immediate

What is immediate?

The 'immediate' npm package is designed to execute functions asynchronously, as soon as possible, but outside of the current call stack. It uses a set of clever tricks to defer the execution of a function until the stack is clear. It is useful for scheduling tasks to run after the current event loop tick without the overhead of using setTimeout with a delay of 0.

What are immediate's main functionalities?

Asynchronous Execution

This feature allows you to schedule a function to be executed asynchronously, immediately after the current call stack is clear, but before the next event loop tick.

var immediate = require('immediate');
immediate(function () {
  console.log('Executed after the current event loop tick');
});

Other packages similar to immediate

asap

The 'asap' npm package is similar to 'immediate' in that it schedules tasks to execute as soon as possible but after the current call stack has cleared. The main difference is in the internal implementation and the specific scheduling mechanisms used by each library.

raf

The 'raf' package stands for requestAnimationFrame, which is used for scheduling animations in web browsers. It can be used to defer tasks until the next repaint, which may be similar to 'immediate' in deferring execution, but it is specifically tied to the browser's frame rate and rendering.

README

Introduction

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.

The Tricks

process.nextTick

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.

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.

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. 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