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Frontend Coding Challenges

Build and debug focused frontend prompts across JavaScript functions, UI exercises, HTML/CSS implementation, and debugging tasks.

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Opens Essential 60 #1: the high-signal debounce function drill.
139 matches
Coding
Debounce Function
Implement `debounce(fn, delay)` by returning a trailing-edge wrapper that clears the previous timer and runs `fn` only a…
IntermediateHighJavaScript
Coding
Box Model: Margin, Padding, Border
Build a card layout that clearly demonstrates the CSS box model. Use padding to add internal breathing room, a border to…
EasyHighCSS
Coding
Semantic Page Layout
Build a semantic page layout using header, nav, main, section, article, and footer. Focus on correct landmarks, heading…
EasyHighHTML
Coding
Flexbox: Responsive Navbar
Practice a realistic frontend interview prompt: build a responsive navigation bar with Flexbox, preserve readable links,…
IntermediateHighCSS
Coding
Forms & Pseudo-classes: Focus, Invalid, Required, Helper Text
Style form fields using :focus, :invalid, and :required to guide users. Show clear focus rings, error borders, and helpe…
IntermediateHighCSS
Coding
Grid: Card Gallery (2→4 Columns)
Practice a realistic frontend interview prompt: create a responsive card gallery with CSS Grid, keep cards readable acro…
IntermediateHighCSS
Coding
React Autocomplete Interview Question
Practice a React autocomplete interview question in a live editor. Build a controlled search input with debounce, stale-…
IntermediateHighReact
Coding
Tables and Accessibility
The goal is to provide a semantic table that screen readers can navigate, with a clear caption and correct header associ…
IntermediateHighHTML
Coding
Theming with CSS Variables: OS Dark Mode + Manual Override
Create a themeable panel using CSS custom properties. Define light defaults on :root, redefine the same tokens for OS da…
IntermediateHighCSS
Coding
Contact Form with Proper Labels
The success message should only appear after a valid submit, so native validation must be enabled and the form action sh…
EasyHighHTML
Coding
Links and Images
Create semantic links and images: use anchor elements with descriptive text, add target/rel when opening new tabs, and i…
EasyHighHTML
Coding
Positioning: Relative/Absolute Badge
This pattern is used in notifications, status indicators, and card metadata, so the badge should remain anchored even as…
EasyHighCSS
Coding
SEO: Essential <head> Metadata
You’re given a simple marketing page that renders fine in the browser, but it’s missing production-grade <head> metadata…
EasyHighHTML
Coding
Shallow Clone (Object or Array)
Implement shallowClone(value) in JavaScript. Your function should accept an object or array and return a new top-level c…
EasyHighJavaScript
Coding
Sleep / Delay Promise
Implement sleep(ms) that returns a Promise resolving after ms milliseconds. Use it to pause async flows like retries, ex…
EasyHighJavaScript
Coding
Update Array at Index (Immutable)
Return a new array with one index updated, leaving the original array untouched. This is the core immutable update patte…
EasyHighJavaScript
Coding
Create a Deferred Promise (For Async Tests)
Implement createDeferred() in JavaScript. Your function should return { promise, resolve, reject } so async tests can co…
IntermediateHighJavaScript
Coding
Create a Spy Function (Test Double)
Implement createSpy(impl?) to produce a callable spy that records every call (args + this) and optionally delegates to a…
IntermediateHighJavaScript
Coding
Data Helper 1: Get Value by Path
Implement `getByPath(obj, path, fallback?)` that safely reads a nested value from an object. This is extremely common in…
IntermediateHighJavaScript
Coding
Fetch JSON with Timeout + Abort
Implement fetchJson(url, options) that fetches JSON with built-in timeout and abort support. It should abort on timeout,…
IntermediateHighJavaScript
Coding
Fluid Type & Spacing with clamp()
Implement fluid typography and spacing with `clamp()`. Use `clamp(min, preferred, max)` to scale values across viewport…
IntermediateHighCSS
Coding
Forms: Validation and Required Fields
Build a signup form using native HTML5 validation: required fields, type constraints, minlength, and pattern. The browse…
IntermediateHighHTML
Coding
Implement `instanceof`
Recreate `instanceof` by walking the prototype chain and checking if it reaches `Constructor.prototype`. Handle null/pri…
IntermediateHighJavaScript
Coding
Promise.any (First Fulfilled)
Implement promiseAny(promises) that behaves like Promise.any. It should resolve with the first fulfilled value, ignoring…
IntermediateHighJavaScript
Coding
Run With a Performance Budget (Sync or Async)
Implement runWithPerfBudget(fn, budgetMs, now?) to time a sync or async function and report whether it stayed within a b…
IntermediateHighJavaScript
Coding
Sanitize href URL (Block javascript: XSS)
Implement sanitizeHrefUrl(input) to safely use user-provided URLs in an <a href="...">. Use an allowlist for safe scheme…
IntermediateHighJavaScript
Coding
Take Latest (Abort Previous Requests)
Implement takeLatest(fn) that wraps an async function and guarantees only the most recent call can resolve. Each new cal…
IntermediateHighJavaScript
Coding
Implement Function.prototype.bind
Implement `myBind(fn, thisArg, ...boundArgs)` that behaves like `Function.prototype.bind`. It must: 1) Bind `this` to `t…
HardHighJavaScript
Coding
Implement the `new` Operator
Implement a myNew helper that mimics the new operator. Create an object linked to the constructor prototype, call the co…
HardHighJavaScript
Coding
Check if Two Arrays Are Equal
Check whether two arrays are equal by length and by value at each index. This is a shallow equality check for primitive…
EasyMediumJavaScript
Coding
Cleanup Bag (Dispose Subscriptions)
Implement createCleanupBag() that helps prevent memory leaks by tracking cleanup functions (like removing event listener…
EasyMediumJavaScript
Coding
Create a Counter Function
Support an optional initial value and keep the API simple so consumers can call inc/dec/get without exposing the interna…
EasyMediumJavaScript
Coding
Create Object with Prototype
Implement objectCreate(proto) to return a new object whose prototype is `proto`. Use a temporary constructor function or…
EasyMediumJavaScript
Coding
Display & Centering: Center Inline Content
This is a horizontal alignment task, so avoid flex unless vertical alignment is explicitly required by the prompt. Use t…
EasyMediumCSS
Coding
Escape HTML for Safe Text
Implement escapeHtml(text) to convert &, <, >, " and ' into HTML entities so user-provided text can be safely inserted i…
EasyMediumJavaScript
Coding
Falsy Bouncer
Remove all falsy values from an array while preserving order. Use a boolean filter to drop false, 0, empty string, null,…
EasyMediumJavaScript
Coding
Flatten One Level
This should not deep-flatten; only the first nesting level is removed, matching Array.prototype.flat(1). Use it to norma…
EasyMediumJavaScript
Coding
Inline vs Block Elements
Demonstrate the difference between block and inline elements using pure HTML. Show that block elements start new lines w…
EasyMediumHTML
Coding
Lists and Navigation
Build a semantic navigation bar using `nav > ul > li > a` with hash links. Use CSS `:target` to show only one section at…
EasyMediumHTML
Coding
Once (Function Wrapper)
Implement once(fn) that returns a wrapper which executes `fn` only on the first call. Cache and return the result on sub…
EasyMediumJavaScript
Coding
Querystring Helper 1: Implement `parseQueryString`
Implement `parseQueryString(qs)` that converts a URL querystring into a plain object. This is very common in frontend wo…
EasyMediumJavaScript
Coding
Selectors & Text: Hero Title + Lead + Emphasis (Interview Warm-up)
Style a small hero header using both element and class selectors. Establish base typography on element selectors, then o…
EasyMediumCSS
Coding
Todo List (Component with Local State)
Create a local-state todo list: add, toggle, and remove items. Keep input state controlled, generate stable ids, and upd…
EasyMediumReact
Coding
Format Date in Time Zone (YYYY-MM-DD)
Implement formatDateInTimeZone(date, timeZone) to return a YYYY-MM-DD string for a given Date (or timestamp) in a specif…
IntermediateMediumJavaScript
Coding
Implement arrayForEach (no prototype mutation)
You should also ensure the callback is invoked with the correct thisArg and that the method returns undefined like the n…
IntermediateMediumJavaScript
Coding
Measure Function Duration (Profiling Wrapper)
Implement measure(fn, now?) to profile execution time. It should run a function, measure the duration using the provided…
IntermediateMediumJavaScript
Coding
Poll Until (with Timeout + Abort)
Implement pollUntil(check, { interval, timeout, signal }) that repeatedly runs a check until it returns a truthy value.…
IntermediateMediumJavaScript
Coding
Storage TTL Cache (localStorage/sessionStorage)
Implement createTtlStorage(storage, now?) that returns { set, get, remove }. It should store JSON values with an optiona…
IntermediateMediumJavaScript
Coding
Stream to Text (ReadableStream -> string)
Implement streamToText(stream, options) that reads a ReadableStream<Uint8Array> and returns the full string. Use TextDec…
IntermediateMediumJavaScript
Coding
Transitions & Transforms: Lift on Hover
Create a hover lift effect with `transform` and `transition`. Combine a slight translate/scale with a deeper `box-shadow…
IntermediateMediumCSS
Coding
Create an LRU Cache (Bounded Memory)
Implement createLruCache(maxSize) to keep a bounded, eviction-based cache. This is a classic way to prevent "memory leak…
HardMediumJavaScript
Coding
Capitalize Words
Capitalize the first letter of every word while preserving spacing. Handle multiple spaces and punctuation without colla…
EasyMediumJavaScript
Coding
Check if an Object or Array is Empty
Implement `isEmpty(obj)` that returns `true` only if the input is an object or array with no own keys/elements, and `fal…
EasyMediumJavaScript
Coding
Clamp
Implement clamp(value, lower, upper) to keep a number within an inclusive range. If the value is below the lower bound,…
EasyMediumJavaScript
Coding
Count Vowels
Implement countVowels(str) that returns how many vowels appear in a string. Normalize case, decide whether to include 'y…
EasyMediumJavaScript
Coding
React Counter (Guarded Decrement)
Build a React counter component with increment/decrement and a zero floor. Use state for the count, disable decrement at…
EasyMediumReact
Coding
Safe JSON Parse (Fallback on Error)
Implement safeJsonParse(text, fallback = null) to safely parse JSON. It should return the parsed value for valid JSON st…
EasyMediumJavaScript
Coding
Sum of Numbers in an Array
Sum only numeric values from an input array, ignoring non-numbers and `NaN`. This reinforces safe type checking, accumul…
EasyMediumJavaScript
Coding
Validate Username (Rules + Edge Cases)
Implement validateUsername(name) to enforce common username rules and handle edge cases safely. Rules: 3–16 chars, lower…
EasyMediumJavaScript
Coding
Warm-Up: Basic Structure
Build a valid HTML document skeleton with `<!doctype html>`, `<html>`, `<head>`, and `<body>`. Include a `<meta charset>…
EasyMediumHTML
Coding
Resolve package.json Exports (Import vs Require)
Implement resolvePackageEntry(pkg, options) to choose the correct entry file for a package based on package.json fields…
HardMediumJavaScript
Coding
Debug: Component Renders Twice on Every Update
Locked
Fix a component that renders twice per click because it stores derived state in `useEffect`. The correct fix is to remov…
IntermediateHighReact
Coding
React Debounced Search with Fake API
Locked
Build a React debounced search input with fake API calls. Use useEffect + setTimeout cleanup to cancel pending debounce…
IntermediateHighReact
Coding
Reusable Child Component with @Input/@Output (Two-way Binding)
Locked
Expose a value input and a valueChange output to support two-way binding, and keep the child component stateless so the…
IntermediateHighAngular
Coding
useEffectOnce: Run an Effect Only Once
Locked
This hook should run the effect only on mount and clean up on unmount, even under React StrictMode development behavior.…
IntermediateHighReact
Coding
Remove Duplicates
Locked
Implement uniqueArray(arr) that removes duplicates while preserving order. Use a Set for O(n) time and clarify how to tr…
EasyHighJavaScript
Coding
Sort Numbers with Array.prototype.sort
Locked
Implement a function `sortNumbers(arr, ascending = true)` that returns a **new array** of numbers sorted numerically in…
EasyHighJavaScript
Coding
Contact Form (Component + HTTP)
Locked
Build a contact form using a React component with controlled inputs and basic validation. You will only work in `src/App…
IntermediateHighReact
Coding
Custom setTimeout/clearTimeout Timer Manager
Locked
Build a timer registry with mySetTimeout, myClearTimeout, and myClearAllTimeouts. Return your own numeric ids, store nat…
IntermediateHighJavaScript
Coding
Data Helper 2: Set Value by Path
Locked
Implement setByPath(obj, path, value) that returns a new object/array tree with the value written at the nested path. No…
IntermediateHighJavaScript
Coding
Delegated Event Handler (E)
Locked
Implement `delegate(root, eventType, selector, handler)` for **event delegation**. Requirements: 1) Attach **one** event…
IntermediateHighJavaScript
Coding
Event Emitter (Mini Implementation)
Locked
Build a tiny event emitter with on, off, and emit. Store listeners per event, allow multiple listeners, and ensure off r…
IntermediateHighJavaScript
Coding
Group By
Locked
Group an array of items by a key (function or property). Build an object or Map where each key maps to an array of items…
IntermediateHighJavaScript
Coding
Implement Array.prototype.filter
Locked
Recreate the native `Array.prototype.filter()` method as `myFilter`. It should call a provided callback once for each ex…
IntermediateHighJavaScript
Coding
Implement Array.prototype.map
Locked
Recreate the native `.map()` method **without using it**. Your `myMap` must call `callbackFn(value, index, array)` for e…
IntermediateHighJavaScript
Coding
Implement Array.prototype.reduce
Locked
Recreate the native `.reduce()` method **without using it**. Your `myReduce` must call the reducer with `(accumulator, c…
IntermediateHighJavaScript
Coding
Implement lodash.get
Locked
Implement `get(obj, path, defaultValue)` where `path` is a string like `user.profile.name` or `a[0].b`. Return `defaultV…
IntermediateHighJavaScript
Coding
Implement Promise.all
Locked
Implement a function `promiseAll(promises)` that behaves like `Promise.all`. It should take an array of values or promis…
IntermediateHighJavaScript
Coding
Invite Chips Input (Tags + Autocomplete)
Locked
Build a Material-like invite field in React. As users type, show autocomplete suggestions, convert selections into remov…
IntermediateHighReact
Coding
Memoization
Locked
Implement memoize(fn) that caches results for repeated calls. Choose a cache key strategy, discuss memory growth/evictio…
IntermediateHighJavaScript
Coding
Modal: Native <dialog> Confirm (Accessible Naming)
Locked
Build a native confirm dialog for deleting a project using the HTML <dialog> element. The dialog must be accessible (pro…
IntermediateHighHTML
Coding
Multi-step Signup Form
Locked
Build a 3-step signup flow in React that collects basic info, address info, and then shows a read-only summary before su…
IntermediateHighReact
Coding
React Nested Checkbox Tree (Parent–Child Sync)
Locked
Build a small React UI that renders a parent checkbox and multiple child checkboxes. The parent controls all children, a…
IntermediateHighReact
Coding
React Nested Comments (Infinite Replies, Single Active Reply Input)
Locked
Build a React nested comments panel with infinite replies. Users can add top-level comments and reply to any comment, cr…
IntermediateHighReact
Coding
React Paginated Data Table
Locked
Build a simple paginated data table in React that shows a static list of users. Display 5 rows per page, with Previous /…
IntermediateHighReact
Coding
React Shopping Cart Mini
Locked
Build a small shopping cart UI in React. Show a list of products, let the user add them to the cart, adjust quantities,…
IntermediateHighReact
Coding
Throttle Function
Locked
Implement a function `throttle(fn, interval)` that ensures `fn` is executed at most once during every `interval` millise…
IntermediateHighJavaScript
Coding
Chat UI with Streaming Response
Locked
Build a simplified ChatGPT-like interface that streams an assistant response token-by-token. The goal is to model chat s…
HardHighReact
Coding
Concurrency-Limited Map (order-preserving)
Locked
Implement a concurrency-limited async map that preserves order. Run at most N promises at a time, resolve results in inp…
HardHighJavaScript
Coding
Deep Clone
Locked
Create a deep clone of an object/array so nested structures are copied by value. Discuss limits (functions, dates, cycle…
HardHighJavaScript
Coding
Deep Equal
Locked
Implement a function `deepEqual(a, b)` that returns `true` when two values are deeply equal — primitives by value and ob…
HardHighJavaScript
Coding
Delegated Event Handler (M)
Locked
Implement `delegate(root, eventType, selector, handler, options?)` with **advanced event delegation**. Requirements: 1)…
HardHighJavaScript
Coding
Querystring Helper 3: Implement `parseQueryStringAdvanced`
Locked
Implement `parseQueryStringAdvanced(qs)` that converts a URL querystring into a **nested** object structure. This mirror…
HardHighJavaScript
Coding
Add Large Integers as Strings
Locked
Add two non-negative integers represented as strings and return the sum as a string. You may not convert the entire stri…
EasyMediumJavaScript
Coding
Find Maximum Occurring Character
Locked
Find the most frequent character in a string using a frequency map. Track counts in one pass and return the character wi…
EasyMediumJavaScript
Coding
Image Slider (Dots + Previous/Next)
Locked
Build a simple React image slider with previous/next buttons and dot navigation. Show the current slide, its title, and…
EasyMediumReact
Coding
Merge Two Sorted Arrays
Locked
Merge two already-sorted arrays into a single sorted array. Do not call sort on the combined output; use a two-pointer m…
EasyMediumJavaScript
Coding
Reverse a String
Locked
Reverse a string by iterating from the end to the start and building a new string, without using `Array.prototype.revers…
EasyMediumJavaScript
Coding
Selector Polyfill 1: Implement `matchesSimple`
Locked
Implement `matchesSimple(node, selector)` — a tiny subset of `Element.matches()`. This is extremely common in frontend i…
EasyMediumJavaScript
Coding
3Sum (find unique triplets that sum to zero)
Locked
Given an array of integers, return all unique triplets `[a, b, c]` such that `a + b + c = 0`. Triplets should be unique,…
IntermediateMediumJavaScript
Coding
Add Two Promises
Locked
This is a small async composition task and should return a Promise that resolves to the numeric sum once both inputs set…
IntermediateMediumJavaScript
Coding
Build a DOM Renderer from a Nested Object
Locked
Implement render(node) to convert a JSON-like DOM description into real DOM nodes using createElement/createTextNode. Co…
IntermediateMediumJavaScript
Coding
Clear All Timers + Render DOM Elements
Locked
Solve two independent tasks: implement clearAllTimeouts for a timer registry and render a DOM tree from a nested object.…
IntermediateMediumJavaScript
Coding
Compose Function
Locked
Implement `compose(...fns)` that returns a new function applying the provided functions from **right to left**. The righ…
IntermediateMediumJavaScript
Coding
Curry Function
Locked
Implement curry(fn) to transform a multi-argument function into chained calls. Collect arguments across invocations, all…
IntermediateMediumJavaScript
Coding
DOM Tree Walk 2: Find Closest Ancestor
Locked
Implement `closestWithin(node, selector, boundary)` for a **DOM-like** tree. This is a simplified `.closest()` used cons…
IntermediateMediumJavaScript
Coding
Find a Node in a DOM Tree (DFS/BFS)
Locked
Implement findNode(root, predicate) to traverse the DOM and return the first matching element. Use DFS/BFS with early ex…
IntermediateMediumJavaScript
Coding
Find the Corresponding Node in Twin DOM Trees
Locked
Given two identical DOM trees and a node in tree A, return the corresponding node in tree B. Traverse both trees in sync…
IntermediateMediumJavaScript
Coding
Flatten with Depth
Locked
Implement flatten(arr, depth) that flattens nested arrays up to a specific depth. Depth controls how many nesting levels…
IntermediateMediumJavaScript
Coding
Querystring Helper 2: Implement `stringifyQueryString`
Locked
Implement `stringifyQueryString(params)` that converts an object into a URL querystring. This is common in FE screens (b…
IntermediateMediumJavaScript
Coding
React Accordion / FAQ Component
Locked
Build a simple FAQ (accordion) UI in React using component state. Clicking a question should toggle its answer. By defau…
IntermediateMediumReact
Coding
React Filterable / Searchable User List
Locked
Filter a React user list by search text and role. Derive filtered results without mutating the source array, and show an…
IntermediateMediumReact
Coding
React Transfer List (Select + Move Between Two Lists)
Locked
Build a React transfer list UI with two panels and checkbox selection. Users select items in one list and move them to t…
IntermediateMediumReact
Coding
Selector Polyfill 2: Implement `closestSimple`
Locked
Implement `closestSimple(node, selector)` — a DOM-like polyfill for `Element.closest()` using **only** `parentNode` trav…
IntermediateMediumJavaScript
Coding
Abortable Helpers (Timeout + Composed Abort)
Locked
Provide utilities to combine timeouts with AbortController, and to compose multiple signals (abort if any fires). All he…
HardMediumJavaScript
Coding
Data Helper 3: Deep Clone with Cycles
Locked
Implement `deepClone(value)` that returns a **deep copy** of the input while correctly handling **circular references**.…
HardMediumJavaScript
Coding
Delegated Event Handler (H)
Locked
Implement delegated event handling: attach one listener to a parent, match events by selector, and call the handler with…
HardMediumJavaScript
Coding
React Snake Game (Grid + Food + Collision)
Locked
Build a Snake game in React using a grid board. Support keyboard controls, food spawning, score updates, and game-over h…
HardMediumReact
Coding
Streaming NDJSON Parser (Web Streams)
Locked
Build an NDJSON streaming parser that reads chunked text and emits JSON objects line by line. Handle partial lines acros…
HardMediumJavaScript
Coding
Angular Chessboard Click/Highlight (N×N Board)
Locked
Build an interactive N×N chessboard in Angular. Clicking a cell should highlight it and clear the previous selection. Ad…
EasyMediumAngular
Coding
Median of Array
Locked
Return the median of a list of numbers. For even length, return the average of the two middle values. Do not mutate the…
EasyMediumJavaScript
Coding
React Chessboard Click/Highlight (N×N Board)
Locked
Build an interactive N×N chessboard in React. Clicking a cell should highlight it and clear the previous selection. Incl…
EasyMediumReact
Coding
React Dynamic Table (Rows × Columns)
Locked
Generate a dynamic table in React from row/column inputs. Normalize values, rebuild the grid on action, and render consi…
EasyMediumReact
Coding
React Like Button (Toggle + Counter)
Locked
Build a React Like button that toggles between liked/unliked and updates a count. Clicking should add or subtract one ba…
EasyMediumReact
Coding
React Progress Bar (0–100 with Threshold Colors)
Locked
Build a React progress bar for values 0–100 with +10%/−10% controls. Clamp progress to the valid range and change the ba…
EasyMediumReact
Coding
React Star Rating Widget
Locked
Implement a reusable <StarRating /> widget in React. It should render a row of clickable stars, allow the user to select…
EasyMediumReact
Coding
Tic-Tac-Toe (Component State + Winner Detection)
Locked
Build Tic-Tac-Toe with React state: a 9-cell board, alternating turns, win/draw detection, and a reset button. Prevent o…
EasyMediumReact
Coding
Valid Anagram
Locked
Return true if two strings are anagrams (same characters with the same counts). Treat strings as case-sensitive and incl…
EasyMediumJavaScript
Coding
Vue Chessboard Click/Highlight (N×N Board)
Locked
Build an interactive N×N chessboard in Vue 3. Clicking a cell should highlight it and clear the previous selection. Add…
EasyMediumVue
Coding
DOM Tree Walk 1: Collect Text Nodes
Locked
Implement `collectText(node)` that traverses a **DOM-like** tree and returns **all text content** (nodeType `3`) in **do…
IntermediateMediumJavaScript
Coding
Implement Stack and Queue in JavaScript
Locked
Implement Stack (push/pop/peek/size) and Queue (enqueue/dequeue/peek/size). Use a head index or deque to keep queue oper…
IntermediateMediumJavaScript
Coding
Maze Traversal (Find a Path in a Grid)
Locked
Given a 2D grid of 0/1 (0=open, 1=wall), determine if a path exists from start to end using 4-direction moves. Use BFS/D…
IntermediateMediumJavaScript
Coding
React Dynamic Counter Buttons (Grow-on-Click)
Locked
In React, let users add or remove step buttons dynamically, and have each button update the counter by its own step valu…
IntermediateMediumReact
Coding
React Tabs / Multi-View Switcher
Locked
Implement React tabs with a single activeTab state. Buttons update the value and conditional rendering shows only the ac…
IntermediateMediumReact
Coding
React Theme Toggle with Persisted Light/Dark Mode
Locked
Add a global light/dark theme toggle using React Context. The selected theme should be stored in localStorage and restor…
IntermediateMediumReact
Coding
Recover a BST with Two Swapped Nodes
Locked
Two nodes in a Binary Search Tree were swapped by mistake. Implement `recoverBST(root)` to fix the tree **in-place** (sw…
IntermediateMediumJavaScript
Coding
DOM Tree Walk 3: Basic Selector Engine (Hard)
Locked
Implement a small selector engine: `queryAll(root, selector)` that returns all matching nodes in **document order** from…
HardMediumJavaScript
Coding
Selector Polyfill 3: Implement `queryAllSimple` (Hard)
Locked
Implement `queryAllSimple(root, selector)` — a DOM-like polyfill for `querySelectorAll`. This is a high-likelihood front…
HardMediumJavaScript
Coding
Check if a String Is a Palindrome
Locked
Normalize the input (lowercase + strip non-alphanumerics) and check if it reads the same forwards and backwards using tw…
EasyLowJavaScript
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