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A cross-browser implementation of the new setImmediate API.

branch: master

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A NobleJS production


setImmediate.js is a highly cross-browser implementation of the setImmediate and clearImmediate APIs, currently a W3C draft spec from the Web Performance Working Group. setImmediate allows scripts to yield to the browser, executing a given operation asynchronously, in a manner that is typically more efficient and consumes less power than the usual setTimeout(..., 0) pattern.

setImmediate.js runs at “full speed” in the following browsers and environments, using various clever tricks:

  • Internet Explorer 6+
  • Firefox 3+
  • WebKit
  • Opera 9.5+
  • Node.js
  • Web workers in browsers that support MessageChannel, which I can't find solid info on.

In all other browsers we fall back to using setTimeout, so it's always safe to use.

The Tricks


In Node.js versions below 0.9, setImmediate is not available, but process.nextTick is, so we use it to shim support for a global setImmediate. In Node.js 0.9 and above, setImmediate is available.

Note that we check for actual Node.js environments, not emulated ones like those produced by browserify or similar. Such emulated environments often already include a process.nextTick shim that's not as browser-compatible as setImmediate.js.


In Firefox 3+, Internet Explorer 9+, all modern WebKit browsers, and Opera 9.5+, postMessage is available and provides a good way to queue tasks on the event loop. It's quite the abuse, using a cross-document messaging protocol within the same document simply to get access to the event loop task queue, but until there are native implementations, this is the best option.

Note that Internet Explorer 8 includes a synchronous version of postMessage. We detect this, or any other such synchronous implementation, and fall back to another trick.


Unfortunately, postMessage has completely different semantics inside web workers, and so cannot be used there. So we turn to MessageChannel, which has worse browser support, but does work inside a web worker.

<script> onreadystatechange

For our last trick, we pull something out to make things fast in Internet Explorer versions 6 through 8: namely, creating a <script> element and firing our calls in its onreadystatechange event. This does execute in a future turn of the event loop, and is also faster than setTimeout(…, 0), so hey, why not?


In the browser, include it with a <script> tag; pretty simple.

In Node.js, do

npm install setimmediate



somewhere early in your app; it attaches to the global.


Reference and Reading

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