immediate

Introduction

immediate.js is a microtask library, based on NobleJS's setImmediate, but stealing the best ideas from Cujo's When and RSVP.

immediate takes the tricks from setImmedate and RSVP and combines them with the schedualer from when.

Note versions 2.6.5 and earlier were strictly speaking a 'macrotask' library not a microtask one, see this for the difference, if you need a macrotask library, I got you covered.

The Tricks

process.nextTick

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.

MutationObserver

This is what RSVP uses, it's very fast, details on MDN

postMessage

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.

Also note that Internet Explorer 10 has a bug relating to refusing to yield the queue which effects setImmediate, postMessage and MessageChannel

MessageChannel

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?

Tricks we don't use

setImmediate

We avoid this process.nextTick in node is better suited to our needs and in Internet Explorer 10 there is a broken version of setImmediate we avoid using this.

Usage

In the browser, include it with a <script> tag; pretty simple. Creates a global called immediate which should act like setImmediate. It also has a method called clear which should act like clearImmediate.

In Node.js, do

npm install immediate

then

var immediate = require("immediate");

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

Reference and Reading

• Efficient Script Yielding W3C Editor's Draft
• W3C mailing list post introducing the specification
• IE Test Drive demo
• Introductory blog post by Nicholas C. Zakas
• I wrote a couple blog pots on this, part 1 and part 2