Frontend Developer technical round: JS, browser and React

Frontend technical rounds sample across the stack that runs in the browser. Frontend engineers who cover the boundary with UI/UX Designer Jobs — layout, accessibility, motion — tend to interview strongest at product companies.

• JavaScript core: closures, the event loop, promises and async/await, prototypes and 'this'.
• Browser internals: the DOM, rendering, reflow versus repaint, and event delegation.
• CSS: the box model, flexbox and grid, specificity, and responsive layout.
• Framework depth: React rendering, hooks, keys and reconciliation, or the equivalent in your framework.

The signal they want is someone who understands why the browser behaves as it does, not only which API to call.

Certain conceptual questions recur because they reveal depth quickly.

'Explain the event loop and the output of this setTimeout/promise snippet.'

They test whether you understand the microtask versus macrotask queue.

'What causes a re-render in React and how do you prevent unnecessary ones?'

They probe state, props, memoisation and key usage.

'Why does this CSS not centre the element?'

A practical layout debug that tests real understanding, not memorised tricks.

Question: 'What is a closure and where would you use one?'

Strong: 'A closure is a function that retains access to variables from the scope in which it was defined, even after that scope has returned. I use closures for data privacy, like a counter factory, and in event handlers and hooks to capture state. The gotcha is stale closures in React effects, which is why the dependency array matters.' It defines, applies, and names a real pitfall.

Weak: 'A closure is a function inside a function.' It is technically incomplete, gives no use case, and misses why closures matter. Interviewers use the follow-up to expose the shallow understanding.

Prioritise depth in fundamentals over breadth of libraries.

• Master the event loop, closures and async behaviour until you can predict output.
• Be able to debug CSS layout and explain specificity live.
• Understand your framework's render cycle well enough to optimise it.

Because interviewers push follow-ups on every answer, rehearse explaining concepts aloud with examples. A free AI voice mock interview on InterviewPrep generates a frontend mock from your CV and a real job description, then scores your explanation clarity, pace and filler words, so your answers stay precise when the questioning goes deeper.