## Any Promise
[](http://travis-ci.org/kevinbeaty/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.
### Usage with global Promise:
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){...})
}
```
### Usage with registration:
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.
### Optional Application Registration
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})
```
### Library Usage
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](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) 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.
### Support for old Node.js versions
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.
### Related
- [any-observable](https://github.com/sindresorhus/any-observable) - `any-promise` for Observables.
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