Why do browsers throttle JavaScript timers?

Summary

Even if you’ve been doing JavaScript for a while, you might be surprised to learn that setTimeout(0) is not really setTimeout(0). Instead, it could run 4 milliseconds later: const start = performance.now() setTimeout(() => { // Likely 4ms console.log(performance.now() - start) }, 0) Nearly a decade ago when I was on the Microsoft Edge team, it was explained to me that browsers did this to avoid “abuse.” I.e. there are a lot of websites out there that spam setTimeout, so to avoid draining the user’s battery or blocking interactivity, browsers set a special “clamped” minimum of 4ms. This also explains why some browsers would bump the throttling for devices on battery power (16ms in the case of legacy Edge), or throttle even more aggressively for background tabs (1 second in Chrome!). One question always vexed me, though: if setTimeout was so abused, then why did browsers keep introducing new timers like setImmediate (RIP), Promises, or even new fanciness like scheduler.postTask()? If setTimeout had to be nerfed, then wouldn’t these timers suffer the same fate eventually? I wrote a long post about JavaScript timers back in 2018, but until recently I didn’t have a good reason to revisit this question. Then I was doing some work on fake-indexeddb, which is a pure-JavaScript implementation of the IndexedDB API, and this question reared its head. As it turns out, IndexedDB wants to auto-commit transactions when there’s no outstanding work in the event loop – in other words, after all microtasks have finished, but before any tasks (can I cheekily say “macro-tasks”?) have started. To accomplish this, fake-indexeddb was using setImmediate in Node.js (which shares some similarities with the legacy browser version) and setTimeout in the browser. In Node, setImmediate is kind of perfect, because it runs after microtasks but immediately before any other tasks, and without clamping. In the browser, though, setTimeout is pretty sub-optimal: in one benchmark, I was seeing Chrome tak...