setTimeout with 0ms is not instant -- event loop minimum delay

setTimeout(fn, 0) does not run fn immediately. There is a minimum delay of 4ms in browsers (after 5 nested timeouts). In Node.js, setImmediate runs before setTimeout(fn, 0).

setTimeout(fn, 0) queues fn for the next macrotask. Use queueMicrotask(fn) or Promise.resolve().then(fn) for microtask timing (runs before next render). Use requestAnimationFrame(fn) for before-paint timing. In Node.js: process.nextTick (before I/O) > microtask > setImmediate > setTimeout(0).

The event loop processes microtasks (promises) before macrotasks (timers). setTimeout always creates a macrotask, so even 0ms delay waits for the current microtask queue to drain.