Channel4

Dead simple communicating sequential processes for Javascript (like Clojurescript core.async, or Go channels).

npm install channel4

Usage

let channel = Channel();
Channel.put(channel, 'World!');
Channel.take(channel, (value) => {
  console.log('Hello ');
});
 
// prints `Hello World!`

Motivation

Consider the following code:

const listenerFn = (event) => {
  event.preventDefault();
  doSomething(event);
};
 
document.querySelector('button').addEventListener('click', listenerFn);

There's a one-to-one relationship between the producer addEventListener and the consumer listenerFn. Also the producer knows 1) there's a single listener attached, 2) how the data is feeded to the listener and 3) when the data is going to be processed. Adding a second listener introduces some challenges:

document.querySelector('button').addEventListener('click', (e) => {
  listenerFn() && otherListenerFn();
});
 
// or
 
document.querySelector('button').addEventListener('click', listenerFn);
document.querySelector('button').addEventListener('click', otherListenerFn);

It's immediately obvious that this couples the producer and consumer: when you introduce another listener, the producer has to cater for it.

This is poor separation of concerns.

But what if you could decouple consumers from producers? What if the producer could send the message without the need to worry about who's consuming it? What if the consumer could consume messages at its own peace?

You won't need to fiddle with such poor code, that's for sure. Decoupling would also lead to better and easier testing.

As you might have guessed by now channels can help you decouple producers and consumers. The decoupling is obtained through a simple queue.

The producer places items of work on the queue for later processing. The consumer is free to remove the work item from the queue at any time.

Producer and consumer only have to know about the channel to communicate. Also, multiple producers can put values for multiple consumers to take.

let channel = Channel();
document.querySelector('button')
  .addEventListener('click', Channel.put.bind(null, channel));
 
channel.take(channel, (value) => console.log('Hello World'));

In the example above, addEventListener isn't aware that there's a consumer listening to click events.

let channel = Channel();
document.querySelector('a')
  .addEventListener('click', Channel.put.bind(null, channel));
document.querySelector('button')
  .addEventListener('click', Channel.put.bind(null, channel));
 
channel.take(channel, (value) => console.log('Hello'));

In this other example, the consumer isn't aware that multiple producers are placing items of work on the queue.

API

Channel.put :: (channel, a) -> channel

Put a value into a channel and return the channel. The result in a noop when called on closed channel.

let channel = Channel();
Channel.put(channel, 46);

Channel.take :: (channel, (a) -> b) -> channel

Take values from the channel and fire the callback. The result in a noop when called on closed channel.

let channel = Channel();
Channel.take(channel, console.log.bind(console, 'received: '));
Channel.put(channel, 47);
 
// prints `received: 47`

Channel.close :: (channel) -> channel

Close the current channel. This is a shorthand for Channel.put(channel, Channel.END).

let channel = Channel();
Channel.take(channel, (value) => {
  if (value === Channel.END) console.log('Closed!');
});
Channel.close(channel);
 
// prints `Closed!`
 
...
 
Channel.put(channel, 46); // has no effect
Channel.take(channel, () => ()); // callback will never fire

Channel.pipe :: (input, output, (a) -> b, keepOpen) -> output

Pipe all the incoming values from the input to the output channel. If the input channel is closed, the output channel is kept open unless it is specified otherwise. You can apply a transformation function while piping the values.

let input = Channel();
let output = Channel();
Channel.pipe(input, output, (x) => x/2);
Channel.take(output, console.log.bind(console, 'received: '))
Channel.put(input, 44);
 
// prints: `received: 22`

Channel.demux :: ([channel], output, (a) -> b, keepOpen) -> output

Merge all values from an array of channels into the output. If one of the input channels is closed, the output channel is kept open unless it is specified otherwise. You can apply a transformation function while merging the values.

let one = Channel();
let two = Channel();
let output = Channel();
Channel.demux([one, two], output);
Channel.take(output, console.log.bind(console, 'received: '));
Channel.take(output, console.log.bind(console, 'received: '));
Channel.put(one, 1);
Channel.put(two, 2);
 
// prints `received: 1` and `received 2`

Channel.mux :: (input, [channel], (a) -> b, keepOpen) -> [channel]

Broadcast all the values from input channel to an array of channels. If the input channel is closed, the output channels are kept open unless it is specified otherwise. You can apply a transformation function to the value before it is broadcasted.

let one = Channel();
let two = Channel();
let input = Channel();
Channel.mux(input, [one, two]);
Channel.take(one, console.log.bind(console, 'received: '));
Channel.take(two, console.log.bind(console, 'received: '));
Channel.put(input, 1);
 
// prints `received: 1` and `received 1`

Channel.END

Emitted from the channel on close.

let channel = Channel();
Channel.take(channel, (value) {
  if (value === END) console.log('Closed!');
})
Channel.put(channel, Channel.END);
 
// prints `Closed!`

Channel.KEEP_OPEN

Keep the output channel open during Channel.pipe, Channel.demux or Channel.mux as a response to an incoming Channel.END value.

let input = Channel();
let output = Channel();
Channel.pipe(input, output, ((x) => x), Channel.KEEP_OPEN);
Channel.take(output, console.log.bind(console, 'received: '))
Channel.put(input, Channel.END);
Channel.put(output, 44);
 
// prints: `received: 44`

Channel.CLOSE_BOTH

Close the output channel when the input receives a Channel.end value.

let input = Channel();
let output = Channel();
Channel.pipe(input, output, ((x) => x), Channel.CLOSE_BOTH);
Channel.take(output, (value) => {
  if (value === CLOSED) console.log('Closed!');
});
Channel.put(input, Channel.END);
 
// prints: `Closed!`

Resources

This project is heavily inspired by:

• CSP and transducers in JavaScript
• Taming the Asynchronous Beast with CSP Channels in JavaScript
• Go-flavored JavaScript
• Raynos/value-event
• The Producer Consumer Pattern