A metronome that cannot drift.

JavaScript timers drift by milliseconds, get throttled to a crawl in a background tab, and animation frames stop altogether when the tab is hidden. The audio hardware has its own clock, which does none of that. Every click is handed to the audio hardware stamped with an exact time on that clock, and it plays then, whether or not the page is keeping up.

A timer wakes up every 25 milliseconds and looks a tenth of a second ahead. Any beat falling inside that window is scheduled with the audio hardware and forgotten about. If a wake-up is late, the beat is still scheduled for the same moment, so the lateness is absorbed instead of added to the beat.

Each beat time is the previous beat time plus 60 divided by the tempo, worked out in advance. Nothing reads the wall clock to decide when the next click happens, so there is no error to accumulate. Change the tempo while it runs and the new spacing applies from the next unscheduled beat, about a tenth of a second later.

The swinging arm reads the same audio clock the clicks are scheduled on and works out where it should be. It reaches an extreme exactly on each beat because it is following the schedule, not making it. If frames drop, the arm catches up and the clicks are untouched. Come back from a background tab and it fast-forwards past the beats you missed rather than firing them all at once.