---

title: "Window: requestIdleCallback() method"

short-title: requestIdleCallback()

slug: Web/API/Window/requestIdleCallback

page-type: web-api-instance-method

browser-compat: api.Window.requestIdleCallback

---

{{APIRef("HTML DOM")}}

The **`window.requestIdleCallback()`** method queues a function

to be called during a browser's idle periods. This enables developers to perform

background and low priority work on the main thread, without impacting

latency-critical events such as animation and input response. Functions are generally

called in first-in-first-out order; however, callbacks which have a `timeout`

specified may be called out-of-order if necessary in order to run them before the

timeout elapses.

You can call `requestIdleCallback()` within an idle callback function to

schedule another callback to take place no sooner than the next pass through the event

loop.

> [!NOTE]

> A `timeout` option is strongly recommended for required work,

> as otherwise it's possible multiple seconds will elapse before the callback is fired.

## Syntax

```js-nolint

requestIdleCallback(callback)

requestIdleCallback(callback, options)

```

### Parameters

- `callback`

- : A reference to a function that should be called in the near future, when the event

loop is idle. The callback function is passed an {{domxref("IdleDeadline")}} object

describing the amount of time available and whether or not the callback has been run

because the timeout period expired.

- `options` {{optional_inline}}

- : Contains optional configuration parameters. Currently only one property is defined:

- `timeout`

- : If the number of milliseconds represented by this parameter has elapsed and the callback has not already been called, then a task to execute the callback is queued in the event loop (even if doing so risks causing a negative performance impact). `timeout` must be a positive value or it is ignored.

### Return value

An ID which can be used to cancel the callback by passing it into the

{{domxref("window.cancelIdleCallback()")}} method.

## Examples

See our [complete example](/en-US/docs/Web/API/Background_Tasks_API#example)

in the article [Cooperative Scheduling of Background Tasks API](/en-US/docs/Web/API/Background_Tasks_API).

## Specifications

{{Specifications}}

## Browser compatibility

{{Compat}}

## See also

- {{domxref("Window.cancelIdleCallback()")}}

- {{domxref("IdleDeadline")}}

- {{domxref("Window.setTimeout()")}}

- {{domxref("Window.setInterval()")}}

- {{domxref("Window.requestAnimationFrame()")}}