useEventListener in Vue
Vue interview exercise on Composables, difficulty 2 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Write useEventListener(target, event, handler) where target is an EventTarget or a ref holding one (or null): add the listener in onMounted, remove it in onUnmounted, and return a `stop()` function that removes it early. A ref target is read when mounting (that is when a template ref is filled).
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.