Frontend Development › JavaScript & TypeScript

Microtasks vs Macrotasks

Why promise callbacks run before setTimeout callbacks.

Also known as: microtasks, macrotasks, microtask queue, task queue vs microtask queue, queueMicrotask

The event loop has more than one queue. Microtasks and macrotasks (just “tasks”) get different priority, which explains why a resolved promise’s callback runs before a setTimeout(..., 0) callback.

The rule

After each macrotask finishes (and whenever the call stack empties), the engine runs all pending microtasks, including any new ones they add, before moving to the next macrotask or letting the browser render.

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

The synchronous lines run first. Then microtasks (the promise). Then the next macrotask (the timer).

await is built on promises, so the code after an await is a microtask:

async function f() {
  console.log("a");
  await null;
  console.log("c");     // a microtask
}
f();
console.log("b");
// a, b, c

(Node.js has two more special cases: process.nextTick callbacks run even before promise microtasks, and setImmediate has its own phase.)

Consequences

• Microtasks can starve everything else. A microtask that schedules another microtask forever blocks timers, events and rendering, because the queue never empties. An endless Promise.resolve().then(loop) freezes the page.
• Rendering happens between macrotasks, not between microtasks. If you want the browser to paint before continuing, yield with a macrotask (setTimeout) or requestAnimationFrame.
• Ordering bugs: code that “usually works” can depend on whether a callback was a microtask or a task. Know which you’re using.
• Predictable promise chains: the order of .then callbacks follows the order they were queued.

Python’s asyncio has a related model (a ready queue run by the loop), but without the microtask/macrotask distinction. This is mainly a JavaScript topic.