trough

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

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

Install

This package is ESM only: Node 12+ is needed to use it and it must be imported instead of required.

npm:

npm install trough

Use

import fs from 'fs'
import path from 'path'
import {trough} from 'trough'

const 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:

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

This package exports the following identifiers: trough and wrap. There is no default export.

trough()

Create a new Trough.

wrap(middleware, callback)(…input)

Call middleware with all input. If middleware accepts more arguments than given in input, an extra done function is passed in after the input when calling it. In that case, done must be called.

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

• If middleware throws an error, returns a promise that is rejected, or calls the given done function with an error, callback is called 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-nullish values except for the first one (the error) overwrite the output values

Trough

A pipeline.

Trough#run([input…, ]done)

Run the pipeline (all use()d middleware). Calls 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(), called with an error if a middleware function rejected, passed, or threw one, or the output of the last middleware function.

Trough#use(fn)

Add fn, a middleware function, to the pipeline.

function fn([input…, ][next])

A middleware function called with the output of its predecessor.

Synchronous

If fn returns or throws an error, the pipeline fails and done is called 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:

import {trough} from 'trough'

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

Yields:

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

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

import {trough} from 'trough'

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

Yields:

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

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

import {trough} from 'trough'

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

Yields:

null 'even more value' 'untouched'

Promise

If fn returns a promise, and that promise rejects, the pipeline fails and done is called 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:

import {trough} from 'trough'

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

Yields:

Got: val

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

import {trough} from 'trough'

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

Yields:

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 called with that value.

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

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

import {trough} from 'trough'

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

Yields:

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

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

import {trough} from 'trough'

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

Yields:

null 'some' 'values'

License

MIT © Titus Wormer