JavaScript interview questions

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

Implement lodash xor — return the values that appear in exactly one of the given arrays, with duplicates removed.

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 countdown latch whose wait resolves once countDown has been called the required number of times.

Implement a setInterval wrapper that returns a function which stops the interval when invoked.

Implement a setTimeout wrapper that returns a function you can call to cancel the pending callback.

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 composes functions right-to-left so the output of each feeds into the next.

Implement get/set/remove over document.cookie, handling encoding and expiry.

Fetch successive pages with an opaque cursor, accumulating items and tracking whether more remain.

Implement a function that, given several values, returns a function that yields each one in turn and loops forever.

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

Implement a function that delays invoking another function until a specified period of inactivity has passed.

Implement a deep clone of a DOM node and its subtree (element, text, and attributes) without cloneNode.

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

Implement a function that returns the height (max depth) of a DOM element's subtree.

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 delegated event listener that matches events bubbling up from descendants against a selector.

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

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 builds an object from an array of [key, value] pair tuples.

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

Implement a function that builds a unique CSS selector path from the root to a given element.

Implement a function returning the set of distinct tag names used in a DOM subtree, in document order.

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 a jQuery-style helper that gets or sets inline CSS properties on a DOM element.

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 a factory that takes a starting integer and returns a function that returns the next value on each call.

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 a function that finds the element immediately to the right of a target at the same tree depth.

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 returns the count of arguments passed to it at call time.

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

Implement a function that wraps a callback so it runs at most one time and reuses that result thereafter.

Run async tasks in sequence, feeding each result into the next, stopping the chain on the first rejection.

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

Implement a function that returns a promise resolving after the specified number of milliseconds.

Implement a cache whose entries expire a fixed time after their last access, not after being set.

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 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.

Implement a function that wraps occurrences of a search term within a string in highlight markup.

Implement times — invoke a function n times and collect the results into an array.

Implement a breadth-first traversal of a DOM tree, returning elements level by level.

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

Buffer bursty streamed text and emit it one character at a time at a steady cadence, with flush and stop controls.

Implement a function that returns a new array with duplicate values removed while preserving original order.

Implement a hook that exposes a boolean value alongside setTrue, setFalse, and toggle helpers.

Implement a hook that fires a callback whenever the user clicks anywhere in the window, cleaning up on unmount.

Implement a hook returning the last copied text and an async copy function built on the Clipboard API.

Implement a hook that exposes a numeric count along with increment, decrement, and reset helpers.

Rewrite useCounter so the returned helpers keep stable identities across renders.

Implement a hook that returns the current value of a sequence and advances to the next on demand.

Implement a hook that steps through an array with wraparound, exposing the current item and next/prev/set.

Implement a hook that falls back to a default value whenever the underlying state becomes null or undefined.

Implement a hook that sets document.title and restores the previous title when the component unmounts.

Implement a hook that runs the given effect a single time after mount, regardless of subsequent renders.

Implement a hook that swaps the page favicon imperatively to a given URL.

Implement a hook that exposes a ref plus a function to focus the attached element on demand.

Implement a hook that reports whether a referenced element is currently being hovered.

Implement a hook that returns true only on the first render and false on every render after.

Implement a hook returning a function that reports whether the component is still mounted, to guard async setState.

Implement a hook that returns a ref always holding the newest value, so callbacks and effects can read it without going stale.

Implement a useState-like hook that persists to localStorage, with SSR-safe init and cross-tab sync.

Implement a hook that holds an object in state and exposes helpers to merge updates or reset its value.

Implement a hook that remembers and returns the value a state held on the previous render.

Implement a hook that counts how many times a component has rendered, without causing extra renders.

Implement a useState-like hook backed by sessionStorage, with a graceful fallback when it is unavailable.

Implement a hook that stores a Set in state and exposes add, delete, has, and clear helpers.

Implement a useState-like hook that also returns a reset function which restores the initial value.

Implement a hook that holds a boolean and returns a stable toggle function to flip its value.

Implement a useEffect variant that skips the first render and fires only on subsequent updates.

Implement a hook that returns the current window width and height, updating on resize.

Implement without — return a copy of an array with the given values excluded, matched by SameValueZero.

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 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.

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.

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.

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.

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 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 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.

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).

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.

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 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.

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

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

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.

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

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.

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 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 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.

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

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

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.

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

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.

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.

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 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 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.

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)).

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.

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 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.

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.

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.

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.

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

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.

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

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.

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

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 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.

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

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.

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.

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

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

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

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.

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

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 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.

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.

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.

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.

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

Build an emitter whose emit awaits async listeners, with once, off, and serial or parallel dispatch.

Implement Node-style async runners — run error-first tasks in parallel or in series and collect their results.

Implement a read-write lock that allows many concurrent readers or one exclusive writer without starving writers.

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

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

Run a queue of tasks in time slices, yielding control between them so the main thread stays responsive.

Implement a curry helper that turns a variadic function into one that can be called repeatedly with any number of arguments.

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

Batch and cache load(key) calls made in one tick into a single batch function call, preserving order.

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.

Reorder a list by pointer drag, computing the target index from the cursor over item midpoints.

Build an in-memory query builder inspired by Drizzle with table helpers, filters, sorting, and pagination.

Extend an in-memory query builder inspired by Drizzle with select projections and column aliases.

Extend an in-memory query builder inspired by Drizzle with innerJoin and leftJoin between related tables.

Implement a function that finds all DOM elements matching an ancestor-to-descendant tag-name hierarchy.

Tokenize an HTML string and build a nested node tree, handling attributes, void, and nested tags.

Run a queue of tasks in time-sliced batches, yielding between slices to keep the frame responsive.

Implement produce so mutating a draft proxy yields a new immutable state with structural sharing.

Spell out a non-negative integer in English words — break it into groups of three digits and name each group with its scale word.

Implement JSON.parse — a recursive-descent parser that turns a JSON string into the corresponding JavaScript value.

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

Reconcile two keyed child lists into a minimal set of DOM inserts, moves, and removals.

Implement a function that finds the deepest common ancestor whose hiding would conceal every target element.

Implement an async map that processes an array with a bounded number of in-flight tasks.

Implement an async middleware composer that chains functions Koa-style, threading a shared context through next.

Build MobX-style observables with autorun and computed that re-run when the data they read changes.

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

Build a framework-agnostic query cache with request dedup, staleTime, subscribers, and invalidation.

Implement a minimal Observable with subscribe/unsubscribe and of/from plus map and filter operators.

Transform concurrent insert and delete operations so replicas converge to the same text.

Apply optimistic updates immediately and roll them back on failure, even with concurrent mutations.

Parse a truncated JSON string into the best-valid value so far by completing open strings and containers.

Build reactive state with a Proxy that tracks reads and re-runs registered effects on write.

Build a redux-saga-style interpreter that runs a generator of declarative effects and resumes it with each result.

Implement a function that converts a list of styled text ranges into a canonical, properly nested HTML string.

Build a mini reactive-streams library: a cold Observable and pipeable operators composed with a .pipe() chain.

Extend a tiny schema validator to handle nested object and array shapes plus optional fields.

Build a replicated growable array for text whose positional-id inserts and deletes converge across replicas.

Build a spreadsheet that recomputes dependent cells when inputs change and detects circular references.

Build a state-machine engine with events, guards, entry and exit actions, and context updates.

Render markdown-so-far to HTML, holding and closing unclosed bold, inline code, and fenced code blocks.

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

Implement a slice operation over text annotated with overlapping style ranges, preserving alignment.

Implement a function that replaces a text range and keeps the surrounding style ranges normalized.

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

Implement serialize and deserialize helpers that round-trip values like Date, Map, Set, and BigInt through JSON.

Build serialize and deserialize that preserve referential identity — shared references and circular structures round-trip intact.

Implement a function that walks an HTML document's headings and builds a nested table of contents.

Extend a tiny test runner so it correctly awaits asynchronous specs and asynchronous setup or cleanup hooks.

Implement a function that gathers the text content lying between two DOM nodes in document order.

Compute a tooltip's position around an anchor, flipping and shifting to stay inside the viewport.

Build a class that tracks draft values and commits them as batch checkpoints for undo and redo navigation.

Build a user database that buffers draft mutations and commits them as batch undo and redo checkpoints.

Implement a hook returning a debounced callback with cancel and flush, always calling the latest function.

Implement a pointer-based drag hook that tracks an element's x/y offset with start/move/end handlers.

Implement a form-state hook managing values, touched, errors, validation, and submission.

Implement a hook that loads more pages when a sentinel element scrolls into view, guarding against duplicate loads.

Implement a hook that reports a ref'd element's content-box size via ResizeObserver.

Implement a stale-while-revalidate data hook with an in-memory cache, dedup, and revalidation shared across components.

Implement a hook returning a throttled version of a callback that runs at most once per interval, with leading/trailing calls.

Implement an undo/redo state hook exposing undo, redo, canUndo, canRedo over past/present/future stacks.

Implement a windowing hook that renders only the visible rows of a long list, with correct spacer offsets.

Implement a WebSocket hook exposing connection status, last message, and send, with auto-reconnect and cleanup.

Compute the visible slice and spacer offsets to render only on-screen rows of a huge list.