Window: requestIdleCallback() method

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

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 IdleDeadline object describing the amount of time available and whether or not the callback has been run because the timeout period expired.
• options Optional
  • : 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 cancelIdleCallback() method.

Examples

See our complete example in the article Cooperative Scheduling of Background Tasks API.

Specifications

Browser compatibility

See also

• cancelIdleCallback()
• IdleDeadline
• setTimeout()
• setInterval()
• requestAnimationFrame()