Question 1

Build a Stopwatch

Accepted Answer

**Target Audience:** Beginners preparing for a frontend machine coding round. You know a little JavaScript and React; timestamps, intervals and effect cleanup are explained from zero. **Difficulty:** Easy > **How to read this doc:** Plain language first, then 📌 Interview term: callouts with the vocabulary interviewers expect. Every output under "Verified" was printed by running this exact solution on React 19.3.1 inside in jsdom. ## 1. Why This Eve

Question 2

Build a Lazy-loaded Image

Accepted Answer

**Target Audience:** Frontend candidates preparing for a machine coding round. Assumes basic React (components, props, `useState`); every browser API is explained from zero. **Difficulty:** Medium > **How to read this doc:** Each idea is explained in plain words first, then tagged with 📌 Interview term: so you also learn the vocabulary an interviewer listens for. The output in section 6 was produced by actually running the solution on React 19.3.1, in jsdom and in headless Chromiu

Question 3

Build a debounced Search Bar

Accepted Answer

**Target Audience:** Frontend developers preparing for a machine coding round. Assumes basic React (components, `useState`), nothing about debouncing or request cancellation. **Difficulty:** Medium > **How to read this doc:** Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer listens for. The output in the Verified section was produced by actually running the solution on React 19.3.1 in jsdom with a

Question 4

Build a Virtualized List

Accepted Answer

**Target Audience:** Frontend developers preparing for machine coding rounds who know basic React (components, `useState`, props) but have never built a virtualized list. **Difficulty:** Hard > **How to read this doc:** Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the words an interviewer listens for. The output in the Verified section was produced by actually running the solution on React 19.3.1 in jsdom, and the browser nu

Question 5

Build a Virtualized List with react-window (Fixed Size)

Accepted Answer

**Target Audience:** React developers who already understand the basic windowing maths (if not, start with [Build a Virtualized List](/interview-question/build-a-virtualized-list)) and want to understand, and rebuild, the API of a production library. **Difficulty:** Hard > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The Verified section quotes output from running the solution on React 19.3.1 in jsdom, plus the real react-window 1.8.11 and 2.3.4

Question 6

Build a Live Stock Ticker with throttled updates

Accepted Answer

**Target Audience:** Developers who know React state and effects and want to understand how to keep a UI smooth when data arrives faster than the screen can draw it. **Difficulty:** Hard > **How to read this doc:** Plain language first, then 📌 Interview term: callouts with the vocabulary an interviewer listens for. The render counts in the Verified section were measured by running the real solution in React 19.3.1 under `StrictMode` in jsdom. The market feed is simulated with a se

FRONTEND MACHINE CODINGDECODED.

Easy