Frontend Interview Questions & Coding Practice

Implement an event emitter where subscribe returns a handle whose release method removes that listener.

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

Implement a function that safely reads a nested object path and returns a default when any segment is missing.

Implement get and set — read and write a nested value by a path like 'a.b[0].c', creating missing containers on set.

Implement invert and invertBy — swap an object's keys and values, grouping collisions with invertBy.

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

Implement a localStorage wrapper that stores values with a TTL and returns null once an entry has expired.

Implement mapKeys and mapValues — transform an object's keys or values while preserving its structure.

Build a simplified in-memory ORM with model delegates that expose basic CRUD operations.

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

Implement pick, omit, pickBy, and omitBy — build a subset of an object by a key list or by a predicate.

Implement a function that flattens a nested object into a single-level object using dot-delimited keys.

Build a Turtle class that tracks position and heading as you issue movement and turn commands on a 2D plane.

Implement helpers that accurately identify non-primitive JavaScript values such as arrays, plain objects, dates, and maps.

Implement a function that rebuilds a nested object from a flat map of dot-delimited keys.

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

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

Implement an instanceof check — walk the prototype chain to see whether a constructor's prototype appears in it.

Implement Object.create — make a new object with a given prototype and optional property descriptors.

Implement a Backbone-style Model class that stores attributes and emits change events per key.

Implement a deep clone that copies nested values and correctly handles circular references using a visited map.

Implement a function that determines whether two values are structurally equal across nested objects and arrays.

Build a simplified in-memory ORM with field selection and eagerly included related records.

Implement structuredClone — deep-clone a value including Map, Set, Date, RegExp, typed arrays, and circular references.

Implement a function that patches shallow style objects over a text range without clobbering unrelated keys.

Implement a function that checks whether a string of brackets is properly opened and closed in order.

Convert a Roman numeral string to its integer value, handling subtractive pairs like IV and IX.

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

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

Convert a number's string representation between any two bases from 2 to 36 — parse the digits in the source base, then re-emit them in the target base.

Add and multiply arbitrarily large non-negative integers given as decimal strings, using digit-by-digit schoolbook arithmetic with carries.

Shift each letter by a fixed amount with wraparound, preserving case and passing non-letters through — with ROT13 as the self-inverse case.

Implement a function that counts how many ways a digit string can be decoded into letters using a 1 to 26 mapping.

Compute the minimum number of insertions, deletions, and replacements to turn one string into another — the Levenshtein distance via a 2D dynamic-programming table.

Convert between Excel column titles and numbers in both directions — A is 1, Z is 26, AA is 27 — the classic bijective base-26 system with no zero digit.

Generate every well-formed combination of n pairs of parentheses by backtracking — add an open bracket while any remain, and a close bracket only while it stays balanced.

Build a translation engine with nested key lookup, variable interpolation, pluralization, and locale fallback.

Parse and format ICU MessageFormat strings with plural, select, and nested argument substitution.

Validate IPv4 dotted-quad and IPv6 addresses, handling octet ranges, leading zeros, and :: zero-group compression.

Implement a function that finds the longest substring made of one repeated character allowing up to k swaps.