React: practice, test and prove it

React components, props and state, hooks, list keys, controlled inputs and how re-rendering works.

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Start the react test Read the first lesson

React screenings tend to show a small component and ask what it renders after a click, or why a value is not updating. They focus on how state updates are scheduled, when effects run, why lists need stable keys and what causes a re-render. Live coding rounds often ask for a small form or filtered list.

The practice questions here mirror that style. The lessons explain the render cycle and the hooks most interviews return to, with the mistakes reviewers spot first.

Benchmark: The pass mark is 80%, and this skill is commonly screened for front-end developer, React developer and junior full-stack roles. Requirements vary by employer.

Lessons

State, props and how React re-rendersUnderstand what triggers a render, why state updates are not instant, and how to share state between components.Lesson 1

useEffect, useMemo and rendering listsUse effects only for syncing with the outside world, memoise only real costs, and give list items stable keys.Lesson 2

Controlled forms, validation and submit handlingBind inputs to state with one change handler, validate before submit, and avoid the warnings that reveal form mistakes.Lesson 3

Fetching data: loading, errors, cleanup and custom hooksLoad data in an effect with loading and error state, cancel stale requests, and move the logic into a reusable hook.Lesson 4

Interview prep

React interview questionsCommon React questions that check whether you understand rendering and hooks rather than only their syntax.6 questions

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Common questions

Do I need to know class components?

Most new code uses function components and hooks, and the questions here do too. Knowing that older class code exists helps when maintaining an existing project, but it is not the focus.

Is Redux or another state library tested?

Not on this page. The questions cover built-in React state, because you need to understand it before any library makes sense.

Why does my effect run twice in development?

In development Strict Mode, React mounts, unmounts and mounts components again to reveal missing cleanup. It does not happen in a production build, so the fix is a proper cleanup function, not removing Strict Mode.

React test for interviewWhat the screening round looks like, with a 5-question sample.