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Async Iterator Extensions

A library of async iterator extensions for JavaScript including map, reduce, filter, flatMap, pipe and more.


npm install axax # or yarn add axax

Why Axax?

Async iterators are a useful way to handle asynchronous streams. This library adds a number of utility methods similar to those found in lodash, underscore, Ramda or RxJs.

es5 vs esnext

Axax contains both transpiled es5 code as well as esnext code, the difference being that esnext uses the native for await syntax. In nodejs 10.x that gives approximately a 40% speedup.

// use es5 if you want to support more browsers
import { map } from "axax/es5/map"; 

// use esnext if you're only using node 10.x or supporting very new browsers
import { map } from "axax/esnext/map"; 

Reference Documentation



fromEvent turns DOM events into an iterable.

import { fromEvent } from "axax/es5/fromEvent";

const clicks = fromEvent(document, 'click');

for await (const click of clicks) {
    console.log('a button was clicked');

pipe, map, filter, fromLineReader

fromLineReader turns a NodeJS LineReader into an async iterable. The example below prints the lines from a file in upper case after filtering out the empty ones.

// create the line reading async iterable
const lines = fromLineReader(
        input: require("fs").createReadStream("./data/example.txt")

// create a filter that removes empty lines
const notEmpty = filter(line => line.length > 0);

// convert to uppercase
const toUpperCase = map(line => line.toUpperCase());

// go through each of the non empty lines
for await (const line of pipe(notEmpty, toUpperCase)(lines)) {


Subject makes it easy to turn stream of events into an iterable. The code below is essentially how fromEvent was implemented.

import { Subject } from "axax/es5/subject";

const subject = new Subject();

// set up a callback that calls value on the subject
const callback = value => subject.onNext(value);

// attach the callback to the click event
document.addEventListener('click', callback);

// remove the callback when / if the iterable stops
subject.finally(() => document.removeEventListener('click', callback));

// go through all the click events
for await (const click of subject.iterator) {
    console.log('a button was clicked');

Avoiding leaks

It's possible to have an async iterator leak if it never returns a value e.g.:

const subject1 = new Subject();
const subject2 = new Subject();

async function* neverEnds() {
  try {
    for await(const i of subject2.iterator) {
      yield i;
  } finally {
    console.log("never called")

async function* run() {
  for await(const i of merge(subject1.iterator,neverEnds())) {


If you need to be able to cancel async iterators that may never return values, consider Rx or regular Observables for now.

(Thanks to @awto for the example)


Async Iterator Extensions for JavaScript e.g. map, reduce, filter, flatMap, etc.




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