The event loop, explained without hand-waving

One thread, many tricks

JavaScript runs your code on a single thread. Concurrency is an illusion built from a queue and a scheduler.

The order that matters

1. Run the current synchronous stack to completion.
2. Drain all microtasks (promises, queueMicrotask).
3. Render, if the browser wants to.
4. Take one macrotask (timers, I/O) and repeat.

console.log("1");
setTimeout(() => console.log("2"), 0);
Promise.resolve().then(() => console.log("3"));
console.log("4");
// 1, 4, 3, 2

Try the interactive version on the Demos page to watch tasks flow through the queues in real time.