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Open a JS / Node question via Full page for split view: reading on the left, Monaco on the right. After that, your recent questions show up in Continue reading below (saved on this device).

JavaScript & Node.js interview Q&A

Interview tracks

Curated JavaScript and React questions from the bank. Pick from the dropdown, study the theory, then jump into the editor to implement and test yourself.

JavaScript track

DSA you can solve in JS in the editor, vanilla DOM UI tasks, plus JS trivia quizzes.

Choose a problem, read the theory below, then open the workspace to write code and use Run / Submit to self-test.

Interview theory (read before / between attempts)▼

How interviewers think about JavaScript

Expect runtime + language questions alongside small coding exercises. Be ready to narrate your approach before you type: inputs, outputs, edge cases, then complexity.

  • Execution model — Call stack, microtasks (Promises) vs macrotasks (setTimeout), and why order matters in async snippets.
  • Scope & closures — Lexical scope, closure over variables in loops, and common “what logs?” traps.
  • Reference vs value — Objects and arrays passed by reference; mutating vs copying for immutability in React-style updates.
  • Arrays & maps — Two-pointer patterns, prefix sums, hash maps for O(n) lookups — say the invariant you maintain in the loop.
  • DOM & events — Event delegation, preventDefault vs stopPropagation, and basic accessibility (focus, labels).

In the editor: use Run for quick feedback from the sandbox, then Submit when you want automated checks and coaching-style feedback.

React track

React UI labs (CDN template) and React-focused quizzes.

Choose a problem, read the theory below, then open the workspace to write code and use Run / Submit to self-test.

Interview theory (read before / between attempts)▼

React rounds: what to rehearse

UI interviews often mix small component builds with concept questions (hooks, rendering, performance). Read the brief, confirm behaviour with the interviewer, then implement in small steps.

  • Hooks rules — Only call hooks at the top level; stable dependency arrays in useEffect / useMemo / useCallback.
  • State vs props — Lift state when multiple children need the same source of truth; keep derived values computed during render when possible.
  • Reconciliation & keys — Why list keys must be stable; avoid using array index as key when order changes.
  • Controlled inputs — Pattern for forms: value + onChange; tie to validation and accessibility (aria-*, labels).
  • Performance story — When you would memoize, split bundles, or virtualize long lists — tie answers to measurable pain (slow TTI, janky scroll).

UI challenges open a live preview and Monaco editor: edit HTML/JSX-style markup, then use Submit to run structure checks.

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