Frontend Interview Questions & Coding Practice

Implement a utility that conditionally joins class name arguments into a single space-separated string.

Implement a function that tallies how many array elements map to each key produced by an iteratee.

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

Implement a function that locates a target integer in a sorted array that has been rotated at an unknown pivot.

Implement a function that recursively flattens a nested array into a single-level array.

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

Implement a function that buckets array elements into an object keyed by the value an iteratee returns for each.

Implement a function that intersects arrays by comparing values produced by an iteratee, keeping uniqueness.

Implement a function that intersects arrays using a user-supplied comparator to decide equality between elements.

Rotate an n×n matrix 90 degrees clockwise and return the result as a new array.

Implement a function that finds the contiguous subarray with the largest product, handling negatives and zeros.

Implement a function that finds the contiguous subarray with the largest sum.

Implement a function that finds the maximum amount of water that can be trapped between two walls.

Implement orderBy — stably sort objects by multiple keys, each ascending or descending.

Implement partition — split an array into elements that pass a predicate and those that fail, in one pass.

Decide whether an array can split into two subsets with equal sums — a subset-sum dynamic-programming problem over half the total.

Find the k-th smallest element of an unsorted array in average linear time by partitioning, without fully sorting it.

Implement a function that finds the minimum value in a sorted array that has been rotated at an unknown pivot.

Implement a function that returns unique values across input arrays using a custom iteratee to determine identity.

Implement zip, unzip, and zipObject — interleave arrays into tuples and back, padding ragged lengths.

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

Implement lodash pull and pullAll — remove every occurrence of the given values from an array in place, mutating the original.

Implement lodash shuffle and sampleSize with an unbiased Fisher-Yates swap, taking an injectable RNG so shuffles are testable.

Extend the classNames utility to dedupe class names and resolve function arguments lazily.

Implement a function that filters an array of row objects by a flexible field-and-condition specification.

Find the median of two sorted arrays in logarithmic time by binary-searching for the partition that splits all the elements into equal halves.

Implement a function that returns the number of declared parameters of a given function.

Implement a function that returns a new object with every value transformed by a mapper callback.

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 a function that returns the element count of any collection — array, string, object, Map, or Set.

Implement a set of helper functions that report the primitive type of any JavaScript value.

Implement a function that decides whether a value is empty across arrays, strings, objects, Maps, Sets, and primitives.

Implement a function that returns a new object with every key converted to camelCase, recursing into nested objects.

Implement a function that returns a deep copy of an object with all falsy values stripped from nested structures.

Implement a function that checks whether an object satisfies a source schema of predicate validators.

Implement a function that recursively clones a JSON-serializable value without sharing references.

Implement a function that recursively merges two objects, combining nested properties rather than overwriting them.

Implement a function that returns a copy of a value with the given keys stripped from every nested object.

Implement an EventEmitter class supporting on, off, and emit to register and trigger listeners.