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https://github.com/getify/native-promise-only

A polyfill for native ES6 Promises as close as possible (no extensions) to the strict spec definitions.
https://github.com/getify/native-promise-only

javascript polyfill promises

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A polyfill for native ES6 Promises as close as possible (no extensions) to the strict spec definitions.

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# Native Promise Only (NPO)



[![CDNJS](https://img.shields.io/cdnjs/v/native-promise-only.svg)](https://cdnjs.com/libraries/native-promise-only/)



A polyfill for native ES6 Promises as close as possible (no extensions) to the strict spec definitions.



## Intent



The aim of this project is to be the smallest polyfill for Promises, staying as close as possible to what's specified in both [Promises/A+](http://promisesaplus.com) and the [upcoming ES6 specification](http://people.mozilla.org/~jorendorff/es6-draft.html#sec-promise-objects).



An equally important goal is to avoid exposing any capability for promise-state to be mutated externally. The [Known Limitations](#known-limitations) section below explains the trade-offs of that balance.



## Usage



To use this polyfill in the browser, include the "npo.js" file (see the instructions in [Tests/Compliance section](#testscompliance) below for how to build "npo.js" if you don't have it already) with your site's scripts. It's a polyfill, which means it will not overwrite `Promise` if it exists as a global already, so it's safe to include unconditionally.



To use with AMD, import the "npo.js" file module.



To install the polyfill via bower, run:



```

bower install native-promise-only

```



To install the polyfill via npm, run:



```

npm install native-promise-only

```



Then require the module into your node code:



```js

require("native-promise-only");

```



Notice that using the module in this way, we don't assign the module's public API to any variable. **We don't need to**, because it's a polyfill that intentionally patches the global environment (in this case to the `Promise` name) once included.



If you *want* to also have a reference pointing to the same `Promise` global, you *can also* assign the return value from the `require(..)` statement, but it's strongly recommended that you use the same `Promise` name so as to not create confusion:



```js

var Promise = require("native-promise-only");



// Promise === global.Promise; // true!

```



Other than the below [Known Limitations](#known-limitations) discussion ~~and some browser bugs (such as [these](https://gist.github.com/getify/bd11ccf1eff2efdac0fb)) which **this polyfill doesn't suffer from**~~, your promises should operate the same in all JS environments.



Exactly like native promises, here's a quick example of how you create and use the polyfilled promises:



```js

var p = new Promise(function(resolve,reject){

	setTimeout(function(){

		resolve("Yay!");

	},100);

});



p.then(function(msg){

	console.log(msg); // Yay!

});

```



For more on promises, check these blog posts out:



1. Back-story on the hows and whys behind promises (chaining, errors, etc): [multi-part blog post series "Promises"](http://blog.getify.com/promises-part-1/) by [getify](http://twitter.com/getify) (me).

2. Using and enjoying native promises: [JavaScript Promises](http://www.html5rocks.com/en/tutorials/es6/promises/) by [Jake Archibald](http://twitter.com/jaffathecake).



## Known Limitations



A promise object from this polyfill **will be** an instance of the `Promise` constructor, which makes identification of genuine promises easier:



```js

var p = new Promise(..);



p instanceof Promise; // true

```



However, these promise instances don't inherit (delegate to) a *meaningful* `Promise.prototype` object for their methods (there is one, it's just mostly empty).



Consider:



```js

var p = new Promise(..);



Object.getOwnPropertyNames( p ); // [ then, catch ]

Object.getOwnPropertyNames( Promise.prototype ); // [ constructor ]

```



As such, these promises are not really "sub-classable" in the ES6 `class` / `extends` sense, though theoretically you should be able to do that in ES6 with the built-in Promises.



To read a full explanation of why, read [Part 3: The Trust Problem](http://blog.getify.com/promises-part-3/) of my blog post series on Promises.



Briefly, the reason for this deviation is that there's a choice between having delegated methods on the `.prototype` or having private state. Since **the spirit of promises was always to ensure trustability** -- that promises were immutable (from the outside) to everyone except the initial resolver/deferred -- private state is a critically important feature to preserve.



Many other ES6 promise shims/libs seem to have forgotten that important point, as many of them either expose the state publicly on the object instance or provide public accessor methods which can externally mutate a promise's state. Both of these deviations are **intolerable** in my opinion, so this library chose the opposite trade-off: *no ES6 sub-classing*.



Any trade-off is a shame, but this one is the least of a few evils, and probably won't prove to limit very many, as there are only a limited number of use-cases for `extend`ing `Promise` in the ES6 sub-class sense.



## Still Want More?



This project intentionally adheres pretty strictly to the narrow core of [Promises/A+](http://promisesaplus.com) as adopted/implemented by ES6 into the native `Promise()` mechanism.



But it's quite likely that you will [experience a variety of scenarios](http://blog.getify.com/promises-part-5/) in which using *only* native promises might be tedious, limiting, or more trouble than it's worth. There's good reason why most other **Promises/A+** "compliant" libs are actually superset extensions on the narrow core: **because async flow-control is often quite complex in the real world.**



*Native Promise Only* will **NOT** add any of these extra flourishes. Sorry.



**However, I have another project**: [asynquence](http://github.com/getify/asynquence) (async + sequence). It's an abstraction on top of the promises concept (promises are hidden inside), designed to drastically improve the readability and expressiveness of your async flow-control code.



You simply express your async flow-control and *asynquence* creates and chains all the promises for you underneath. **Super simple.**



*asynquence* has a custom implementation for the internal "promises" it uses, and as such does not need native `Promises`, nor does it need/include this polyfill.



Get your feet wet with native promises first, but then when you go looking for something more, consider [asynquence](http://github.com/getify/asynquence) (which is [vastly more powerful](http://davidwalsh.name/asynquence-part-1) and is still only ~2k!).



## Tests/Compliance





    Promises/A+ logo




*Native Promise Only* is "spec compliant" in the sense of passing all tests in the [Promises/A+ Test Suite](https://github.com/promises-aplus/promises-tests).



To run all tests:



1. Either git-clone this repo or run `npm install native-promise-only`, and then switch into that project root.

2. Run `npm install` in the project root to install the dev-dependencies.

3. If you didn't get *native-promise-only* from npm, then from the project root, run `./build.js` or `node build.js` or `npm run build` to generate the minified "npo.js" in the project root.

4. Finally, run `npm test`.



**Note:** Other tests need to be added, such as testing the `Promise()` constructor's behavior, as well as the `Promise.*` static helpers (`resolve(..)`, `reject(..)`, `all(..)`, and `race(..)`), none of which are covered by the Promises/A+ test suite.



Developing a more comprehensive test-suite to augment the  Promises/A+ test suite **is now another primary goal** of this project.



## License



The code and all the documentation are released under the MIT license.



http://getify.mit-license.org/