Medium frontend interview questions

Maximize the value packed into a weight-limited knapsack when each item can be taken at most once — the classic 0/1 dynamic-programming table.

Assign users to weighted experiment variants deterministically, with a traffic gate and independent buckets per experiment.

Fetch with a timeout that aborts the request via AbortController and honors an external abort signal.

Build an accordion component with the correct ARIA roles, states, and properties for screen readers.

Build an analog clock whose hour, minute, and second hands move smoothly to reflect the current time.

Build an analytics client that queues tracked events, flushes them in batches, and retries failed sends with backoff.

Tokenize and evaluate a math expression string with operator precedence, parentheses, and unary minus.

Return an array where each entry is the product of every element except the one at that index, without division and in O(n).

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 — the reduce method from scratch, including the empty-array + initial-value edge cases.

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.

Build an autocomplete that debounces typing, fetches matching results from an API, and handles loading and stale responses.

Implement an async debounce that resolves only the newest call and discards the results of superseded ones.

Implement an async memoize that caches results by key for a TTL and coalesces concurrent calls for the same key.

Implement an async mutex whose acquire returns a release function, serializing access so only one holder runs at a time.

Implement an async semaphore that admits up to N concurrent holders and queues the rest until a permit frees.

Implement a task queue that runs async jobs under a concurrency limit and resolves an idle promise when drained.

Build an image carousel that auto-advances on a timer, pauses on hover, and has manual controls.

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.

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

Buffer events and flush them in batches when a size threshold or a max-wait timer is reached.

Implement before and after — cap how many times a function runs, or delay invoking it until the Nth call.

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

Implement binary search on a sorted array, returning the index of the target or -1 when absent.

Implement a function that returns the kth smallest value in a binary search tree.

Implement a function that finds the lowest common ancestor of two nodes in a binary search tree.

Build a binary tree data structure with the essential insert, traverse, and search operations.

Implement a function that returns the values of a binary tree grouped by level from top to bottom.

Implement a function that reconstructs a binary tree given its preorder and inorder traversal sequences.

Implement a function that determines whether one binary tree appears as a subtree of another.

Build a widget that fetches birth year data from an API and renders it as a histogram of bucketed counts.

Implement a function that returns an array of set-bit counts for every integer from 0 up to n.

Build a Bloom filter — a compact probabilistic set that answers membership with no false negatives but possible false positives, using a bit array and several hash functions.

Implement breadth-first traversal of a directed graph, visiting nodes level by level from a start vertex.

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

Wrap a promise so it can be cancelled — the wrapper rejects on cancel and ignores the underlying settlement.

Convert any string between camelCase, snake_case, kebab-case, and PascalCase by tokenizing it into words first.

Implement a circuit breaker that trips open after repeated failures, rejects fast while open, then probes half-open.

Implement a simplified cva that builds class names from base styles, variant tables, and default values.

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

Implement a function that returns every unique combination of reusable candidates that sums to a target.

Build an editable combobox that filters options as you type and supports full keyboard selection.

Build a keyboard-driven command palette with fuzzy search and arrow-key navigation.

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.

Build a two-player Connect Four game where discs fall to the lowest empty slot and four-in-a-row wins.

Implement a wrapper that intercepts console.log calls and records their arguments for later inspection.

Implement a function that locates the node in a mirrored DOM tree occupying the same path as a given node.

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

Implement a function that counts the number of distinct islands of connected cells in a 2D binary grid.

Sort integers without comparisons — counting sort tallies occurrences into buckets, and radix sort applies it digit by digit from the least significant.

Implement a function that decides whether a set of courses can be finished given their prerequisite pairs.

Build a useState shared by every component that calls it, by subscribing React to a store that lives outside it.

Build a store whose subscribers each re-render only when the slice they selected changes.

Implement Element.matches: test whether an element satisfies a compound CSS selector (tag, id, class, attribute).

Build a currency converter with two linked amount fields that recompute from an exchange rate.

Implement a function that transforms a multi-argument function into a chain of single-argument calls.

Implement a flexible curry where each call can supply any number of arguments until the original arity is reached.

Extend curry with placeholder support, so arguments can be supplied in any position across calls.

Build a users data table that paginates rows with page-size and page-navigation controls.

Build a users data table that lets you sort by any column in ascending or descending order.

Build a calendar date picker that renders a month grid, navigates between months, and selects a day.

Extend debounce with cancel and flush methods to abort or immediately invoke pending calls.

Extend debounce with a maxWait option that forces a call after a maximum delay even under constant activity.

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

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

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

Implement a function that walks an arbitrarily nested structure and applies a transform to every leaf value.

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 a function that removes the nth-from-end node from a singly linked list in one pass.

Build a DI container that resolves a service dependency graph, caches singletons, and detects circular dependencies.

Implement depth-first traversal of a directed graph, exploring each branch fully before backtracking.

Build a double-ended queue with constant-time push and pop at both the front and the back, backed by a doubly linked list.

Build a dice roller that lets you pick a count of six-sided dice, roll them, and display the resulting faces.

Build a 7-segment style digital clock that updates to display the current time.

Implement a function that returns the minimum number of intervals to remove so the remaining ones do not overlap.

Implement a function that counts the number of unique paths a robot can take from the top-left to bottom-right of an m by n grid.

Build a jQuery-style wrapper whose methods return this so calls like addClass().text() chain.

Build an accessible dropdown menu that opens on click, closes on outside-click and Escape, and supports arrow-key navigation.

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

Build an emoji picker with category tabs and a search filter that inserts the chosen emoji.

Implement a function that decides whether you can reach the last index of an array using each element as a max jump.

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

Cache responses by URL and revalidate with an ETag, reusing cached data on a 304 Not Modified.

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

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

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.

Raise a base to an integer exponent in logarithmic time using exponentiation by squaring, handling negative exponents.

Evaluate feature flags with targeting rules and a deterministic percentage rollout that stays sticky per user.

Build a file explorer that renders a nested folder hierarchy and lets you expand and collapse directories.

Build a file explorer that exposes the correct ARIA tree roles, states, and properties for assistive tech.

Build a file explorer that renders the tree as a flat DOM list while preserving nested visual structure.

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

Implement a function that returns the longest palindromic substring contained within a given string.

Implement a function that checks whether a target word can be traced through adjacent cells in a 2D character grid.

Implement a state machine from a config of states and transitions, with send, guards, and entry actions.

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

Implement flow — compose functions left to right so each output feeds the next.

Serialize a form's fields into a nested object, handling checkboxes, multi-selects, and bracket names.

Build a schema-driven form validator with composable per-field rules, cross-field checks, and async validators that report all errors.

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.

Build increment and decrement counters that merge by per-node maxima into a conflict-free convergent total.

Compute the greatest common divisor with Euclid's algorithm and the least common multiple, for two numbers or a whole list.

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.

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

Implement a function that returns all DOM elements whose computed style matches a given property and value.

Implement a function that walks a DOM tree and returns every descendant element matching a given tag name.

Implement a function that produces a deep copy of a connected, undirected graph given a reference node.

Implement a function that counts the number of connected components in an undirected graph given its edges.

Build a grid of cells that light up on click and then deactivate one by one in the reverse order they were activated.

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

Build a client-side router over location.hash with route params and a change subscription.

Build a contribution-style heatmap calendar that colors each day cell by its activity value.

Build a client-side router over the History API (pushState/popstate) with path params.

Implement a simplified HTML sanitizer that strips out unsafe tags and attributes from a given DOM tree.

Implement a function that serializes a tree-shaped object into a pretty-printed HTML string with indentation.

Build an axios-style client whose request and response interceptors transform each call as it passes through the chain.

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.

Dedupe by idempotency key: run a task at most once per key, cache the result with a TTL, and retry on failure.

Implement a function that recursively compares two DOM trees and returns whether they are structurally equal.

Build an image carousel that lets you page through a sequence of images with previous and next controls.

Build an image carousel with smooth slide transitions between images.

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

Build a list that loads more items as the user scrolls near the bottom using an observer.

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

Compute the integer square root — the floor of the real square root — using Newton's method, without Math.sqrt.

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.

Implement a scroll/resize-driven fallback that reports when observed elements enter the viewport.

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

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

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

Implement a function that determines whether a given undirected graph forms a valid tree.

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

Build a job board that fetches and renders recent Hacker News job postings with paginated loading.

Implement chainable addClass, removeClass, toggleClass, and hasClass helpers on a DOM element wrapper.

Resolve a dotted path with array indices and a wildcard against a nested object, returning matches.

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

Implement the jsx() factory (type, props, children) that a JSX transform compiles to.

Build a three-column board where cards drag between columns and each column's count updates.

Swap each image's data-src into src as it nears the viewport, using an IntersectionObserver.

Implement a function that wraps a callback so it runs at most N times and returns the last result thereafter.

Build a singly linked list data structure with the standard insert, remove, and traversal operations.

Implement a function that detects whether a singly linked list contains a cycle.

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

Build a long-polling loop that re-issues a held request as each returns, advances a cursor, backs off on error, and stops cleanly.

Implement a function that returns the length of the longest subsequence common to two input strings.

Implement a function that returns the length of the longest run of consecutive integers found in an unsorted array.

Implement a function that returns the length of the longest strictly increasing subsequence in an array.

Implement a function that returns the length of the longest substring containing no duplicate characters.

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

Implement a fixed-capacity least-recently-used cache with O(1) get and put via a map plus a linked list.

Build a last-write-wins map whose entries merge by timestamp with a deterministic node-id tiebreak.

Implement a factory that returns a counter object with methods to read, increment, decrement, and reset its value.

Implement a function that applies an async mapper to every item in an array and resolves with the results.

Implement a class that records timestamped writes and returns the most recent value at or before a queried timestamp.

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

Parse a subset of Markdown (headings, bold, italics, links, code) into an HTML string.

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

Implement a function that walks a 2D matrix in clockwise spiral order and returns the visited values.

Implement a function that zeroes out every row and column in a matrix that originally contained a zero, ideally in place.

Find the area of the largest all-ones square in a binary matrix, where each cell's square size is one more than the smallest of its top, left, and top-left neighbors.

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 a function that caches results of a single-argument function so repeat calls return instantly.

Implement a memoize helper that caches results for functions called with any number and type of arguments.

Extend memoize with a custom key resolver and cache controls (.clear() and .delete(key)).

Build a memory card-matching game where players flip cards to find matching pairs.

Implement a function that inserts a new interval into a sorted list and merges any resulting overlaps.

Implement a function that merges all overlapping intervals in a list and returns the consolidated set in sorted order.

Implement merge sort recursively by splitting the array and merging the sorted halves back together.

Coalesce all schedule calls made in one synchronous frame into a single flush on the next microtask.

Build atom-based state with primitive and derived atoms that recompute and notify subscribers on change.

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

Extend a simplified in-memory ORM with richer filtering predicates and multi-field sorting.

Build a Redux core with createStore, combineReducers, and applyMiddleware over a subscribe/dispatch loop.

Implement a string template engine with {{ variable }} interpolation and nested-path lookup.

Implement a function that returns the fewest coins needed to make a target amount, or -1 when no combination works.

Implement a function that returns the fewest meeting rooms required to host a list of time intervals without conflicts.

Find the cheapest path from the top-left to the bottom-right of a grid, moving only right or down, by summing each cell with the smaller of the two ways to reach it.

Build a modal dialog with the correct ARIA roles, states, and properties for assistive technology.

Build a modal dialog that closes via the close button, backdrop click, and the Escape key.

Implement a function that returns the k most frequent integers in an array, breaking ties consistently.

Build a multi-select where chosen options become removable chips and the rest stay in the list.

Build a wizard form with next/back navigation, per-step validation, and a progress indicator.

Build a mini MutationObserver that diffs a subtree on demand and reports added and removed nodes.

Implement negate, overEvery, and overSome — build new predicates by combining existing ones.

Implement a function that computes the maximum money you can rob from a row of houses without picking two adjacent ones.

Implement a function that maximizes loot from houses arranged in a circle without robbing two adjacent ones.

Build a threaded comments tree where each comment can be replied to and collapsed recursively.

Implement a registry that associates values with DOM nodes without leaking memory once nodes are gone.

Store entities flat by id, normalizing nested relations and merging updates immutably.

Build a notification center with an unread badge, mark-as-read, and dismiss.

Implement lodash-style set and unset: write or delete a value at a deep object path, creating containers as needed.

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

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

Build a tiny state store with getState, setState, and subscribe that notifies listeners on change.

Implement a function that returns every grid cell from which water can flow to both the Pacific and Atlantic oceans.

Queue writes while offline and flush them in FIFO order when the network returns, retrying failures without dropping or reordering.

Build an observed-remove set where a concurrent add and remove converge with the add winning.

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

Build a one-time-code input of separate boxes with auto-advance, backspace, and paste handling.

Split a string every possible way so that each piece is a palindrome — backtrack over cut positions, extending a piece only while it reads the same both ways.

Implement a function that counts every palindromic substring within a string, including overlapping occurrences.

Implement partial and partialRight — pre-fill some of a function's arguments, with placeholder support.

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.

Generate every permutation and every subset of an array with the same backtracking template — choose, recurse, and undo.

List every letter string a phone number could spell, mapping each digit to its keypad letters and backtracking through the choices.

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

Build a pixel art editor where you pick a color from a palette and paint individual cells on a grid canvas.

Build a WordPress/Tapable-style hook system where plugins tap named actions and filters that run in priority order.

Implement pollUntil — call an async function on an interval until a predicate passes or a timeout elapses.

Test whether a number is prime and generate all primes up to n with the Sieve of Eratosthenes.

Schedule async requests under a concurrency limit, always running the highest-priority waiter next with a stable FIFO tiebreak.

Build a list of progress bars that fill in sequence, each one starting only after the previous bar finishes.

Build a list of progress bars that fill concurrently with at most three running at the same time.

Implement a function that combines the results of two promises into a single value using a merger function.

Implement a promise pool that runs task factories with a concurrency cap and resolves to their results in order.

Implement a function that races a promise against a timer and rejects if the promise doesn't settle in time.

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 a function that wraps a Node-style error-first callback function so it returns a promise.

Extend promisify so the wrapped function can override the resolved value via a custom override hook.

Implement an event bus whose subscriptions support wildcard segments matching any single topic level.

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

Implement querySelectorAll for descendant-combinator selectors, returning matches in document order.

Build a FIFO queue out of two LIFO stacks — pour one stack into the other so the reversed order dequeues oldest-first in amortized constant time.

Implement quick sort recursively, partitioning around a pivot and sorting each side independently.

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

Build a dual-handle range slider that picks a min and max value by dragging each handle.

Implement a sliding-window rate limiter that decides which task timestamps fit within the allowed quota.

Implement a function that sorts positioned canvas elements into natural left-to-right, top-to-bottom reading order.

Implement a function that groups misaligned 2D canvas elements into rows, then sorts them into a natural reading order.

Implement a function that reorders a singly linked list by interleaving nodes from the front and the reversed back half.

Wrap a WebSocket so it auto-reconnects with exponential backoff, queues sends while down, and re-emits its events.

Implement the curried Redux middleware chain with working thunk and logger middleware.

Build memoized selectors that recompute only when their input selections change by reference.

Wrap an async call with retries, backoff, and a per-attempt timeout so transient failures recover.

Build two panes split by a draggable divider that resizes them within min and max bounds.

Build a polling-based ResizeObserver that reports elements whose content-box size has changed.

Implement an interval helper that exposes start, pause, and resume controls while preserving elapsed progress.

Implement retry — re-run a failing async function with a delay that grows exponentially between attempts.

Convert between Roman numerals and integers in both directions, handling subtractive notation like IV, IX, and XL.

Round, floor, and ceil a number to N decimal places without the classic floating-point drift — 1.005 must round to 1.01, not 1.00.

Implement run-length encoding: compress runs of repeated characters into count+char pairs, and decode them back.

Implement a small schema library that validates primitives and flat object shapes with composable rules.

Build a tiny schema validation library that supports chainable min and max rules on primitive types.

Search a matrix whose rows and columns are both sorted by walking a staircase from the top-right corner, eliminating a whole row or column at each step.

Implement a function that decides whether a string can be broken into a sequence of words from a given dictionary.

Parse and compare two semantic version strings — major, minor, patch, and prerelease — returning -1, 0, or 1 with correct prerelease precedence.

Build a cart where quantity changes update line totals and a grand total live.

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

Build reactive signals with automatic dependency tracking so computed values and effects update on change.

Build a signup form that validates user details on submit and posts them to a back-end API.

Implement singleflight so concurrent calls for the same key share one in-flight promise instead of duplicating work.

Implement a Singleton class that always hands back the same instance no matter how many times you construct it.

Turn a string into a URL-safe slug: lowercase, strip accents, and collapse non-alphanumeric runs into single hyphens.

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

Build the classic snake game on a grid with keyboard control, growth on eating, and game over.

Run many logical channels over one WebSocket — route each incoming frame to the right channel and clean up on leave.

Build a list whose items reorder by dragging, with a live drop indicator.

Implement a spreadsheet class that resolves numeric cell values and evaluates simple addition formulas.

Build a spreadsheet class that evaluates left-to-right arithmetic formulas referencing other cells.

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

Decode a Server-Sent Events stream arriving in arbitrary chunks into discrete parsed events.

Return cached data instantly, serving stale values while revalidating them in the background.

Build an interactive star rating with hover preview and half-star support.

Build a star rating component where users click a star to set the rating shown by filled icons.

Build a stopwatch widget that measures elapsed time with start, stop, and reset controls.

Build a pure, immutable reducer that opens an assistant message, appends streamed deltas, and finalizes it.

Implement a function that groups an array of strings so anagrams of each other end up in the same bucket.

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 a function that overwrites the style on a given text range and normalizes the resulting range list.

Implement a curried sum function that accepts numbers one call at a time and returns the running total when invoked with none.

Implement a function that adds two integers using only bitwise operators, without using + or -.

Build a tabs component that shows one panel at a time based on the currently selected tab.

Build a tabs component with the correct ARIA roles, states, and properties for assistive tech.

Implement an html tagged-template function that interpolates values safely into a DOM fragment.

Implement a function that schedules tasks with a cooldown between identical ones and returns the minimum total time.

Run async tasks in dependency order, executing independent ones in parallel and catching cycles.

Implement a template renderer that substitutes Mustache-style placeholders with values from a data object.

Implement a minimal test runner that registers cases, runs each one, and reports pass and fail counts.

Build a small test runner that supports nested describe suites and reports tests with their full spec names.

Extend a tiny test runner with beforeEach and afterEach hooks that are inherited from outer suites.

Implement a function that wraps every occurrence of any term from a list inside highlight markup.

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

Group a number's integer digits with separators — 1234567 to 1,234,567 — handling decimals, negatives, and a custom separator.

Implement a function that limits how often a wrapped function can be invoked within a time window.

Extend throttle with cancel and flush methods plus leading and trailing edge options.

Build a two-player tic-tac-toe game that detects wins and draws on a 3x3 board.

Build a Todo List component with add and delete actions, with focus on functionality over styling.

Track streaming LLM tool calls by id, accumulating argument deltas and guarding each pending, running, and done transition.

Build a component that moves selected items back and forth between two side-by-side lists.

Implement a Trie data structure supporting insert, exact word search, and prefix lookup operations.

Implement a function that returns every unique triplet of distinct indices whose values sum to zero.

Implement lodash truncate — cut a string to a maximum length, append an ellipsis, and optionally break on the last word boundary before the limit.

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

Wire an input and a model object so edits to either side stay in sync via events and a setter.

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

Implement a class that tracks value history and exposes undo and redo navigation across the timeline.

Build a counter that keeps a history of values so you can undo and redo each change.

Build a user database class that supports CRUD operations plus undo and redo across the operation history.

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

Build a disjoint-set structure with near-constant-time union and find, using path compression and union by rank to keep the trees flat.

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

Implement a URLSearchParams-style class with get, getAll, append, set, and toString over an encoded query string.

Implement a hook that manages an array along with helpers to push, remove, update, and clear items.

Implement a hook that runs an async function and tracks its status, value, and error, with a manual execute trigger.

Implement an async hook that exposes a retry function to re-run the last operation on demand.

Implement an optimized useBoolean hook whose returned setters keep stable identity across renders.

Implement a hook that returns the active responsive breakpoint name derived from the current window width.

Implement a hook that fires a callback when a pointer event lands outside a referenced element.

Implement a hook that lets one component work both controlled and uncontrolled, deciding the mode from its props.

Implement a useState backed by a cookie — state the server can also write, with no event when it does.

Implement a hook that drives a countdown timer with start, pause, and reset controls.

Implement a hook that returns a debounced version of a value, updating only after a quiet period.

Implement a hook that manages a list whose items keep stable keys across inserts, removals, and reorders.

Implement a hook returning a stable callback that always sees the latest props and state, avoiding stale closures.

Implement a hook that attaches and cleans up a browser event listener on a target element or window.

Implement a data-fetching hook exposing data, error, and loading, that refetches when the URL changes and ignores stale responses.

Implement a hook wrapping the Geolocation API to expose position, error, and loading state.

Implement a useState variant that also returns a getter for reading the newest state from inside stale closures.

Implement a hook that flags the user as idle after a period without keyboard, pointer, or scroll activity.

Implement a hook that manages a controlled input value alongside its dirty and touched flags.

Implement a hook that reports whether a ref'd element is intersecting the viewport via IntersectionObserver.

Implement a hook that runs a callback on a timer with a configurable, pauseable delay.

Implement a hook that returns whether a given key is currently being pressed on the keyboard.

Implement a hook managing an array with immutable helpers — push, removeAt, updateAt, insertAt, clear, and set.

Implement a hook that fires a callback after a pointer is held down past a delay, cancelling on early release or move.

Implement a hook that wraps a JavaScript Map with set, delete, and clear helpers backed by state.

Implement a hook that returns whether a media query currently matches and updates on changes.

Implement a useState-like hook that runs every incoming value through a mediator before committing it.

Implement a hook that turns a map of state transitions into bound, stable action methods over a reducer.

Implement a hook that reports online/offline status and connection info from the Network Information API.

Implement a hook that manages page state over a total count, exposing page, pageCount, and next/prev/go with clamping.

Implement a hook that remembers the last value that actually differed from the current one, with a customizable comparator.

Implement a hook that tracks the loading, data, and error states of a promise-returning request.

Implement a hook wrapping a FIFO queue with add, remove, clear, and first/last/size accessors.

Implement a hook whose state object you mutate directly, and reckon with what that costs React.

Build a CRUD interface to list, filter, create, edit, and delete users backed by local state.

Implement a hook that injects an external script tag and reports its loading status, deduping repeated sources.

Implement a hook that tracks the window scroll x/y position with a passive listener, throttled with rAF.

Implement a hook that tracks a multi-select over a list, with an all/none/partial header state that stays honest.

Implement a useState variant that records value history and exposes back, forward, and go over it.

Implement a hook that drives a multi-step flow with next, previous, reset, and bounded index controls.

Implement a hook that returns a throttled copy of a value, updating at most once per given interval.

Implement a hook that runs a callback after a given delay and cleans up when the component unmounts.

Implement a useState that lives in the query string, staying shareable without burying the back button.

Implement a useState variant that reports whether the current value passes a validator, without an extra render.

Implement a debugging hook that logs which props changed between renders.

Implement a function that determines whether a binary tree satisfies the binary search tree ordering invariant.

Order distributed events with vector clocks, detecting whether two events are causal or concurrent.

Implement a diff that patches real DOM from an old vnode tree to a new one, minimizing changes.

Implement createElement to build a virtual DOM tree and render to turn it into real DOM nodes.

Resume a dropped connection without gaps — track sequence numbers, request a replay from the last seen, and de-duplicate redelivered messages.

Pick items at random in proportion to their weights using prefix sums and a binary search, with an injectable RNG so picks are testable.

Build the arcade game where moles randomly appear in a grid and players click them to score.

Implement a data structure that supports adding words and searching with a wildcard character that matches any letter.

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