Any Promise


Let your library support any ES 2015 (ES6) compatible Promise and leave the choice to application authors. The application can optionally register its preferred Promise implementation and it will be exported when requiring any-promise from library code.

If no preference is registered, defaults to the global Promise for newer Node.js versions. The browser version defaults to the window Promise, so polyfill or register as necessary.

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Assuming the global Promise is the desired implementation:

``bash


Install any libraries depending on any-promise


$ npm install mz
`

The installed libraries will use global Promise by default.

`js
// in library
var Promise = require('any-promise') // the global Promise

function promiseReturningFunction(){
return new Promise(function(resolve, reject){...})
}
`

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Assuming bluebird is the desired Promise implementation:

`bash


Install preferred promise library

$ npm install bluebird

Install any-promise to allow registration

$ npm install any-promise

Install any libraries you would like to use depending on any-promise

$ npm install mz
`

Register your preference in the application entry point before any other require of packages that load any-promise:

`javascript
// top of application index.js or other entry point
require('any-promise/register/bluebird')

// -or- Equivalent to above, but allows customization of Promise library
require('any-promise/register')('bluebird', {Promise: require('bluebird')})
`

Now that the implementation is registered, you can use any package depending on any-promise:


`javascript
var fsp = require('mz/fs') // mz/fs will use registered bluebird promises
var Promise = require('any-promise') // the registered bluebird promise
`

It is safe to call register multiple times, but it must always be with the same implementation.

Again, registration is optional. It should only be called by the application user if overriding the global Promise implementation is desired.

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As an application author, you can optionally register a preferred Promise implementation on application startup (before any call to require('any-promise'):

You must register your preference before any call to require('any-promise') (by you or required packages), and only one implementation can be registered. Typically, this registration would occur at the top of the application entry point.


#### Registration shortcuts

If you are using a known Promise implementation, you can register your preference with a shortcut:


`js
require('any-promise/register/bluebird')
// -or-
import 'any-promise/register/q';
`

Shortcut registration is the preferred registration method as it works in the browser and Node.js. It is also convenient for using with import and many test runners, that offer a --require flag:

`
$ ava --require=any-promise/register/bluebird test.js
`

Current known implementations include bluebird, q, when, rsvp, es6-promise, promise, native-promise-only, pinkie, vow and lie. If you are not using a known implementation, you can use another registration method described below.


#### Basic Registration

As an alternative to registration shortcuts, you can call the register function with the preferred Promise implementation. The benefit of this approach is that a Promise library can be required by name without being a known implementation. This approach does NOT work in the browser. To use any-promise in the browser use either registration shortcuts or specify the Promise constructor using advanced registration (see below).

`javascript
require('any-promise/register')('when')
// -or- require('any-promise/register')('any other ES6 compatible library (known or otherwise)')
`

This registration method will try to detect the Promise constructor from requiring the specified implementation. If you would like to specify your own constructor, see advanced registration.


#### Advanced Registration

To use the browser version, you should either install a polyfill or explicitly register the Promise constructor:

`javascript
require('any-promise/register')('bluebird', {Promise: require('bluebird')})
`

This could also be used for registering a custom Promise implementation or subclass.

Your preference will be registered globally, allowing a single registration even if multiple versions of any-promise are installed in the NPM dependency tree or are using multiple bundled JavaScript files in the browser. You can bypass this global registration in options:


`javascript
require('../register')('es6-promise', {Promise: require('es6-promise').Promise, global: false})
`

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To use any Promise constructor, simply require it:

`javascript
var Promise = require('any-promise');

return Promise
.all([xf, f, init, coll])
.then(fn);


return new Promise(function(resolve, reject){
try {
resolve(item);
} catch(e){
reject(e);
}
});

`

Except noted below, libraries using any-promise should only use documented functions as there is no guarantee which implementation will be chosen by the application author. Libraries should never call register, only the application user should call if desired.


#### Advanced Library Usage

If your library needs to branch code based on the registered implementation, you can retrieve it using var impl = require('any-promise/implementation'), where impl will be the package name ("bluebird", "when", etc.) if registered, "global.Promise" if using the global version on Node.js, or "window.Promise" if using the browser version. You should always include a default case, as there is no guarantee what package may be registered.




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Node.js versions prior to v0.12 may have contained buggy versions of the global Promise. For this reason, the global Promise is not loaded automatically for these old versions. If using any-promise in Node.js versions versions <= v0.12, the user should register a desired implementation.

If an implementation is not registered, any-promise will attempt to discover an installed Promise implementation. If no implementation can be found, an error will be thrown on require('any-promise'). While the auto-discovery usually avoids errors, it is non-deterministic. It is recommended that the user always register a preferred implementation for older Node.js versions.

This auto-discovery is only available for Node.jS versions prior to v0.12. Any newer versions will always default to the global Promise implementation.

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- any-observable - any-promise` for Observables.