What does queueMicrotask() do?

JavaScriptJunior~2 min read

Definition

javascript

queueMicrotask(callback)

What it does:

Adds the callback function to the microtask queue;
It runs after the current function finishes, but before any setTimeout, setInterval, events, or other macrotasks.

Example

javascript

console.log('A');

queueMicrotask(() => console.log('B'));

console.log('C');

Output:

javascript

A
C
B

Why?

First, all synchronous code runs (A, then C);
Then the Event Loop sees that the stack is empty and runs the microtask (B) before moving on to macrotasks.

What happens under the hood

The current code (on the Call Stack) runs to completion;
Once it finishes, the Event Loop checks the microtask queue;
All microtasks in the queue run in order;
Only then does it take the next macrotask (for example, setTimeout).

Comparison with `Promise.then`

queueMicrotask() works the same way as Promise.resolve().then(callback), but without creating a Promise object, which makes it slightly faster and simpler.

Example - equivalent code:

javascript

Promise.resolve().then(() => console.log('microtask via Promise'));
queueMicrotask(() => console.log('microtask via queueMicrotask'));

Both functions go into the microtask queue and run before any timers.

Example with a timer

javascript

setTimeout(() => console.log('timeout'), 0);

queueMicrotask(() => console.log('microtask'));

console.log('sync');

Output:

javascript

sync
microtask
timeout

Order:

sync - synchronous code,
microtask - microtask queue,
timeout - macrotask queue.

Important to know

1. All microtasks run one after another

If a new microtask is created inside a microtask, it gets added to the queue and runs before moving on to a macrotask.

javascript

queueMicrotask(() => {
  console.log('A');
  queueMicrotask(() => console.log('B'));
});

Output:

javascript

A
B

2. You can "hang" the loop with infinite scheduling

javascript

function loop() {
  queueMicrotask(loop);
}
loop();

Code like this "hangs" the Event Loop: microtasks keep running forever, and no macrotask (setTimeout, clicks, and so on) ever gets to run.

3. Supported everywhere

queueMicrotask() is an ECMAScript 2019 standard, and it is supported by all modern browsers and Node.js.

Summary

Property	Value
What it does	Adds a function to the microtask queue
When it runs	After the current code, but before any macrotasks
Equivalent	`Promise.resolve().then(fn)`
Queue	Microtask Queue
Return value	`undefined`
Use	A lightweight, fast alternative to promises for microtasks

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