## Any Promise

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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.