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JavaScript interview questions

JavaScript questions a Frontend / web developer can be asked, from first job to senior. Open one to see the strongest answer and why it works.

  1. They ask why 0 == '' is true and why you would not write that check. Strongest explanation?

    The strongest answer: Say == coerces types; use === (or Object.is) and treat empty string and zero as separate cases

    Why it works: Naming coercion and choosing === is the junior bar; then handle 0 and '' explicitly if both are valid.

    • Explain coercion, then refuse it in app code.
    • == turns values into a common type first.
    • === compares type and value.
    • If 0 and '' are both legal, check them by name.

  2. typeof null is 'object'. How do you safely check for a real object versus null?

    The strongest answer: Check value !== null && typeof value === 'object', and use Array.isArray when arrays must be excluded

    Why it works: The historic typeof bug is the prompt; the fix is an explicit null check (and array if it matters).

    • Don't trust typeof alone for objects.
    • Reject null explicitly.
    • Decide whether arrays count as objects.
    • Prefer a schema/guard once the shape matters.

  3. An API sometimes returns an empty body. How do you parse it safely?

    The strongest answer: Check status and whether a body exists (content-type / length); only json() when there is JSON; handle parse errors

    Why it works: Empty 204s and HTML error pages are why you look before you parse.

    • Parse only what is JSON.
    • Read status and content-type first.
    • Skip json() on empty bodies.
    • Catch parse errors; never eval.

  4. A form reloads the page when you click Save, and you wanted to POST with fetch. What did you miss?

    The strongest answer: Handle submit on the form, call preventDefault, then fetch — do not rely only on the button click

    Why it works: Enter in a field fires submit, not your button handler. preventDefault on submit, then fetch.

    • Own the submit event.
    • Listen on the form, not only the button.
    • preventDefault, then fetch and handle status.
    • Never put secrets on the query string.

  5. A list will grow to hundreds of rows, each with a Delete control. How do you attach clicks?

    The strongest answer: Listen on the list parent and read the row id from the event target so new rows work without rebinding

    Why it works: Delegation is the vanilla answer: one listener, stable as the list grows. In React, one handler on the parent is the same idea.

    • One listener, many rows.
    • Attach at the list, not each button.
    • Identify the row from the target or a data attribute.
    • In React, the same rule: a parent handler plus a stable id.

  6. fetch('/api/items') resolves but the UI is useless on a 500. What do you check?

    The strongest answer: Say fetch only rejects on network failure — check response.ok or status, then parse the body, then show a real error

    Why it works: This is the classic fetch pitfall: HTTP errors are successful promises. Branch on status first.

    • Treat HTTP status as part of the contract.
    • fetch rejects on network, not on 404/500.
    • Branch on ok/status before parsing.
    • Show a human error; back off if you retry.

  7. Typeahead fires a request per keystroke. How do you fix it without dropping the last query?

    The strongest answer: Debounce the request, cancel in-flight work with AbortController, and send the latest term

    Why it works: Debounce reduces traffic; abort keeps the last term winning.

    • Latest query wins.
    • Debounce the call.
    • Abort the previous request.
    • Apply only the response that matches the current term.

  8. '1 files' and Hebrew plural forms are wrong. What does the i18n layer need?

    The strongest answer: ICU/CLDR plural categories per locale — not an English-only count === 1 check

    Why it works: Pluralization is a locale table, not a boolean. Hebrew, Russian, and Arabic are the usual interview examples.

    • Use real plural rules.
    • ICU messages, not if (count === 1).
    • Translators fill each category.
    • Don't hide the number.