6

Sample questions

CodingJuniorArrays & iteration methods
Write entryPoint(items, size) that splits items into consecutive chunks of length size. The last chunk may be shorter if items doesn't divide evenly. If size is less than 1, return an empty array. Return the chunks as an array of arrays, in the original order.
CodingJuniorArrays & iteration methods
Write entryPoint(records, key) that groups an array of plain objects by the value at records[i][key]. Return a plain object whose keys are the group values converted to strings, and whose values are arrays of the original records, in their original relative order. Records whose
TheoryJuniorArrays & iteration methods
What does Array.prototype.map do differently from .forEach? And what does .filter do? Walk me through what each one returns.
CodingJuniorError handling
Write entryPoint(a, b) that divides a by b, safely.

Internally: throw an Error with message "invalid input" if a or b is not a finite number (use Number.isFinite). Otherwise, throw an
TheoryJuniorError handling
Walk me through what try, catch and finally each do in JavaScript. If a function has a return inside try and another return inside finally, which one wins?
CodingJuniorMaps, Sets & WeakMaps
Write entryPoint(items) that counts how many times each value in the array items occurs, and returns the counts as a plain object mapping the string form of each value to its count (so a number 1 and the string "1" count toward the same key). Build the counts with a Map