trough

trough is middleware.

Contents

• What is this?
• When should I use this?
• Install
• Use
• API
  • trough()
  • wrap(middleware, callback)
  • Callback
  • Middleware
  • Pipeline
  • Run
  • Use
• Compatibility
• Security
• Contribute
• License

What is this?

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.

When should I use this?

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.

Install

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

npm install trough

In Deno with esm.sh:

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

In browsers with esm.sh:

Use

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 

API

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

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

trough()

Create new middleware.

Parameters

There are no parameters.

Returns

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

Parameters

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

Returns

Wrapped middleware (Run).

Callback

Callback function (TypeScript type).

Parameters

• error (Error, optional) — error, if any
• ...output (Array, optional) — output values

Returns

Nothing (undefined).

Middleware

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

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 (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'

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

Yields:

Got: thing

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 (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:

null 'some' 'values'

Parameters

• ...input (Array, optional) — input values

Returns

Output, promise, etc (any).

Pipeline

Pipeline (TypeScript type).

Properties

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

Run

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.

Parameters

• ...input (Array, optional) — input values
• done (Callback) — callback called when done

Returns

Nothing (undefined).

Use

Add middleware (TypeScript type).

Parameters

• middleware (Middleware) — middleware function

Returns

Current pipeline (Pipeline).

Compatibility

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.

Security

This package is safe.

Contribute

Yes please! See How to Contribute to Open Source.

License

MIT © Titus Wormer