Cancellable Timeout
Problem statement
Implement setCancellableTimeout. It should behave like setTimeout, but instead of returning a timer id, it returns a function that cancels the pending callback.
Signature
setCancellableTimeout(callback);
setCancellableTimeout(callback, delay);
setCancellableTimeout(callback, delay, param1);
setCancellableTimeout(callback, delay, param1, param2);Example
let i = 0;
const cancel = setCancellableTimeout(() => {
i++;
}, 100);
cancel();
// i is still 0 after 100msRequirements & constraints
- Match the `setTimeout` argument shape.
- Return a cancel function.
- Do not execute the callback after cancellation.
How to approach Cancellable Timeout
The strategy an interviewer expects you to reach for.
UI code schedules work it may cancel — route changes, unmounts, newer keystrokes. A cancellable timeout returns that cancel handle. Naive move Call setTimeout and forget to expose clearTimeout, or clear the wrong id. Where it breaks Without cancel, callbacks fire after unmount and set state on dead components. Extra args must still forward into the timer callback. The fix Capture the timer id in a closure. Return a cancel function that clears it. Forward ...args to setTimeout. Remember: Always return a cancel; always clearTimeout the same id you created. Say this in the interview: I'd wrap setTimeout, keep the id in a closure, forward args, and return a cancel function that clearTimeouts that id.
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