Tell me about a difficult bug you tracked down in JavaScript. How did you approach it?

Note: The interviewer is testing your debugging method, not your luck. Pick a bug that needed reasoning rather than one you fixed by chance.

Tell it as a narrowing process:

• How you reproduced it. The hardest part of most JavaScript bugs is making them happen on demand — a race that only appears on slow connections, a state bug that only appears after a specific navigation order.
• How you bisected. Commenting out halves, git bisect, or a breakpoint moved progressively earlier until the state was still correct.
• The tools. Conditional breakpoints, the Network tab for a response that was not the shape you assumed, the Performance panel for a rendering problem, or simply logging the value and its typeof.
• The root cause. Good candidates: a stale closure capturing an old value, an await inside a loop that silently serialised requests, mutation of an object someone else held a reference to, or this losing its binding in a callback.

Finish with the test you wrote so the bug could not return.