Observables in JavaScript were largely popularized by libraries such as RxJS and Bacon.js . Jafar Husain, a Netflix tech lead and long-time functional programming advocate who’s also on TC39 has been developing a proposal to bring observables into the core language. The Observable proposal is at stage 1 but marked as ready to move up to stage 2, at the time of this writing.

Observable and the observer API

In this proposal, Observable would be a new built-in that can be used to handle event streams. The Observable constructor takes a callback which defines an event stream. In the following example, our observable returns a stream of events with just 1 and 2 values. The observer.next method can be used to add events to the observable stream.

new Observable(observer => { observer.next( 1 ) observer.next( 2 ) })

We can use observer.error to report errors that occur during stream processing.

new Observable(observer => { observer.error( new Error (`Failed to stream events`)) })

We can use observer.complete to signal when the event stream has come to an end.

new Observable(observer => { observer.next( 1 ) observer.next( 2 ) observer.complete() })

The callback passed to an Observable constructor can return a cleanup function to tear down our observable. This can be useful to remove event listeners, clear timeouts, and similar cleanup tasks. The following observable is a bit more interesting than the last one. It tracks mouse position relative to the page as the user moves the cursor on screen, producing an event stream that describes the cursor position on the page through time.

function mouseTracking () { return new Observable(observer => { const handler = ({ pageX, pageY }) => { observer.next({ x: pageX, y: pageY }) } document .body.addEventListener(`mousemove`, handler) return () => { document .body.removeEventListener(`mousemove`, handler) } }) }

In order for us to subscribe to an observable event stream, we just call the Observable#subscribe method on an observable object instance. Doing so will invoke the callback passed to the Observable constructor in the previous code snippet, binding the event listener and getting the event stream started. Moving the mouse will now result in events being fired into the event stream.

mouseTracking().subscribe({ next({ x, y }) { console .log(`New position: ${ x }, ${ y }`) }, error(err) { console .log(` Error : ${ err }`) }, complete() { console .log(`Done!`) } })

Observable#subscribe returns an object that lets us unsubscribe , executing the cleanup method – if one exists. When we’re no longer interested in events from the observable stream, we’ll unsubscribe and let the observable clean itself up.

const subscription = mouseTracking().subscribe({ next({ x, y }) { console .log(`New position: ${ x }, ${ y }`) }, error(err) { console .log(` Error : ${ err }`) }, complete() { console .log(`Done!`) } }) subscription.unsubscribe()

Observable.of

Observable.of(...items) is a simple static utility helper that creates an Observable out of the provided items . The items are then delivered synchronously when Observable#subscribe is called.

Observable.of( 1 , 2 , 3 , 4 ).subscribe({ next(item) { console .log(item) } })

We can think of Observable.of as the following simplified implementation, where we return a synchronous stream of provided values.

Observable.of = (...items) => { return new Observable(observer => { items.forEach(item => { observer.next(item) }) observer.complete() }) }

Observable.from

This static method casts the provided argument into an Observable . If the provided Object has a Symbol.observable method, then the result of invoking that method is returned.

Observable .from({ [Symbol.observable]() { return Observable.of( 1 , 2 , 3 ) } }) .subscribe({ next(item) { console .log(item) } })

If the provided argument doesn’t implement a Symbol.observable method, then it’s assumed to be an iterable. A synchronous Observable sequence of the iterable is returned.

Observable .from([ 1 , 2 , 3 ]) .subscribe({ next(item) { console .log(item) } })

In this case, Observable.from is similar to Observable.of . We could think of Observable.from as the following simplified implementation.

Observable.from = value => { if ( typeof value[Symbol.observable] === ` function `) { return value[Symbol.observable]() } return Observable.of( Array .from(value)) }

Conclusions

Note that this proposal is still in its infancy, but it’d lay out the foundation for functional programming at the native JavaScript level. Eventually, it may earn the ability to Observable#filter or Observable#map the stream of events, allowing us to focus only on the kinds of events we want to listen for.