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This is RxJS v 4. Find the latest version here

RxJS for Bacon.js Users

Bacon.js is a popular Reactive Programming library which was inspired by RxJS, ReactiveBanana among other libraries.

But, before we get started, why use RxJS over Bacon.js?

Performance

RxJS has always been committed to providing the best performance available on any given JavaScript platform. Whether it is providing a small footprint for an Observable instance, to providing the capability of using setImmediate versus setTimeout versus requestAnimationFrame, all by choosing a different scheduler. In addition, since RxJS is written in regular JavaScript versus CoffeeScript, there are a lot more optimizations that can be made by hand to ensure performance.

To make this more concrete, there is a project called Kefir which is trying to make a more performance oriented version of Bacon.js. Here are some current numbers based upon the memory tests provided by Kefir. Note that these may change over time, but gives you a good indication of memory consumption. Below will be comparison of combineLatest, filter, map, and scan.

stream1.combine(stream2, ->) (1000 samples)
----------------------------------------------------------------
Kefir   w/o subscr. 0.44 KiB   w/ subscr. +0.80 KiB   sum 1.23 KiB
Bacon   w/o subscr. 5.42 KiB   w/ subscr. +6.15 KiB   sum 11.57 KiB
Rx      w/o subscr. 0.43 KiB   w/ subscr. +2.84 KiB   sum 3.26 KiB
-----------------------
Kefir 1.00 1.00 1.00    Bacon 12.45 7.71 9.39    Rx 0.98 3.56 2.65

.filter(->) (1000 samples)
----------------------------------------------------------------
Kefir   w/o subscr. 0.31 KiB   w/ subscr. +0.46 KiB   sum 0.77 KiB
Bacon   w/o subscr. 1.82 KiB   w/ subscr. +2.49 KiB   sum 4.31 KiB
Rx      w/o subscr. 0.37 KiB   w/ subscr. +1.44 KiB   sum 1.81 KiB
-----------------------
Kefir 1.00 1.00 1.00    Bacon 5.91 5.39 5.60    Rx 1.21 3.11 2.35

.map(->) (1000 samples)
----------------------------------------------------------------
Kefir   w/o subscr. 0.30 KiB   w/ subscr. +0.47 KiB   sum 0.77 KiB
Bacon   w/o subscr. 1.81 KiB   w/ subscr. +2.49 KiB   sum 4.30 KiB
Rx      w/o subscr. 0.37 KiB   w/ subscr. +1.43 KiB   sum 1.81 KiB
-----------------------
Kefir 1.00 1.00 1.00    Bacon 6.07 5.31 5.60    Rx 1.24 3.06 2.35

.scan(0, ->) (1000 samples)
----------------------------------------------------------------
Kefir   w/o subscr. 0.30 KiB   w/ subscr. +0.47 KiB   sum 0.76 KiB
Bacon   w/o subscr. 1.68 KiB   w/ subscr. +2.06 KiB   sum 3.75 KiB
Rx      w/o subscr. 0.39 KiB   w/ subscr. +1.13 KiB   sum 1.52 KiB
-----------------------
Kefir 1.00 1.00 1.00    Bacon 5.62 4.44 4.90    Rx 1.29 2.43 1.99

As you'll note, while Kefir does pretty well here, Bacon.js does not. Bacon.js has a fairly heavy Observable object to start with and then subscriptions are fairly heavy as well. With both Kefir and RxJS, the initial size of the Observable is about the same. We'll get to the other fine tuning such as schedulers in the later section.

Interoperability With The Libraries You Use

One of the most important parts of when choosing a library is how well it works with the libraries you already use. Bacon.js ties itself directly to jQuery and Zepto.js for event binding with asEventStream. Not only that, but adding support for your own custom library's binding is difficult, whereas with RxJS it couldn't be easier.

RxJS, on the other hand is more flexible about binding to the libraries you use. For example, if you use jQuery or Zepto.js, Rx.Observable.fromEvent will work perfectly for you. In addition, we also support the other libraries out of the box:

You can also override this behavior in fromEvent so that you use native DOM or Node.js events via the EventEmitter directly by setting the Rx.config.useNativeEvents flag to true, so that it's never in doubt which event system you are using. When using native DOM events, you can attach a listener to one item, or you can attach listeners to a NodeList's children, we figure that out for you without you having to change your code, whereas Bacon.js does not.

In order to support the libraries you use, it's very simple to add your own event binding, for example we can bind our events to the Dojo Toolkit using the Rx.Observable.fromEventPattern method:

require(['dojo/on', 'dojo/dom', 'rx'], function (on, dom, rx) {

  var input = dom.byId('input');

  var source = Rx.Observable.fromEventPattern(
    function addHandler (h) {
      return on(input, 'click', h);
    },
    function delHandler (_, signal) {
      signal.remove();
    }
  );

  var subscription = source.subscribeOnNext(function (x) {
    console.log('Next: Clicked!');
  });

  on.emit(input, 'click');
  // => Next: Clicked!
});

In addition, transducers hold a great amount of potential for high performance querying, and to that end, RxJS has added transducers support via the transduce method.

var t = transducers;

var source = Rx.Observable.range(1, 4);

function increment(x) { return x + 1; }
function isEven(x) { return x % 2 === 0; }

var transduced = source.transduce(t.comp(t.map(increment), t.filter(isEven)));

transduced.subscribe(
  function (x) { console.log('Next: %s', x); },
  function (e) { console.log('Error: %s', e); },
  function ()  { console.log('Completed'); });

// => Next: 2
// => Next: 4
// => Completed

Browser Compatibility

Browser and runtime compatibility is important to RxJS. For example, it can run not only in the browser, but also Node.js, RingoJS, Narwhal, Nashorn, and event Windows Scripting Host (WSH). We realize that there are many users out there without access to the latest browser, so we do the best to accommodate this with our compat builds. These builds bridge all the way back to IE6 for support which includes using attachEvent with event normalization, to DOM Level 1 events. Bacon.js does not have this behavior, and instead falls back to hoping that you use jQuery.

We also want to build RxJS for speed with asynchronous operations, so we optimize for the browser and runtime you are using. For example, if your environment supports setImmediate for immediate execution, we will use that. Else, if you're in Node.js and setImmediate is not available, it falls back to process.nextTick. For browsers and runtimes that do not support setImmediate, we will fall back to postMessage, to MessageChannel, to asynchronous script loading, and finally defaulting back to setTimeout or event WScript.Sleep for Windows Scripting Host.

These compat files are important as we will shim behavior that we require such as Array#extras, Function#bind and so forth, so there is no need to bring in your own compatibility library such as html5shiv, although if they do exist already, we will use the native or shimmed methods directly.

Standards Based

Iterables and Array#extras

RxJS has a firm commitment to standards in JavaScript. Whether it is supporting the Array#extras standard method signatures such as map, filter, reduce, every and some, or to some new emerging standard on collections, RxJS will implement these features accordingly from pull collections to push. Unlike Bacon.js, RxJS conforms to Array#extras syntax to have the callback style of function (item, index, collection) in addition to accepting a thisArg for the calling context of the callback. This helps as you can easily reuse your code from the Array version to Observable version.

An example of forward thinking is the introduction of ES6+ operators on Array with implementations on Observable such as:

  • includes
  • find
  • findIndex
  • from
  • of

In addition, RxJS supports ES6 iterables in many methods which allow you to accept Map, Set, Array and array-like objects, for example in Rx.Observable.from, flatMap, concatMap among others. RxJS also has the capability of converting to Maps and Sets via the toMap and toSet methods if available in your runtime.

This makes the following code possible to yield an array from an observable sequence and have it automatically converted into an observable sequence.

Rx.Observable.range(1, 3)
  .flatMap(function (x, i) { return [x, i]; })
  .subscribeOnNext(function (value) {
    console.log('Value: %o', value);
  });
// => 1
// => 0
// => 2
// => 1
// => 3
// => 2

Generators

Generators also play a big part in the next version of JavaScript. RxJS also takes full advantage of generators as iterables in such methods as the aforementioned Rx.Observable.from, flatMap, concatMap and other methods. For example, you can yield values like the following:

var source = Rx.Observable.from(
  function* () { yield 1; yield 2; yield 3; }()
);

source.subscribeOnNext(function (value) {
  console.log('Next: %s', value);
});

// => 1
// => 2
// => 3

Generators also give the developer pretty powerful capabilities when dealing with asynchronous actions. RxJS introduced the Rx.Observable.spawn method which allows you to write async/await style code over Observables, Promises, Arrays, and just plain objects.

var spawned = Rx.Observable.spawn(function* () {
  var x = yield Rx.Observable.just(42);
  var y = yield Promise.resolve(42);
  console.log(x + y);

  try {
    var source = yield Rx.Observable.throw(new Error('woops'));
    console.log('Source is: %s', source);
  } catch (e) {
    console.log('Catched: %s', e.message);
  }
});

spawned.subscribe();
// => 84
// => Source is: Error: woops

Promises

Promises have been a great way for developers to express single value asynchronous values. With ES6, they have now been standardized and are starting to appear in all browsers. RxJS also supports Promise values as arguments in many places as noted in our Bridging to Promises documentation. No more having to call Rx.Observable.fromPromise everywhere, RxJS automatically converts them for you.

var source = Rx.Observable.fromEvent(input, 'keyup')
  .map(function (e) { return e.target.value; })
  .flatMapLatest(function (text) {
    return queryPromise(text);
  });

RxJS also allows an Observable sequence to be converted to an ES6 compliant Promise with the toPromise method.

Rx.Observable.just(42)
  .toPromise()
  .then(function (value) {
    console.log(value);
  });
// => 42

Swappable Concurrency Layer

RxJS can easily be described in three pieces. First the Observer and Observable objects, secondly by the operators for composition on top of them, and finally a swappable concurrency layer which allows you to swap out your concurrency model at any time. This last part is key which distinguishes RxJS from many other libraries. There are a number of advantages to this approach that may be subtle at first, but invaluable as you start to use them.

The first advantage is that you can switch where callbacks are executed, for example, instead of using setImmediate or any of its fallbacks, you can execute the callback on window.requestAnimationFrame for smooth animations.

// Available in RxJS-DOM project
var scheduler = Rx.Scheduler.requestAnimationFrame;

function render(value) {
  // Do something to render the value
}

Rx.Observable.range(1, 100, scheduler)
  .subscribe(render);

Schedulers also have advantages with virtual time, which means we can say what time it really is. This is great for deterministic testing in which you can verify the behavior of every single operator. RxJS, through the TestScheduler can record when items happen and what values were yielded, thus no need for asynchronous testing.

var scheduler = new TestScheduler();

var xs = scheduler.createHotObservable(
  Rx.ReactiveTest.onNext(201, 1),
  Rx.ReactiveTest.onCompleted(202)
);

var results = scheduler.startWithCreate(function () {
  return xs.map(function (x, i) { return x + x + i });
});

// Some custom collection assertion for values
collectionAssert.assertEqual(results.messages, [
  Rx.ReactiveTest.onNext(201, 2),
  Rx.ReactiveTest.onCompleted(202)
]);

// Some custom collection assertion for subscriptions
collectionAssert.assertEqual(xs.subscriptions, [
  Rx.ReactiveTest.subscribe(200, 202)
])

Every operator within RxJS has tests written in this style where all data is easily verified. Not only can we create observables through the TestScheduler, but we can create Promises via the createResolvedPromise and createRejectedPromise methods.

Virtual time has more advantages as well. Imagine if you had some historical stock data that you wanted to run through a simulation. Using the Historical scheduler, you can easily accomplish this.

var scheduler = new Rx.HistoricalScheduler(new Date(2014, 1));

var source = new Rx.Subject();

getStockData().forEach(function (stock) {
  scheduler.scheduleWithAbsolute(stock.date, function () {
    stock.onNext(stock);
  });
});

// Calculate with the data
source.groupBy(function (stock) { return stock.symbol; })
  /* Do something with the data */
  .subscribe(function (info) {
    // Process the data
  });

Backpressure

In our applications, we consume a lot of data from external sources. But, what if our consumers (Observers) cannot handle the load from the observable sequence? RxJS has a number of mechanisms to handle this in the Backpressure and Observable Sequences documentation. This could come in the form of lossy operations such as debounce, throttleFirst, sample, pausable, to loss-less operations such as pausableBuffered, buffer, etc.

Build What You Want

RxJS has a rather large surface area, so we give you the ability to build RxJS with only the things you want and none of the things you don't via the rx-cli project. For example, we can build a compat version of RxJS with only the map, flatMap, takeUntil, and fromEvent methods, which keeps your RxJS version lean and mean.

rx --lite --compat --methods map,flatmap,takeuntil,fromevent

Long Stack Trace Support

Debugging programming with callbacks can be quite cumbersome. To that end, RxJS has introduced a notion of "Long Stack Traces" which allows you to quickly isolate your code from the plumbing not only of RxJS, but also Node.js and the browser cruft, thus getting you to the real cause of the issue.

For example, without "long stack trace" support, typically, an error would look like the following:

var Rx = require('rx');

var source = Rx.Observable.range(0, 100)
  .timestamp()
  .map(function (x) {
    if (x.value > 98) throw new Error();
    return x;
  });

source.subscribeOnError(
  function (err) {
    console.log(err.stack);
  });
$ node example.js

  Error
  at C:\GitHub\example.js:6:29
  at AnonymousObserver._onNext (C:\GitHub\rxjs\dist\rx.all.js:4013:31)
  at AnonymousObserver.Rx.AnonymousObserver.AnonymousObserver.next (C:\GitHub\rxjs\dist\rx.all.js:1863:12)
  at AnonymousObserver.Rx.internals.AbstractObserver.AbstractObserver.onNext (C:\GitHub\rxjs\dist\rx.all.js:1795:35)
  at AutoDetachObserverPrototype.next (C:\GitHub\rxjs\dist\rx.all.js:9226:23)
  at AutoDetachObserver.Rx.internals.AbstractObserver.AbstractObserver.onNext (C:\GitHub\rxjs\dist\rx.all.js:1795:35)
  at AnonymousObserver._onNext (C:\GitHub\rxjs\dist\rx.all.js:4018:18)
  at AnonymousObserver.Rx.AnonymousObserver.AnonymousObserver.next (C:\GitHub\rxjs\dist\rx.all.js:1863:12)
  at AnonymousObserver.Rx.internals.AbstractObserver.AbstractObserver.onNext (C:\GitHub\rxjs\dist\rx.all.js:1795:35)
  at AutoDetachObserverPrototype.next (C:\GitHub\rxjs\dist\rx.all.js:9226:23)

This can be remedied using "long stack trace" support by setting the following flag:

Rx.config.longStackSupport = true;

Then we can run our program again using this support

$ node example.js

  Error
  at C:\GitHub\example.js:6:29
  From previous event:
  at Object.<anonymous> (C:\GitHub\example.js:3:28)
  From previous event:
  at Object.<anonymous> (C:\GitHub\example.js:4:4)
  From previous event:
  at Object.<anonymous> (C:\GitHub\example.js:5:4)

Many Examples and Tutorials

As people try to learn RxJS, it's always great to have examples to get them started. To that end, RxJS ships a number of examples out of the box including simple scenarios such as Autocomplete, Follow the Mouse, Drawing, to more complete examples like databinding using a little demo project called TKO, to a complete game of Alphabet Invasion.

Want to learn RxJS at your own pace? We also have tutorials for that as well called LearnRx which will walk you through the basics of learning to compose arrays, and then how that applies to observable sequences.

There are also many community resources to learn RxJS from videos, to presentations, to examples with integration with such libraries as AngularJS and React.

Extensive Documentation

As stated before, RxJS has a rather large surface area so sometimes it's hard to know where to start. To that end, RxJS has complete API documentation, as well as a Getting Started Guide as well as Guidelines, a How Do I? section as well as a growing list of comparisons with other libraries.

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