React Interview Prep

Exercises

0/55 done

55 results

Given an array of numbers, return a new array with only the even numbers, each doubled

ArraysEasy

Write a function that checks whether a string is a palindrome (ignoring case and non-alphanumeric characters)

StringsEasy

Without a framework, toggle a CSS class on a button click and update its text accordingly

DOMEasy

Fetch user data from an API and gracefully handle the error if the request fails

Async / PromisesEasy

Implement a live character counter for a textarea that warns when approaching a limit

DOMEasy

Write a function that removes duplicate values from an array, preserving the original order

ArraysEasy

Find the first non-repeating character in a string and return its index. If it doesn't exist, return -1.

ArraysEasy

Write a script to detect when a user has scrolled to the bottom of the page.

DOMEasy

How do you check if an object is empty?

ObjectsEasy

Implement a function `zip` that merges two arrays into an array of pairs.

Functional ProgrammingEasy

Explain the 'this' keyword in JavaScript by providing a context-switching example.

Prototype & ClassesEasy

How do you cancel a fetch request?

Async / PromisesEasy

Write a function that generates a random hex color.

LogicEasy

Move all zeros in an array to the end while maintaining the relative order of non-zero elements.

ArraysEasy

Write a function that checks if an element is currently visible in the viewport.

Web APIsEasy

Build a tooltip that appears on hover above an element and hides when the mouse leaves.

DOMEasy

Implement an accordion — clicking a heading expands its content panel and collapses any other open panel.

DOMEasy

Read a user-selected image file and display a preview before uploading it.

Web APIsEasy

Write a `formatCurrency` function and a `formatRelativeTime` function used to render data in a UI.

CSS & LayoutEasy

Flatten a nested object one level deep — { a: { b: 1, c: 2 }, d: 3 } → { b: 1, c: 2, d: 3 }

ObjectsMedium

Group an array of people objects by their department into an object of arrays

ArraysMedium

Write a counter factory that returns increment, decrement, and reset functions sharing private state

ClosuresMedium

Implement a function that accepts any number of arguments and returns their sum — handle nested arrays too

FunctionsMedium

Fix the classic loop-closure bug so each button logs its own index when clicked

ClosuresMedium

Implement a Stack and a Queue using a plain JavaScript array, with proper error handling

Data StructuresMedium

Implement a function to deeply merge two objects (objB's properties should overwrite objA's)

ObjectsMedium

Write a function to find the longest common prefix string amongst an array of strings.

StringsMedium

Rotate an array to the right by `k` steps. Example: `[1,2,3,4,5], k=2` -> `[4,5,1,2,3]`.

ArraysMedium

Given a string of brackets, determine if the order is valid. Example: `({[]})` is valid, `(]` is not.

AlgorithmsMedium

Implement a simple Linked List node and a function to reverse the list.

Data StructuresMedium

Write a function to find the first occurrence of a string within another (like `indexOf`), without using built-ins.

StringsMedium

What is 'Partial Application' and how is it different from Currying?

FunctionsMedium

Create a 'Private' variable using a function constructor (pre-ES6 classes).

ClosuresMedium

Build a drag-and-drop list where items can be reordered by dragging. Log the new order after each drop.

DOMMedium

Build an autocomplete input: debounce the user's keystrokes, fetch matching results, and cancel stale requests.

Async / PromisesMedium

Implement infinite scroll: load the next page of items from an API when the user reaches the bottom of the list.

Web APIsMedium

Write a function that reads CSS custom properties (variables) at runtime and generates a printable theme report.

CSS & LayoutMedium

Build a multi-step form with Next/Back buttons, validation on each step, and a summary on the final step.

DOMMedium

Write a promisified sleep() then use it inside an async function to retry a failing operation up to 3 times

Async / PromisesHard

Implement debounce() — a wrapper that delays a function until the user stops calling it

FunctionsHard

Implement a memoize() utility that caches a function's results by its arguments

Functional ProgrammingHard

Implement pipe() to compose functions left-to-right, then build a data-transformation pipeline with it

Functional ProgrammingHard

Build an EventEmitter class (on, off, once, emit) without Node's built-in EventEmitter

Prototype & ClassesHard

Find all pairs in an array that sum to a target value — solve it in O(n) using a Set

AlgorithmsHard

Write a deepClone() function that correctly handles nested objects, arrays, Dates, and circular references

Objects & ImmutabilityHard

Re-implement Promise.all and Promise.allSettled from scratch

Prototype & ClassesHard

Create a function `runInParallel` that takes an array of async tasks and a limit, ensuring no more than 'limit' tasks run at once.

Async / PromisesHard

Implement a function `curry(fn)` that converts a function like `sum(a, b, c)` into `sum(a)(b)(c)`.

FunctionsHard

Find the maximum subarray sum (Kadane's Algorithm).

ArraysHard

Implement a virtual list (windowed rendering) that renders only the visible rows of a 10,000-item list.

DOMHard

Implement a client-side request queue with a concurrency limit and automatic retry on failure.

Async / PromisesHard

Build a real-time collaborative cursor tracker using BroadcastChannel to sync cursor positions across browser tabs.

Web APIsHard

Implement a rich-text `contenteditable` toolbar: bold, italic, link insertion, and undo history using execCommand alternatives.

DOMHard

Write a `measureRender` utility that uses the Performance API to benchmark how long a DOM update takes, then log a report.

PerformanceHard

Implement offline-first data persistence: save form data to IndexedDB and sync it when the network comes back online.

Web APIsHard

1 / 55

Easy

Q1.Given an array of numbers, return a new array with only the even numbers, each doubled

solution.js

Loading editor…

Run your code

Hit Run to see console output here

Coding

Coding