setImmediate API

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HTML4 (~15ms Timers)
HTML5 (4ms Timers)
setImmediate
About The setImmediate API

Web developers commonly use the setTimeout API to break apart long running JavaScript operations. This approach allows the browser to process outstanding work, such as layouts and paints, and then waits for the specified period of time before calling back into JavaScript. Itís an important design pattern when building responsive web applications.

Once the browser has yielded control it relies on an interrupt from the underlying operating system and hardware to receive control and issue the JavaScript callback. Having longer durations between these interrupts allows hardware to enter low power states which significantly decreases power consumption. By default the Microsoft Windows operating system and Intel based processors use 15.6ms resolutions for these interrupts (64 interrupts per second). This allows Intel based processors to enter their lowest power state. For this reason web developers have traditionally only been able to achieve 64 callbacks per second when using setTimeout(0) when using HTML4 browsers including earlier editions of Internet Explorer and Mozilla Firefox.

Over the last two years browsers have attempted to increase the number of callbacks per second that JavaScript developers can receive through the setTimeout and setInterval APIís by changing the power conscious Windows system settings and preventing hardware from entering low power states. The HTML5 specification has gone to the extreme of recommending 250 callbacks per second. This high frequency can result in a 40% increase in power consumption, impacting battery life, operating expenses, and the environment. In addition, this approach does not address the core performance problem of improving CPU efficiency and scheduling.

The new setImmediate API, which is semantically comparable to the setTimeout and setInterval APIís, was designed by the W3C Web Performance Working Group to address the core performance problem without negatively impacting power consumption. Web developers can use the setImmediate API to break apart long running JavaScript operations and will receive a callback immediately after the browser has processed outstanding work. Itís the best of both worlds - the efficiency of native scheduling and the power conservation of low frequencies. Using setImmediate is a great replacement for most cases where developers are intentionally using setTimeout(0) today.

This demo visually sorts 250 items using the Quick Sort algorithm once per callback, making one swap during each browser callback. The more browser callbacks per second the faster the items will be sorted. While this is not how you would efficently implement a sorting algoritm it's a fun way to visualize the impact of callbacks per second. Try sorting using the HTML4 resolution of 15.6ms per callback, the HTML5 resolution of 4ms per callback, and the new setImmediate approach of immediate callbacks. Special thanks to Erik Kay and Mike Belshe from the Google Chrome engineering team for the quick sort test used to demonstrate the benefits of the new setImmediate API.