Frontend Interview Questions & Coding Practice

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 a custom Array.prototype.square method that returns a new array with each value squared.

Implement a function that splits an array into groups of a given size, with the last group holding the remainder.

Implement a function that constrains a number to lie within an inclusive minimum and maximum range.

Implement a function that returns a new array with every falsy value stripped out.

Implement a function that consolidates rows sharing the same user into a single grouped record.

Implement a function that returns the elements from the first array that do not appear in the second.

Implement a function that drops trailing array elements as long as a predicate keeps returning truthy.

Implement a function that drops leading array elements as long as a predicate keeps returning truthy.

Implement a function that replaces array elements with a given value between an optional start and end index.

Implement a function that checks whether an array contains any duplicate numbers.

Implement a function that returns the index of the first array element matching a predicate, or -1 if none match.

Implement a function that returns the index of the last array element matching a predicate, or -1 if none match.

Implement a function that returns the single missing number from a sorted sequence of integers.

Find the first failing version among 1..n where a monotonic isBad check flips from false to true — a boundary binary search using the fewest checks.

Implement a function that builds an object from an array of [key, value] pair tuples.

Implement a function that checks whether a number lies within a half-open range, swapping bounds when needed.

Implement a function that returns the unique values present in every input array.

Implement lodash keyBy — index a collection into an object keyed by an iteratee's return, keeping the last item for each key.

Implement a function that joins an array of strings into a grammatical list with commas and a conjunction.

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

Implement a function that returns the array element producing the largest value when run through an iteratee.

Implement a function that computes the arithmetic mean of the numbers in an array.

Implement a function that returns the array element producing the smallest value when run through an iteratee.

Implement nth — return the element at index n, counting from the end when n is negative, so head is nth(0) and last is nth(-1).

Implement a function that computes the maximum profit obtainable from one buy and one sell of a stock.

Implement a function that finds two numbers in an array that add up to a given target.

Implement a function that builds an array of numbers from start up to end, stepping by a given increment.

Implement a function that builds an array of numbers from end down to start in descending order.

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

Implement lodash sumBy and meanBy — total and average an array by an iteratee that maps each element to a number.

Implement take and takeWhile — grab the leading n elements, or the leading run that satisfies a predicate.