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 import
ed
instead of require
d.
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