The Hidden Bug Breaking Most JavaScript Timers
If you've ever set a 5-minute timer in a browser tab, switched to another tab, and come back to find the alarm fired at 5:08 instead of 5:00, or didn't fire at all, you've hit one of the most underdiagnosed bugs in client-side timing. The fix isn't in your timer logic. It's in scheduling the alarm itself.
I ran into this building Timerjoy, a static Next.js site with 1,400+ timer pages. The naive version of my 25-minute timer worked perfectly in foreground tabs and missed by 5 to 30 seconds in backgrounded tabs. Here is the actual mechanism, the diagnostic data, and the two-layer fix that brought drift to under 50 milliseconds regardless of tab state.
The textbook timer (and why it fails)
The way most tutorials teach a JavaScript countdown:
function startTimer(durationMs, onTick, onComplete) { let remaining = durationMs const interval = setInterval(() => { remaining -= 1000...
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