setImmediate.js

A YuzuJS production

Introduction

setImmediate.js is a highly cross-browser implementation of the setImmediate and clearImmediate APIs, proposed by Microsoft to 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.

Macrotasks and Microtasks

The setImmediate API, as specified, gives you access to the environment's task queue, sometimes known as its "macrotask" queue. This is crucially different from the microtask queue used by web features such as MutationObserver, language features such as promises and Object.observe, and Node.js features such as process.nextTick. Each go-around of the macrotask queue yields back to the event loop once all queued tasks have been processed, even if the macrotask itself queued more macrotasks. Whereas, the microtask queue will continue executing any queued microtasks until it is exhausted.

In practice, what this means is that if you call setImmediate inside of another task queued with setImmediate, you will yield back to the event loop and any I/O or rendering tasks that need to take place between those calls, instead of executing the queued task as soon as possible.

If you are looking specifically to yield as part of a render loop, consider using requestAnimationFrame; if you are looking solely for the control-flow ordering effects, use a microtask solution such as asap.

The Tricks

process.nextTick

In Node.js versions below 0.9, setImmediate is not available, but process.nextTick is—and in those versions, process.nextTick uses macrotask semantics. So, we use it to shim support for a global setImmediate.

In Node.js 0.9 and above, process.nextTick moved to microtask semantics, but setImmediate was introduced with macrotask semantics, so there's no need to polyfill anything.

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.

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.

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?

Usage

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

In Node.js, do

npm install --save setimmediate

then

1require("setimmediate");  // (somewhere early in your app; it attaches to the global scope.)

Demo

• Quick sort demo

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