# trough

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> **trough** /trôf/ — a channel used to convey a liquid.

`trough` is like [`ware`][ware] with less sugar, and middleware functions can
change the input of the next.

## Install

[npm][]:

```sh
npm install trough
```

## Use

```js
var fs = require('fs')
var path = require('path')
var trough = require('trough')

var pipeline = trough()
  .use(function(fileName) {
    console.log('Checking… ' + fileName)
  })
  .use(function(fileName) {
    return path.join(process.cwd(), fileName)
  })
  .use(function(filePath, next) {
    fs.stat(filePath, function(err, stats) {
      next(err, {filePath, stats})
    })
  })
  .use(function(ctx, next) {
    if (ctx.stats.isFile()) {
      fs.readFile(ctx.filePath, next)
    } else {
      next(new Error('Expected file'))
    }
  })

pipeline.run('readme.md', console.log)
pipeline.run('node_modules', console.log)
```

Yields:

```txt
Checking… readme.md
Checking… node_modules
Error: Expected file
    at ~/example.js:21:12
    at wrapped (~/node_modules/trough/index.js:93:19)
    at next (~/node_modules/trough/index.js:56:24)
    at done (~/node_modules/trough/index.js:124:12)
    at ~/node_modules/example.js:14:7
    at FSReqWrap.oncomplete (fs.js:153:5)
null <Buffer 23 20 74 72 6f 75 67 68 20 5b 21 5b 42 75 69 6c 64 20 53 74 61 74 75 73 5d 5b 74 72 61 76 69 73 2d 62 61 64 67 65 5d 5d 5b 74 72 61 76 69 73 5d 20 5b ... >
```

## API

### `trough()`

Create a new [`Trough`][trough].

#### `trough.wrap(middleware, callback[, …input])`

Call `middleware` with all input.
If `middleware` accepts more arguments than given in input, and extra `done`
function is passed in after the input when calling it.
It must be called.

The first value in `input` is called the main input value.
All other input values are called the rest input values.
The values given to `callback` are the input values, merged with every non-nully
output value.

*   If `middleware` throws an error, returns a promise that is rejected, or
    calls the given `done` function with an error, `callback` is invoked with
    that error
*   If `middleware` returns a value or returns a promise that is resolved, that
    value is the main output value
*   If `middleware` calls `done`, all non-nully values except for the first one
    (the error) overwrite the output values

### `Trough`

A pipeline.

#### `Trough#run([input…, ]done)`

Run the pipeline (all [`use()`][use]d middleware).
Invokes [`done`][done] on completion with either an error or the output of the
last middleware.

> Note!
> as the length of input defines whether [async][] functions get a `next`
> function, it’s recommended to keep `input` at one value normally.

##### `function done(err?, [output…])`

The final handler passed to [`run()`][run], invoked with an error if a
[middleware function][fn] rejected, passed, or threw one, or the output of the
last middleware function.

#### `Trough#use(fn)`

Add `fn`, a [middleware function][fn], to the pipeline.

##### `function fn([input…, ][next])`

A middleware function invoked with the output of its predecessor.

###### Synchronous

If `fn` returns or throws an error, the pipeline fails and `done` is invoked
with that error.

If `fn` returns a value (neither `null` nor `undefined`), the first `input` of
the next function is set to that value (all other `input` is passed through).

The following example shows how returning an error stops the pipeline:

```js
var trough = require('trough')

trough()
  .use(function(val) {
    return new Error('Got: ' + val)
  })
  .run('some value', console.log)
```

Yields:

```txt
Error: Got: some value
    at ~/example.js:5:12
    …
```

The following example shows how throwing an error stops the pipeline:

```js
var trough = require('trough')

trough()
  .use(function(val) {
    throw new Error('Got: ' + val)
  })
  .run('more value', console.log)
```

Yields:

```txt
Error: Got: more value
    at ~/example.js:5:11
    …
```

The following example shows how the first output can be modified:

```js
var trough = require('trough')

trough()
  .use(function(val) {
    return 'even ' + val
  })
  .run('more value', 'untouched', console.log)
```

Yields:

```txt
null 'even more value' 'untouched'
```

###### Promise

If `fn` returns a promise, and that promise rejects, the pipeline fails and
`done` is invoked with the rejected value.

If `fn` returns a promise, and that promise resolves with a value (neither
`null` nor `undefined`), the first `input` of the next function is set to that
value (all other `input` is passed through).

The following example shows how rejecting a promise stops the pipeline:

```js
var trough = require('trough')

trough()
  .use(function(val) {
    return new Promise(function(resolve, reject) {
      reject('Got: ' + val)
    })
  })
  .run('val', console.log)
```

Yields:

```txt
Got: val
```

The following example shows how the input isn’t touched by resolving to `null`.

```js
var trough = require('trough')

trough()
  .use(function() {
    return new Promise(function(resolve) {
      setTimeout(function() {
        resolve(null)
      }, 100)
    })
  })
  .run('Input', console.log)
```

Yields:

```txt
null 'Input'
```

###### Asynchronous

If `fn` accepts one more argument than the given `input`, a `next` function is
given (after the input).  `next` must be called, but doesn’t have to be called
async.

If `next` is given a value (neither `null` nor `undefined`) as its first
argument, the pipeline fails and `done` is invoked with that value.

If `next` is given no value (either `null` or `undefined`) as the first
argument, all following non-nully values change the input of the following
function, and all nully values default to the `input`.

The following example shows how passing a first argument stops the pipeline:

```js
var trough = require('trough')

trough()
  .use(function(val, next) {
    next(new Error('Got: ' + val))
  })
  .run('val', console.log)
```

Yields:

```txt
Error: Got: val
    at ~/example.js:5:10
```

The following example shows how more values than the input are passed.

```js
var trough = require('trough')

trough()
  .use(function(val, next) {
    setTimeout(function() {
      next(null, null, 'values')
    }, 100)
  })
  .run('some', console.log)
```

Yields:

```txt
null 'some' 'values'
```

## License

[MIT][license] © [Titus Wormer][author]

<!-- Definitions -->

[build-badge]: https://img.shields.io/travis/wooorm/trough.svg

[build]: https://travis-ci.org/wooorm/trough

[coverage-badge]: https://img.shields.io/codecov/c/github/wooorm/trough.svg

[coverage]: https://codecov.io/github/wooorm/trough

[downloads-badge]: https://img.shields.io/npm/dm/trough.svg

[downloads]: https://www.npmjs.com/package/trough

[size-badge]: https://img.shields.io/bundlephobia/minzip/trough.svg

[size]: https://bundlephobia.com/result?p=trough

[npm]: https://docs.npmjs.com/cli/install

[license]: license

[author]: https://wooorm.com

[ware]: https://github.com/segmentio/ware

[trough]: #trough-1

[use]: #troughusefn

[run]: #troughruninput-done

[fn]: #function-fninput-next

[done]: #function-doneerr-output

[async]: #asynchronous