then-callback/README.md

Wrap a promise to allow passing callback to `.then` of given promise, also works as normal `.then`

# [then-callback][author-www-url] [![npmjs.com][npmjs-img]][npmjs-url] [![The MIT License][license-img]][license-url] 

> Wrap a promise to allow passing callback to `.then` of given promise, also works as normal `.then`

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## Install
```
npm i then-callback --save
```


## Usage
> For more use-cases see the [tests](./test.js)

### [thenCallback](./index.js#L43)
> Basically, you just pass a promise and it returns promise, but `.then` from the returned promise can accept a callback function, instead of two arguments as usual - one for result, one for error.  
Notice that it also works as normal `.then` if you pass two arguments.

- `<promise>` **{Promise}**
- `return` **{Promise}**

**Example**

```js
const thenCallback = require('then-callback')
const promise = Promise.resolve(123)

thenCallback(promise).then((err, res) => {
  console.log('err:', err) // => null
  console.log('res:', res) // => 123
})

// but also works as normal `.then`
thenCallback(promise).then((res) => {
  console.log('res:', res) // => 123
}, (err) => {
  console.log('err:', err)
})
```

You just need to wrap a promise, then you can use `.then` as usual, or to pass callback-style function to it as first argument.

**Example**

```js
const fs = require('fs')
const redolent = require('redolent')
const readFile = redolent(fs.readFile)
const thenCallback = require('then-callback')

const thenCallback = require('then-callback')
var promise = readFile('package.json', 'utf8')
promise = thenCallback(promise)

promise.then((err, res) => {
  if (err) return console.error(err)
  console.log(JSON.parse(res).name) //=> 'then-callback'
})
```

Otherwise, if you pass two arguments and second is function, it will work as normal `.then`.

**Example**

```js
const thenCallback = require('then-callback')
const NativePromise = require('native-or-another')()

// rejected promise
var promise = NativePromise.reject(new Error('foo bar'))
thenCallback(promise).then(console.log, console.error) //=> Error: foo bar

// resolved promise
var promise = NativePromise.resolve(123)
thenCallback(promise).then(console.log, console.error) //=> '123'
```


## Related
- [always-promise](https://github.com/hybridables/always-promise): Promisify, basically, everything. Generator function, callback-style or synchronous function; sync function that returns child process, stream or observable; directly passed promise, stream or child process.
- [letta](https://github.com/hybridables/letta): Let's move to promises! Drop-in replacement for `co@4`, but on steroids. Accepts sync, async and generator functions.
- [native-or-another](https://github.com/tunnckoCore/native-or-another): Always will expose native `Promise` if available, otherwise `Bluebird` but only if you don't give another promise module like `q` or `promise` or what you want.
- [native-promise](https://github.com/tunnckoCore/native-promise): Get native `Promise` or falsey value if not available.


## Contributing
Pull requests and stars are always welcome. For bugs and feature requests, [please create an issue](https://github.com/hybridables/then-callback/issues/new).  
But before doing anything, please read the [CONTRIBUTING.md](./CONTRIBUTING.md) guidelines.


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[npmjs-url]: https://www.npmjs.com/package/then-callback
[npmjs-img]: https://img.shields.io/npm/v/then-callback.svg?label=then-callback

[license-url]: https://github.com/hybridables/then-callback/blob/master/LICENSE.md
[license-img]: https://img.shields.io/badge/license-MIT-blue.svg


[codeclimate-url]: https://codeclimate.com/github/hybridables/then-callback
[codeclimate-img]: https://img.shields.io/codeclimate/github/hybridables/then-callback.svg

[travis-url]: https://travis-ci.org/hybridables/then-callback
[travis-img]: https://img.shields.io/travis/hybridables/then-callback.svg

[coveralls-url]: https://coveralls.io/r/hybridables/then-callback
[coveralls-img]: https://img.shields.io/coveralls/hybridables/then-callback.svg

[david-url]: https://david-dm.org/hybridables/then-callback
[david-img]: https://img.shields.io/david/hybridables/then-callback.svg

[standard-url]: https://github.com/feross/standard
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[author-www-url]: http://www.tunnckocore.tk
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[author-npm-url]: https://www.npmjs.com/~tunnckocore
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[author-twitter-url]: https://twitter.com/tunnckoCore
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[author-github-url]: https://github.com/tunnckoCore
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[new-message-url]: https://github.com/tunnckoCore/ama
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