Observables for ECMAScript
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README.md

ECMAScript Observable

This proposal introduces an Observable type to the ECMAScript standard library. The Observable type can be used to model push-based data sources such as DOM events, timer intervals, and sockets. In addition, observables are:

  • Compositional: Observables can be composed with higher-order combinators.
  • Lazy: Observables do not start emitting data until an observer has subscribed.

Example: Observing Keyboard Events

Using the Observable constructor, we can create a function which returns an observable stream of events for an arbitrary DOM element and event type.

function listen(element, eventName) {
    return new Observable(observer => {
        // Create an event handler which sends data to the sink
        let handler = event => observer.next(event);

        // Attach the event handler
        element.addEventListener(eventName, handler, true);

        // Return a function which will cancel the event stream
        return () => {
            // Detach the event handler from the element
            element.removeEventListener(eventName, handler, true);
        };
    });
}

We can then use standard combinators to filter and map the events in the stream, just like we would with an array.

// Return an observable of special key down commands
function commandKeys(element) {
    let keyCommands = { "38": "up", "40": "down" };

    return listen(element, "keydown")
        .filter(event => event.keyCode in keyCommands)
        .map(event => keyCommands[event.keyCode])
}

Note: The "filter" and "map" methods are not included in this proposal. They may be added in a future version of this specification.

When we want to consume the event stream, we subscribe with an observer.

let subscription = commandKeys(inputElement).subscribe({
    next(val) { console.log("Received key command: " + val) },
    error(err) { console.log("Received an error: " + err) },
    complete() { console.log("Stream complete") },
});

The object returned by subscribe will allow us to cancel the subscription at any time. Upon cancelation, the Observable's cleanup function will be executed.

// After calling this function, no more events will be sent
subscription.unsubscribe();

Motivation

The Observable type represents one of the fundamental protocols for processing asynchronous streams of data. It is particularly effective at modeling streams of data which originate from the environment and are pushed into the application, such as user interface events. By offering Observable as a component of the ECMAScript standard library, we allow platforms and applications to share a common push-based stream protocol.

Implementations

Running Tests

To run the unit tests, install the es-observable-tests package into your project.

npm install es-observable-tests

Then call the exported runTests function with the constructor you want to test.

require("es-observable-tests").runTests(MyObservable);

API

Observable

An Observable represents a sequence of values which may be observed.

interface Observable {

    constructor(subscriber : SubscriberFunction);

    // Subscribes to the sequence with an observer
    subscribe(observer : Observer) : Subscription;

    // Subscribes to the sequence with callbacks
    subscribe(onNext : Function,
              onError? : Function,
              onComplete? : Function) : Subscription;

    // Returns itself
    [Symbol.observable]() : Observable;

    // Converts items to an Observable
    static of(...items) : Observable;

    // Converts an observable or iterable to an Observable
    static from(observable) : Observable;

}

interface Subscription {

    // Cancels the subscription
    unsubscribe() : void;

    // A boolean value indicating whether the subscription is closed
    get closed() : Boolean;
}

function SubscriberFunction(observer: SubscriptionObserver) : (void => void)|Subscription;

Observable.of

Observable.of creates an Observable of the values provided as arguments. The values are delivered synchronously when subscribe is called.

Observable.of("red", "green", "blue").subscribe({
    next(color) {
        console.log(color);
    }
});

/*
 > "red"
 > "green"
 > "blue"
*/

Observable.from

Observable.from converts its argument to an Observable.

  • If the argument has a Symbol.observable method, then it returns the result of invoking that method. If the resulting object is not an instance of Observable, then it is wrapped in an Observable which will delegate subscription.
  • Otherwise, the argument is assumed to be an iterable and the iteration values are delivered synchronously when subscribe is called.

Converting from an object which supports Symbol.observable to an Observable:

Observable.from({
    [Symbol.observable]() {
        return new Observable(observer => {
            setTimeout(() => {
                observer.next("hello");
                observer.next("world");
                observer.complete();
            }, 2000);
        });
    }
}).subscribe({
    next(value) {
        console.log(value);
    }
});

/*
 > "hello"
 > "world"
*/

let observable = new Observable(observer => {});
Observable.from(observable) === observable; // true

Converting from an iterable to an Observable:

Observable.from(["mercury", "venus", "earth"]).subscribe({
    next(value) {
        console.log(value);
    }
});

/*
 > "mercury"
 > "venus"
 > "earth"
*/

Observer

An Observer is used to receive data from an Observable, and is supplied as an argument to subscribe.

All methods are optional.

interface Observer {

    // Receives the subscription object when `subscribe` is called
    start(subscription : Subscription);

    // Receives the next value in the sequence
    next(value);

    // Receives the sequence error
    error(errorValue);

    // Receives a completion notification
    complete();
}

SubscriptionObserver

A SubscriptionObserver is a normalized Observer which wraps the observer object supplied to subscribe.

interface SubscriptionObserver {

    // Sends the next value in the sequence
    next(value);

    // Sends the sequence error
    error(errorValue);

    // Sends the completion notification
    complete();

    // A boolean value indicating whether the subscription is closed
    get closed() : Boolean;
}