parallel

execute an async transform and yield in order of completion

Usage

package requires a system that supports async-iteration, either natively or via down-compiling

Install

npm install @async-generators/parallel --save
yarn add @async-generators/parallel

This package's main entry points to a commonjs distribution.

Additionally, the module entry points to a es2015 distribution, which can be used by build systems, such as webpack, to directly use es2015 modules.

Api

parallel(source, selector [,limit=20] [,limited])

parallel() iterates source and executes selector(item, index) concurrently. Upon resolution of selector promise, the result is yielded.

selector(item) is awaited in a non-blocking fashion and parallel results are yielded in the order of completion.

If the the number of concurrent tasks is greater-than or equal-to limit further iteration of source is halted until other tasks complete. Upon halting, the limited(item, index) method is called (sync). If limited returns true the item is ignored not not passed to selector

source must have a [Symbol.asyncIterator] or [Symbol.iterator] property. If both are present only [Symbol.asyncIterator] is used.

parallel(source [,limit=20] [,limited])

parallel() iterates source and awaits each item concurrently. Upon resolve, the result is yielded. source items are awaited in a non-blocking fashion and the results are yielded in the order of completion.

source must have a [Symbol.asyncIterator] or [Symbol.iterator] property. If both are present only [Symbol.asyncIterator] is used. source should ideally yield executing promises, but technically can yield an object - although this makes the call to parallel entirely redundant.

Example

example.js

const parallel = require('@async-generators/parallel').default;

async function* source() {
  yield 1; yield 2; yield 3; yield 4;
}

function delay(item) {
  let duration = {
    1: 500,
    2: 200,
    3: 1000,
    4: 700
  }
  return new Promise(r => setTimeout(r, duration[item ]));
}

async function main() {
  for await (let item of
    parallel(source(), async (value) => {
      await delay(value);
      return value;
    })
  ) {
    console.log(item);
  }
}

main();

Execute with the latest node.js:

node --harmony-async-iteration example.js

output:

2
1
4
3

Typescript

This library is fully typed and can be imported using:

import parallel from '@async-generators/parallel');

It is also possible to directly execute your properly configured typescript with ts-node:

ts-node --harmony_async_iteration example.ts