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GitHub - wooorm/trough: `trough` is middleware

trough is middleware.

trough is like ware with less sugar. Middleware functions can also change the input of the next.

The word trough (/trôf/) means a channel used to convey a liquid.

You can use this package when you’re building something that accepts “plugins”, which are functions, that can be sync or async, promises or callbacks.

This package is ESM only. In Node.js (version 16+), install with npm:

In Deno with esm.sh:

import {trough, wrap} from 'https://esm.sh/trough@2'

In browsers with esm.sh:

<script type="module">
  import {trough, wrap} from 'https://esm.sh/trough@2?bundle'
</script>

import fs from 'node:fs'
import path from 'node:path'
import process from 'node:process'
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 (error, stats) {
      next(error, {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:22:12
    at wrapped (~/node_modules/trough/index.js:111:16)
    at next (~/node_modules/trough/index.js:62:23)
    at done (~/node_modules/trough/index.js:145:7)
    at ~/example.js:15:7
    at FSReqCallback.oncomplete (node:fs:199:5)
null <Buffer 23 20 74 72 6f 75 67 68 0a 0a 5b 21 5b 42 75 69 6c 64 5d 5b 62 75 69 6c 64 2d 62 61 64 67 65 5d 5d 5b 62 75 69 6c 64 5d 0a 5b 21 5b 43 6f 76 65 72 61 ... 7994 more bytes>

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

It exports the TypeScript types Callback, Middleware, Pipeline, Run, and Use.

Create new middleware.

There are no parameters.

Pipeline.

wrap(middleware, callback)

Wrap middleware into a uniform interface.

You can pass all input to the resulting function. callback is then called with the output of middleware.

If middleware accepts more arguments than the later given in input, an extra done function is passed to it after that input, which must be called by middleware.

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

middleware (Middleware) — function to wrap
callback (Callback) — callback called with the output of middleware

Wrapped middleware (Run).

Callback function (TypeScript type).

error (Error, optional) — error, if any
...output (Array<unknown>, optional) — output values

Nothing (undefined).

A middleware function called with the output of its predecessor (TypeScript type).

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 (thing) {
    return new Error('Got: ' + thing)
  })
  .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 (thing) {
    throw new Error('Got: ' + thing)
  })
  .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 (thing) {
    return 'even ' + thing
  })
  .run('more value', 'untouched', console.log)

Yields:

null 'even more value' 'untouched'

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

Yields:

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:

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 (thing, next) {
    next(new Error('Got: ' + thing))
  })
  .run('thing', console.log)

Yields:

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

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

import {trough} from 'trough'

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

Yields:

...input (Array<any>, optional) — input values

Output, promise, etc (any).

Pipeline (TypeScript type).

run (Run) — run the pipeline
use (Use) — add middleware

Call all middleware (TypeScript type).

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.

...input (Array<any>, optional) — input values
done (Callback) — callback called when done

Nothing (undefined).

Add middleware (TypeScript type).

middleware (Middleware) — middleware function

Current pipeline (Pipeline).

This projects is compatible with maintained versions of Node.js.

When we cut a new major release, we drop support for unmaintained versions of Node. This means we try to keep the current release line, trough@2, compatible with Node.js 12.

This package is safe.

Yes please! See How to Contribute to Open Source.

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