JavaScript Developer Interview Questions UK: 8 Real Questions and What Interviewers Want

Before your JavaScript Developer interview, it helps to see the questions from the interviewer's side. We have picked eight from our set of 12 and added the reason each one gets asked.

Eight JavaScript Developer interview questions, and what the interviewer is thinking

1. Explain closures in JavaScript and give a practical example of where you would use one.

Why they ask: Closures are the most classic JavaScript question and underpin many patterns.

2. How does the event loop work in JavaScript, and what is the difference between microtasks and macrotasks?

Why they ask: Understanding execution order is key to debugging asynchronous behaviour.

3. How do promises and async/await work, and how do you handle errors and parallel requests?

Why they ask: Daily work involves asynchronous code, so errors here are costly.

4. Explain how 'this' works in JavaScript and how arrow functions change it.

Why they ask: The behaviour of this is a frequent source of bugs and a favourite interview topic.

5. How do you approach testing JavaScript code, and which tools do you use?

Why they ask: Testing discipline distinguishes mid-level developers from beginners.

6. A page you built is slow on mobile devices. How would you find and fix the problem?

Why they ask: Performance diagnosis is a practical skill for JavaScript developers.

7. Tell me about a time you debugged a difficult JavaScript issue.

Why they ask: They want to see systematic debugging and persistence.

8. Why did you choose to specialise in JavaScript, and how do you keep your skills up to date?

Why they ask: They want to gauge genuine passion and learning habits in a fast-moving ecosystem.

A sample answer to question 1

Explain closures in JavaScript and give a practical example of where you would use one.

A closure is a function that retains access to variables from the scope in which it was created, even after that outer function has returned. Because JavaScript uses lexical scoping, the inner function carries a reference to its surrounding environment. A practical example is a function factory or private state: createCounter returns an increment function that updates a count variable which no one else can access directly. I use closures for debouncing input handlers, where the timer ID is held in the closure, for memoisation caches, and for event handlers that need contextual data. They are also how React hooks such as useState and useEffect capture values, which can cause stale closure bugs if dependency arrays are wrong. One thing to watch is memory: closures keep referenced variables alive, so you should avoid unintentionally capturing large objects, and remove event listeners when no longer needed.

Common mistake to avoid: Reciting a definition without an example; confusing closures with scope only

Want the rest?

Full model answers, key points and common mistakes for all 12 questions are on the JavaScript Developer interview questions page. The first five are free, no sign-up needed.