Polyfill interview questions

Implement findLast and findLastIndex — scan an array from the end for the last element matching a predicate.

Implement Array.prototype.myAt — return the element at a given index, supporting negative offsets from the end.

Implement Array.prototype.every — return false as soon as an element fails the predicate, true when empty.

Implement Array.prototype.filter from scratch as a method on the array prototype.

Implement Array.prototype.flat — flatten nested arrays to a given depth (default 1, Infinity for all levels), dropping holes.

Implement Array.prototype.forEach — run a callback on each element as (value, index, array), skipping sparse holes.

Implement Array.prototype.includes — test membership with SameValueZero so NaN matches, honoring negative fromIndex.

Implement Array.prototype.indexOf — first index of a value by strict equality, supporting a negative fromIndex.

Implement Array.prototype.some — return true as soon as any element passes the predicate, false when empty.

Implement Map.groupBy — group an iterable into a Map whose keys can be any value, preserving key insertion order.

Implement Object.entries — return an object's own enumerable string-keyed [key, value] pairs in insertion order.

Implement Object.fromEntries — build an object from any iterable of [key, value] pairs, with later keys winning.

Implement Object.is — same-value equality that treats NaN as equal to itself and keeps +0 distinct from -0.

Implement Promise.race so it settles with the outcome of whichever input promise settles first.

Implement a function that returns a Promise already rejected with the given reason.

Implement String.prototype.padStart — pad a string to a target length on the left, truncating a multi-char pad.

Implement String.prototype.repeat — join a string n times, throwing RangeError on a negative or infinite count.

Implement String.prototype.trim — strip leading and trailing whitespace, including Unicode whitespace, from a string.

Implement Array.fromAsync: build a promise of an array from a sync or async iterable, awaiting each value in order.

Implement Array.prototype.myConcat — merge arrays and values into a new array, spreading nested arrays one level deep.

Implement Array.prototype.find and findLast — return the first or last element matching a predicate, or undefined.

Implement Array.prototype.flatMap — map each element, then flatten the mapped results by one level.

Implement Array.prototype.myMap — return a new array with the callback applied to each element, skipping holes.

Implement Array.prototype.myReduce — fold an array down to one value, handling sparse arrays and missing initial values.

Implement Array.prototype.reduceRight — fold an array right-to-left, throwing on an empty array with no initial value.

Implement Array.prototype.splice — remove and insert elements in place, returning the removed ones, with clamped indices.

Implement base64 encoding and decoding from scratch — pack bytes into 6-bit groups, map to the alphabet, and handle padding.

Recursively freeze an object and every nested object and array, making the whole structure deeply immutable.

Wire up classical inheritance the pre-class way — link prototypes with Object.create, restore the constructor, and call the superclass constructor.

Implement Function.prototype.myApply — invoke the function with a given `this` and an array of arguments.

Implement Function.prototype.myBind — return a new function with a fixed `this` and optional preset arguments.

Implement Function.prototype.myCall — invoke the function with a given `this` and a list of arguments.

Implement a function that returns all DOM elements containing every one of the specified class names.

Implement the ES2023 change-by-copy trio — toSorted, toReversed, and with — that copy an array instead of mutating it.

Build lazy, chainable iterator helpers — map, filter, take, drop, and flatMap — that pull one value at a time.

Implement a function that converts a JavaScript value into its JSON string representation.

Implement Object.assign — copy own enumerable string and symbol properties from sources onto a target, skipping nullish.

Implement Promise.all so it resolves with an array of results or rejects on the first rejection.

Implement Promise.allSettled from scratch, resolving to an array of fulfilled or rejected outcomes for each input.

Implement Promise.any so it resolves with the first fulfilled value or rejects with an AggregateError.

Implement Promise.resolve — wrap a value in a fulfilled promise, returning thenables unchanged and adopting their state.

Implement Promise.withResolvers — return a new promise alongside its external resolve and reject functions.

Implement replaceAll and matchAll — replace every occurrence of a substring or global pattern, and iterate every match with its capture groups.

Reimplement JavaScript's parseInt — skip leading whitespace, read an optional sign and radix prefix, consume valid digits for the base, and stop at the first invalid character.

Implement newOperator — reproduce the four steps the new keyword performs, honoring the object-return override.

Implement JSON.stringify with full spec support including replacer, indentation, and circular reference detection.