zousan

Zousan

A Lightning Fast, Yet Very Small Promise A+ Compliant Implementation

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There are already several Promise implementations out there, and modern browsers even have built-in Promises, but none met my goals, which are:

1. Exceedingly Fast - it had to be the fastest possible implementation, so it could be used excessively throughout a codebase with impunity. (Even games!)
2. Extremely Small - I will be using this in all my future projects, including mobile, and so fast and efficient code is essential. Also, the less code there is, the less can go wrong.
3. Clearly written and Documented - I want to clearly see (and clearly demonstrate to others) how the code works, both to build confidence in its correctness, and to make it easier to fix/maintain if/when necessary.
4. Usable Everywhere - I required compatability with browsers (both new and old), mobile devices, Node and even make a best effort to work in unknown environments.
5. Simple Build - No dependencies, few files, dog bone simple. (is that a phrase?)

Usage

Zousan is Promise A+ 1.1 compliant, so any documentation for spec-compliant promises applies to Zousan. There are a couple small additions though - see below. Briefly, the spec-compliant API is:

###Constructor

Create a new Promise (often, this promise is returned from a function that provides some asynchronous resource)

	var promise = new Zousan(function(resolve, reject) {
		// ... perform some asynchronous operation ...
		//  load the value to return into "value"
		if(success)
			resolve(value);
		else
			reject(Error("error message goes here"));
	});

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###then()

To use this promise to obtain the value:

	promise.then(function(value) { // this function is called when promise is resolved
			// do something with your value, you deserve it!

		}, function(err) { // this function is called when promise is rejected
		// bummer...

	});

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Zousan does have a couple additional features which are not required by the spec, but are very useful when working with promises. Be aware that if you use this "extension" API, your code may not be compatible with other Promise implementations:

###catch(errorFn)

catch(errorFn) is equivalent to then(null, errorFn) and is just easier to identify - allowing you to adopt the pattern of always ending then chains with a catch, like so:

	getJSON("data.json") 		// hypothetical function which returns a promise
		.then(lookupItems) 		//   takes the data and obtains extra data about items
		.then(updateCount)		//   update item count using host service
		.then(displayResults)	//   update user view of results
		.catch(reportErr);		// Catch any errors occuring in any steps above.

This pattern helps you to remember to always catch any errors produced within your promise chains. Although this isn't part of the Promise A+, it is a very common addition and is present in the ES6 (ECMAScript 2015) draft spec.

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###all(promiseArray)

The other addition is a utility function called all() which takes an array of promises and returns a single promise that will resolve when all promises in the array resolve. The value passed to you is an array with the values from each promise respectively. If any promise within the passed array rejects, this will reject with the same error as the original rejection.

It is available by calling Zousan.all() (i.e. does not require a promise instance).

For example, to obtain data from a list of sources:

	// define an array with our data locations
	var sources = ["data1.json", "data2.json", "data3.json"];

	// Next, obtain an array of promises using hypothetical getJSON function
	var dataProm = sources.map(getJSON);

	// When all promises resolve, we call processData with array of results
	Zousan.all(dataProm).then(processData, reportError);

This function is also present in the ECMAScript 2015 draft spec.

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###Convenience resolve() / reject() as Instance Methods

The spec-compliant manner of resolving/rejecting a promise is to call the methods handed back to the constructor function argument, as shown in the constructor example above. Often it is more convenient (or cleaner) to resolve/reject a promise outside the constructor. For this purpose, Zousan provides resolve and reject methods right on the promise instance. So this pattern is available:

	var promise = new Zousan();
	if(success)
		promise.resolve(value);
	else
		promise.reject(Error("error message goes here"));

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###suppressUncaughtRejectionError flag

By default, Zousan will log a message to the console if a promise is rejected and that rejection is not "caught". Generally, it is best to use the catch() pattern shown above, which will ensure all rejections are handled. If you forget, this will help remind you.

If you wish to suppress this warning, you can turn it off globally via:

	Zousan.suppressUncaughtRejectionError = true;

FAQ

Q: What does "Zousan" mean?

Well, if you had a 3-year-old Japanese child, you would know, now wouldn't you!? "Zou" is the Japanese word for "Elephant". "San" is an honorific suffix placed after someone's name or title to show respect. Children (and other kawaii people) often put "san" after animal names as a sign of respect to the animals.. and just to be kawaii.

Here is a video that might help

And if you need more guidance (or just enjoy these as much as I do) here is another - Zousan Da-ta!!

Q: Ok, cute - but why name it after an Elephant?

Because elephants never forget. So you can depend on them to keep their promises!

Q: Just how fast is it?

I set up a jsperf comparison between:

• Zousan (2,053 bytes minified)
• Bluebird (72,282 bytes minified) - Considered the king of high-performance Promises
• When (12,474 bytes minified) - Long established and high performance Promise shim
• PinkySwear (842 bytes minified) - The smallest compliant Promise immplementation I've come across
• convenant (3,335 bytes) - A Promise implementation written in CoffeeScript
• Native Promises - Built into all recent browsers except IE.

Note: Graph illustrates operations per second, so longer bars are better.