Chain functions as transform streams.
BSD-3-CLAUSE License
stream-chain
creates a chain of streams out of regular functions, asynchronous functions, generator functions, existing Node streams, and Web streams, while properly handling backpressure. The resulting chain is represented as a Duplex stream, which can be combined with other streams the usual way. It eliminates a boilerplate helping to concentrate on functionality without losing the performance especially make it easy to build object mode data processing pipelines.
Originally stream-chain
was used internally with stream-fork and stream-json to create flexible data processing pipelines.
stream-chain
is a lightweight, no-dependencies micro-package with TS typings. It is distributed under New BSD license.
import chain from 'stream-chain';
// or: const chain = require('stream-chain');
import fs from 'node:fs';
import zlib from 'node:zlib';
import {Transform} from 'node:stream';
// this chain object will work on a stream of numbers
const pipeline = chain([
// transforms a value
x => x * x,
// returns several values
x => chain.many([x - 1, x, x + 1]),
// waits for an asynchronous operation
async x => await getTotalFromDatabaseByKey(x),
// returns multiple values with a generator
function* (x) {
for (let i = x; i > 0; --i) {
yield i;
}
return 0;
},
// filters out even values
x => x % 2 ? x : null,
// uses an arbitrary transform stream
new Transform({
objectMode: true,
transform(x, _, callback) {
callback(null, x + 1);
}
}),
// transform to strings
x => '' + x,
// compress
zlib.createGzip()
]);
// the chain object is a regular stream
// it can be used with normal stream methods
// log errors
pipeline.on('error', error => console.log(error));
// use the chain object, and save the result to a file
dataSource.pipe(pipeline).pipe(fs.createWriteStream('output.txt.gz'));
Making processing pipelines appears to be easy: just chain functions one after another, and we are done. Real life pipelines filter objects out and/or produce more objects out of a few ones. On top of that we have to deal with asynchronous operations, while processing or producing data: networking, databases, files, user responses, and so on. Unequal number of values per stage, and unequal throughput of stages introduced problems like backpressure, which requires algorithms implemented by streams.
While a lot of API improvements were made to make streams easy to use, in reality, a lot of boilerplate is required when creating a pipeline. stream-chain
eliminates most of it.
npm i --save stream-chain
# or: yarn add stream-chain
All documentation can be found in the wiki. It document in details the main function and various utilities and helpers that can simplify stream programming. Additionally it includes a support for JSONL (line-separated JSON files).
An object that is returned by chain()
is based on Duplex. It chains its dependents in a single pipeline optionally binding error
events.
Many details about this package can be discovered by looking at test files located in tests/
and in the source code (src/
).
chain(fns[, options])
The factory function accepts the following arguments:
fns
is an array of functions, arrays or stream instances.
If a value is a function, it is a candidate for a Transform stream (see below for more details), which calls this function with two parameters: chunk
(an object), and an optional encoding
. See Node's documentation for more details on those parameters.
Regular value:
undefined
or null
, no value shall be passed.// produces no values:
x => null
x => undefined
// produces one value:
x => x
Special value:
If it is an instance of Promise or "thenable" (an object with a method called then()
), it will be waited for. Its result should be a regular value.
// delays by 0.5s:
x => new Promise(
resolve => setTimeout(() => resolve(x), 500))
If it is an instance of a generator or "nextable" (an object with a method called next()
), it will be iterated according to the generator protocol. The results should be regular values.
// produces multiple values:
class Nextable {
constructor(x) {
this.x = x;
this.i = -1;
}
next() {
return {
done: this.i <= 1,
value: this.x + this.i++
};
}
}
x => new Nextable(x)
next()
can return a Promise
according to the asynchronous generator protocol.
Any thrown exception will be caught and passed to a callback function effectively generating an error event.
// fails
x => { throw new Error('Bad!'); }
If it is an asynchronous function, it can return a regular value.
// delays by 0.5s:
async x => {
await new Promise(resolve => setTimeout(() => resolve(), 500));
return x;
}
If it is a generator function, each yield should produce a regular value.
// produces multiple values:
function* (x) {
for (let i = -1; i <= 1; ++i) {
if (i) yield x + i;
}
return x;
}
If it is an asynchronous generator function, each yield should produce a regular value.
// produces multiple values:
async function* (x) {
for (let i = -1; i <= 1; ++i) {
if (i) {
await new Promise(resolve => setTimeout(() => resolve(), 50));
yield x + i;
}
}
return x;
}
If a value is an array, its items are assumed to be functions, streams or other such arrays. The array is flattened, all individual items are included in a chain sequentially.
If a value is a valid stream, it is included as is in the pipeline.
'data'
event is not used in this case, the instance resumes itself automatically. Read about it in Node's documentation:
(Since 3.1.0) If a value is a web stream object (like ReadableStream
or WritableStream
), it is adapted to a corresponding Node stream and included in the pipeline.
options
is an optional object detailed in the Node's documentation.
The default options is this object:
{writableObjectMode: true, readableObjectMode: true}
If options
is specified it is copied over the default options.
Always make sure that writableObjectMode
is the same as the corresponding object mode of the first stream, and readableObjectMode
is the same as the corresponding object mode of the last stream.
Additionally the following custom properties are recognized:
skipEvents
is an optional boolean flag. If it is falsy (the default), 'error'
events from all streams are forwarded to the created instance. If it is truthy, no event forwarding is made. A user can always do so externally or in a constructor of derived classes.noGrouping
is an optional boolean flag. If it is falsy (the default), all subsequent functions are grouped together using the gen()
utility for improved performance. If it is specified and truthy, all functions will be wrapped as streams individually. This mode is compatible with how the 2.x version works.An instance can be used to attach handlers for stream events.
const pipeline = chain([x => x * x, x => [x - 1, x, x + 1]]);
pipeline.on('error', error => console.error(error));
dataSource.pipe(pipeline);
BSD-3-Clause
clearFunctionList()
.
const
-ness in the async generator branch (thx Patrick Pang).
take
, takeWhile
, skip
, skipWhile
, fold
, scan
, Reduce
, comp
.
'finish'
event instead of _final()
.