immediate.js is a cross between NobleJS's setImmediate, Cujo's When, and RSVP.
immediate takes the tricks from setImmedate and RSVP and combines them with the schedualer from when to make a low latency polyfill.
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.
This is what RSVP uses, it's very fast, details on MDN
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.
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. 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.
- 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