practicebasic
Stopwatch with intervals
Build a stopwatch you can start, pause, and reset. Use a ref to store the interval ID — not state — and always clean up in the cleanup function so the timer doesn't stay alive when unmounted.
Objectives
- 1.elapsed (ms) state that starts at 0
- 2.running (boolean) state that controls the start/pause button
- 3.When running, use setInterval every 10ms with functional update: setElapsed(e => e + 10)
- 4.Store the interval ID in a useRef — not in state
- 5.The useEffect cleanup calls clearInterval with the stored ID
- 6.
resetbutton sets elapsed to 0 and pauses - 7.Output format: mm:ss.cc (centiseconds)
your code
Hint
Return a cleanup function from useEffect that calls clearInterval. The useEffect should depend on [running] so it restarts when pausing/resuming.
Related concepts
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