baconjs

A small functional reactive programming lib for JavaScript.

npm install baconjs
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Bacon.js

A small functional reactive programming lib for JavaScript.

Turns your event spaghetti into clean and declarative feng shui bacon, by switching from imperative to functional. It's like replacing nested for-loops with functional programming concepts like map and filter. Stop working on individual events and work with event streams instead. Combine your data with merge and combine. Then switch to the heavier weapons and wield flatMap and combineTemplate like a boss.

It's the _ of Events. Too bad the symbol ~ is not allowed in JavaScript.

Here's the stuff.

You can also check out my entertaining (LOL), interactive, solid-ass slideshow.

And remember to give me feedback on the bacon! Let me know if you've used it. Tell me how it worked for you. What's missing? What's wrong? Please contribute!

Build Status

Table of contents

Install

You can download the latest generated javascript.

Version 0.7.2 can also be found from cdnjs hosting:

http://cdnjs.cloudflare.com/ajax/libs/bacon.js/0.7.2/bacon.js
http://cdnjs.cloudflare.com/ajax/libs/bacon.js/0.7.2/bacon.min.js

Visual Studio users can obtain version 0.7.2 via NuGet Packages https://www.nuget.org/packages/Bacon.js/0.7.2

If you're targeting to node.js, you can

npm install baconjs

For bower users:

bower install bacon

Intro

The idea of Functional Reactive Programming is quite well described by Conal Elliot at Stack Overflow.

Bacon.js is a library for functional reactive programming. Or let's say it's a library for working with events and dynamic values (which are called Properties in Bacon.js).

Anyways, you can wrap an event source, say "mouse clicks on an element" into an EventStream by saying

var cliks = $("h1").asEventStream("click")

Each EventStream represents a stream of events. It is an Observable object, meaning that you can listen to events in the stream using, for instance, the onValue method with a callback. Like this:

cliks.onValue(function() { alert("you clicked the h1 element") })

But you can do neater stuff too. The Bacon of bacon.js is in that you can transform, filter and combine these streams in a multitude of ways (see API below). The methods map, filter, for example, are similar to same functions in functional list programming (like Underscore). So, if you say

var plus = $("#plus").asEventStream("click").map(1)
var minus = $("#minus").asEventStream("click").map(-1)
var both = plus.merge(minus)

.. you'll have a stream that will output the number 1 when the "plus" button is clicked and another stream outputting -1 when the "minus" button is clicked. The both stream will be a merged stream containing events from both the plus and minus streams. This allows you to subscribe to both streams with one handler:

both.onValue(function(val) { /* val will be 1 or -1 */ })

In addition to EventStreams, bacon.js has a thing called Property, that is almost like an EventStream, but has a "current value". So things that change and have a current state are Properties, while things that consist of discrete events are EventStreams. You could think mouse clicks as an EventStream and mouse position as a Property. You can create Properties from an EventStream with scan or toProperty methods. So, let's say

function add(x, y) { return x + y }
var counter = both.scan(0, add)
counter.onValue(function(sum) { $("#sum").text(sum) })

The counter property will contain the sum of the values in the both stream, so it's practically a counter that can be increased and decreased using the plus and minus buttons. The scan method was used here to calculate the "current sum" of events in the both stream, by giving a "seed value" 0 and an "accumulator function" add. The scan method creates a property that starts with the given seed value and on each event in the source stream applies the accumulator function to the current property value and the new value from the stream.

Properties can be very conventiently used for assigning values and attributes to DOM elements with JQuery. Here we assign the value of a property as the text of a span element whenever it changes:

property.assign($("span"), "text")

Hiding and showing the same span depending on the content of the property value is equally straightforward

function hiddenForEmptyValue(value) { return value == "" ? "hidden" : "visible" }
property.map(hiddenForEmptyValue).assign($("span"), "css", "visibility")

In the example above a property value of "hello" would be mapped to "visible", which in turn would result in Bacon calling

$("span").css("visibility", "visible")

For an actual tutorial, please check out my blog posts

API

Creating streams

$.asEventStream(eventName) creates an EventStream from events on a jQuery or Zepto.js object. You can pass optional arguments to add a jQuery live selector and/or a function that processes the jQuery event and its parameters, if given, like this:

$("#my-div").asEventStream("click", ".more-specific-selector")
$("#my-div").asEventStream("click", ".more-specific-selector", function(event, args) { return args[0] })
$("#my-div").asEventStream("click", function(event, args) { return args[0] })

Bacon.fromPromise(promise [, abort]) creates an EventStream from a Promise object such as JQuery Ajax. This stream will contain a single value or an error, followed immediately by stream end. You can use the optional abort flag (i.e. ┬┤fromPromise(p, true)┬┤ to have the abort method of the given promise be called when all subscribers have been removed from the created stream. Check out this example.

Bacon.fromEventTarget(target, eventName [, eventTransformer]) creates an EventStream from events on a DOM EventTarget or Node.JS EventEmitter object. You can also pass an optional function that transforms the emitted events' parameters.

Bacon.fromEventTarget(document.body, "click").onValue(function() { alert("Bacon!") })

Bacon.fromCallback(f [, args...]) creates an EventStream from a function that accepts a callback. The function is supposed to call its callback just once. For example:

Bacon.fromCallback(function(callback) {
  setTimeout(function() {
    callback("Bacon!")
  }, 1000)
})

This would create a stream that outputs a single value "Bacon!" and ends after that. The use of setTimeout causes the value to be delayed by 1 second.

You can also give any number of arguments to fromCallback, which will be passed to the function. These arguments can be simple variables, Bacon EventStreams or Properties. For example the following will output "Bacon rules":

bacon = Bacon.constant('bacon')
Bacon.fromCallback(function(a, b, callback) {
  callback(a + ' ' + b);
}, bacon, 'rules').log();

Bacon.fromCallback(object, methodName [, args...]) a variant of fromCallback which calls the named method of a given object.

Bacon.fromNodeCallback(f [, args...]) behaves the same way as Bacon.fromCallback, except that it expects the callback to be called in the Node.js convention: callback(error, data), where error is null if everything is fine. For example:

var Bacon = require('baconjs').Bacon,
    fs = require('fs');
var read = Bacon.fromNodeCallback(fs.readFile, 'input.txt');
read.onError(function(error) { console.log("Reading failed: " + error); });
read.onValue(function(value) { console.log("Read contents: " + value); });

Bacon.fromNodeCallback(object, methodName [, args...]) a variant of fromNodeCallback which calls the named method of a given object.

Bacon.fromPoll(interval, f) polls given function with given interval. Function should return Events: either Bacon.Next or Bacon.End. Polling occurs only when there are subscribers to the stream. Polling ends permanently when f returns Bacon.End.

Bacon.once(value) creates an EventStream that delivers the given single value for the first subscriber. The stream will end immediately after this value. You can also send send an Bacon.Error event instead of a value: Bacon.once(new Bacon.Error("fail")).

Bacon.fromArray(values) creates an EventStream that delivers the given series of values (given as array) to the first subscriber. The stream ends after these values have been delivered. You can also send Bacon.Error events, or any combination of pure values and error events like this: `Bacon.fromArray([1, new Bacon.Error()])

Bacon.interval(interval, value) repeats the single element indefinitely with the given interval (in milliseconds)

Bacon.sequentially(interval, values) creates a stream containing given values (given as array). Delivered with given interval in milliseconds.

Bacon.repeatedly(interval, values) repeats given elements indefinitely with given interval in milliseconds. For example, repeatedly(10, [1,2,3]) would lead to 1,2,3,1,2,3... to be repeated indefinitely.

Bacon.never() creates an EventStream that immediately ends.

Bacon.later(delay, value) creates a single-element stream that produces given value after given delay (milliseconds).

new Bacon.EventStream(subscribe) creates an EventStream with the given subscribe function.

property.changes creates a stream of changes to the Property. The stream does not include an event for the current value of the Property at the time this method was called.

property.toEventStream() creates an EventStream based on this Property. The stream contains also an event for the current value of this Property at the time this method was called.

new Bacon.Bus() creates a pushable/pluggable stream (see Bus section below)

Pro tip: you can also put Errors into streams created with the constructors above, by using an Bacon.Error object instead of a plain value.

Bacon.fromBinder for custom streams

If none of the factory methods above apply, you may of course roll your own EventStream by using Bacon.fromBinder.

Bacon.fromBinder(subscribe) The parameter subscribe is a function that accepts a sink which is a function that your subcribe funtion can "push" events to.

For example:

var stream = Bacon.fromBinder(function(sink) {
  sink("first value")
  sink([new Bacon.Next("2nd"), new Bacon.Next("3rd")])
  sink(new Bacon.Next(function() {
    return "This one will be evaluated lazily"
  }))
  sink(new Bacon.Error("oops, an error"))
  sink(new Bacon.End())
  return function() {
     // unsub functionality here, this one's a no-op
  }
})
stream.log()

As shown in the example, you can push

  • A plain value, like "first value"
  • An Event object including Bacon.Error (wraps an error) and Bacon.End (indicates stream end).
  • An array of event objects at once

See another example.

The subscribe function must return a function. Let's call that function unsubscribe. The returned function can be used by the subscriber to unsubscribe and it should release all resources that the subscribe function reserved.

The sink function may return Bacon.more or Bacon.noMore. It may also return undefined or anything else. Iff it returns Bacon.noMore, the subscriber must be cleaned up just like in case of calling the unsubscribe function.

The EventStream will wrap your subscribe function so that it will only be called when the first stream listener is added, and the unsubscibe function is called only after the last listener has been removed. The subscribe-unsubscribe cycle may of course be repeated indefinitely, so prepare for multiple calls to the subscribe function.

A note about the new Bacon.Next(..) constructor: You can use it like

new Bacon.Next("value")

But the canonical way would be

new Bacon.Next(function() { return "value") })

The former version is safe only when you know that the actual value in the stream is not a function.

The idea in using a function instead of a plain value is that the internals on Bacon.js take advantage of lazy evaluation by deferring the evaluations of values created by map, combine.

Bacon.noMore The opaque value sink function may return. See Bacon.fromBinder.

Bacon.more The opaque value sink function may return. See Bacon.fromBinder.

Common methods in EventStreams and Properties

Both EventStream and Property share the Observable interface, and hence share a lot of methods. Common methods are listed below.

observable.onValue(f) subscribes a given handler function to the observable. Function will be called for each new value. This is the simplest way to assign a side-effect to an observable. The difference to the subscribe method is that the actual stream values are received, instead of Event objects. stream.onValue and property.onValue behave similarly, except that the latter also pushes the initial value of the property, in case there is one.

observable.onError(f) subscribes a callback to error events. The function will be called for each error in the stream.

observable.onEnd(f) subscribes a callback to stream end. The function will be called when the stream ends. Just like subscribe, this method returns a function for unsubscribing.

observable.map(f) maps values using given function, returning a new EventStream. Instead of a function, you can also provide a constant value. Further, you can use a property extractor string like ".keyCode". So, if f is a string starting with a dot, the elements will be mapped to the corresponding field/function in the event value. For instance map(".keyCode") will pluck the keyCode field from the input values. If keyCode was a function, the result stream would contain the values returned by the function. The Function Construction rules below apply here.

stream.map(property) maps the stream events to the current value of the given property. This is equivalent to property.sampledBy(stream).

observable.mapError(f) maps errors using given function. More specifically, feeds the "error" field of the error event to the function and produces a Next event based on the return value. The Function Construction rules below apply here. You can omit the argument to produce a Next event with undefined value.

observable.errors() returns a stream containing Error events only. Same as filtering with a function that always returns false.

observable.skipErrors() skips all errors.

observable.mapEnd(f) Adds an extra Next event just before End. The value is created by calling the given function when the source stream ends. Instead of a function, a static value can be used. You can omit the argument to produce a Next event with undefined value.

observable.filter(f) filters values using given predicate function. Instead of a function, you can use a constant value (true to include all, false to exclude all) or a property extractor string (like ".isValuable") instead. Just like with map, indeed.

observable.filter(property) filters values based on the value of a property. Event will be included in output iff the property holds true at the time of the event.

observable.takeWhile(f) takes while given predicate function holds true. Function Construction rules apply.

observable.takeWhile(property) takes values while the value of a property holds true.

observable.take(n) observable.take(n) takes at most n elements from the stream. Equals to Bacon.never() if n <= 0.

observable.takeUntil(stream) takes elements from source until a Next event appears in the other stream. If other stream ends without value, it is ignored

observable.skip(n) skips the first n elements from the stream

observable.delay(delay) delays the stream/property by given amount of milliseconds. Does not delay the initial value of a Property.

var delayed = source.delay(2)
source:    asdf----asdf----
delayed:   --asdf----asdf--

observable.throttle(delay) throttles stream/property by given amount of milliseconds. Events are emitted with the minimum interval of delay. The implementation is based on stream.bufferWithTime. Does not affect emitting the initial value of a Property.

Example:

var throttled = source.throttle(2)
source:    asdf----asdf----
throttled: --s--f----s--f--

observable.debounce(delay) throttles stream/property by given amount of milliseconds, but so that event is only emitted after the given "quiet period". Does not affect emitting the initial value of a Property. The difference of throttle and debounce is the same as it is in the same methods in jQuery.

Example:

source:             asdf----asdf----
source.debounce(2): -----f-------f--

observable.debounceImmediate(delay) passes the first event in the stream through, but after that, only passes events after a given number of milliseconds have passed since previous output.

Example:

source:                      asdf----asdf----
source.debounceImmediate(2): a-d-----a-d-----

observable.doAction(f) returns a stream/property where the function f is executed for each value, before dispatching to subscribers. This is useful for debugging, but also for stuff like calling the preventDefault() method for events. In fact, you can also use a property-extractor string instead of a function, as in ".preventDefault".

observable.not() returns a stream/property that inverts boolean values

observable.flatMap(f) for each element in the source stream, spawn a new stream using the function f. Collect events from each of the spawned streams into the result EventStream. This is very similar to selectMany in RxJs. Note that instead of a function, you can provide a stream/property too. Also, the return value of function f can be either an Observable (stream/property) or a constant value. The result of flatMap is always an EventStream.

The Function Construction rules below apply here.

stream.flatMap() can be used conveniently with Bacon.once() and Bacon.never() for converting and filtering at the same time, including only some of the results.

Example - converting strings to integers, skipping empty values:

stream.flatMap(function(text) {
    return (text != "") ? parseInt(text) : Bacon.never()
})

observable.flatMapLatest(f) like flatMap, but instead of including events from all spawned streams, only includes them from the latest spawned stream. You can think this as switching from stream to stream. Note that instead of a function, you can provide a stream/property too.

The Function Construction rules below apply here.

observable.flatMapFirst(f) like flatMap, but doesn't spawns a new stream only if the previously spawned stream has ended.

observable.scan(seed, f) scans stream/property with given seed value and accumulator function, resulting to a Property. For example, you might use zero as seed and a "plus" function as the accumulator to create an "integral" property. Instead of a function, you can also supply a method name such as ".concat", in which case this method is called on the accumulator value and the new stream value is used as argument.

Example:

var plus = function (a,b) { return a + b }
Bacon.sequentially(1, [1,2,3]).scan(0, plus)

This would result to following elements in the result stream:

seed value = 0
0 + 1 = 1
1 + 2 = 3
3 + 3 = 6

When applied to a Property as in r = p.scan(seed, f), there's a (hopefully insignificant) catch: The starting value for r depends on whether p has an initial value when scan is applied. If there's no initial value, this works identically to EventStream.scan: the seed will be the initial value of r. However, if r already has a current/initial value x, the seed won't be output as is. Instead, the initial value of r will be f(seed, x). This makes sense, because there can only be 1 initial value for a Property at a time.

observable.fold(seed, f) is like scan but only emits the final value, i.e. the value just before the observable ends. Returns a Property.

observable.reduce(seed, f) synonym for fold.

observable.diff(start, f) returns a Property that represents the result of a comparison between the previous and current value of the Observable. For the initial value of the Observable, the previous value will be the given start.

Example:

var distance = function (a,b) { return Math.abs(b - a) }
Bacon.sequentially(1, [1,2,3]).diff(0, distance)

This would result to following elements in the result stream:

1 - 0 = 1
2 - 1 = 1
3 - 2 = 1

observable.zip(other, f) return an EventStream with elements pair-wise lined up with events from this and the other stream. A zipped stream will publish only when it has a value from each stream and will only produce values up to when any single stream ends.

Be careful not to have too much "drift" between streams. If one stream produces many more values than some other excessive buffering will occur inside the zipped observable.

Example 1:

var x = Bacon.fromArray([1, 2])
var y = Bacon.fromArray([3, 4])
x.zip(y, function(x, y) { return x + y })

# produces values 4, 6

Example 2:

You can use zip to combine observables that are pairwise synchronized from e.g. projections or sampling by the same property, while avoiding the double-processing that would happen recombining with combine.

var x = obs.map('.x')
var y = obs.map('.y')
x.zip(y, makeComplex)

observable.slidingWindow(max [, min]) returns a Property that represents a "sliding window" into the history of the values of the Observable. The result Property will have a value that is an array containing the last n values of the original observable, where n is at most the value of the max argument, and at least the value of the min argument. If the min argument is omitted, there's no lower limit of values.

For example, if you have a stream s with value a sequence 1 - 2 - 3 - 4 - 5, the respective values in s.slidingWindow(2) would be [] - [1] - [1,2] - [2,3] - [3,4] - [4,5]. The values of s.slidingWindow(2,2)would be [1,2] - [2,3] - [3,4] - [4,5].

observable.log() logs each value of the Observable to the console. It optionally takes arguments to pass to console.log() alongside each value. To assist with chaining, it returns the original Observable. Note that as a side-effect, the observable will have a constant listener and will not be garbage-collected. So, use this for debugging only and remove from production code. For example:

myStream.log("New event in myStream")

or just

myStream.log()

observable.combine(property2, f) combines the latest values of the two streams or properties using a two-arg function. Similarly to scan, you can use a method name instead, so you could do a.combine(b, ".concat") for two properties with array value. The result is a Property.

observable.withStateMachine(initState, f) lets you run a state machine on an observable. Give it an initial state object and a state transformation function that processes each incoming event and returns and array containing the next state and an array of output events. Here's an an example, where we calculate the total sum of all numbers in the stream and output the value on stream end:

Bacon.fromArray([1,2,3])
  .withStateMachine(0, function(sum, event) {
    if (event.hasValue())
      return [sum + event.value(), []]
    else if (event.isEnd())
      return [undefined, [new Bacon.Next(sum), event]]
    else
      return [sum, [event]]
  })

observable.decode(mapping) decodes input using the given mapping. Is a bit like a switch-case or the decode function in Oracle SQL. For example, the following would map the value 1 into the the string "mike" and the value 2 into the value of the who property.

property.decode({1 : "mike", 2 : who})

This is actually based on combineTemplate so you can compose static and dynamic data quite freely, as in

property.decode({1 : { type: "mike" }, 2 : { type: "other", whoThen : who }})

The return value of decode is always a Property.

observable.awaiting(otherObservable) creates a Property that indicates whether observable is awaiting otherObservable, i.e. has produced a value after the latest value from otherObservable. This is handy for keeping track whether we are currently awaiting an AJAX response:

var showAjaxIndicator = ajaxRequest.awaiting(ajaxResponse)

observable.endOnError() ends the Observable on first Error event. The error is included in the output of the returned Observable.

observable.endOnError(f) ends the Observable on first Error event for which the given predicate function returns true. The error is included in the output of the returned Observable. The Function Construction rules apply, so you can do for example .endOnError(".serious").

observable.withHandler(f) lets you do more custom event handling: you get all events to your function and you can output any number of events and end the stream if you choose. For example, to send an error and end the stream in case a value is below zero:

if (event.hasValue() && event.value() < 0) {
  this.push(new Bacon.Error("Value below zero"));
  return this.push(end());
} else {
  return this.push(event);
}

Note that it's important to return the value from this.push so that the connection to the underlying stream will be closed when no more events are needed.

observable.name(newName) sets the name of the observable. Overrides the default implementation of toString and inspect. Returns itself.

observable.withDescription(param...) Sets the structured description of the observable. The toString and inspect methods use this data recursively to create a string representation for the observable. This method is probably useful for Bacon core / library / plugin development only.

For example:

var src = Bacon.once(1)
var obs = src.map(function(x) { return -x })
console.log(obs.toString())
--> Bacon.once(1).map(function)
obs.withDescription(src, "times", -1)
console.log(obs.toString())
--> Bacon.once(1).times(-1)

EventStream

Bacon.EventStream a stream of events. See methods below.

stream.subscribe(f) subscribes given handler function to event stream. Function will receive Event objects (see below). The subscribe() call returns a unsubscribe function that you can call to unsubscribe. You can also unsubscribe by returning Bacon.noMore from the handler function as a reply to an Event.

stream.onValue(f) subscribes a given handler function to event stream. Function will be called for each new value in the stream. This is the simplest way to assign a side-effect to a stream. The difference to the subscribe method is that the actual stream values are received, instead of Event objects. The Function Construction rules below apply here. Just like subscribe, this method returns a function for unsubscribing.

stream.onValues(f) like onValue, but splits the value (assuming its an array) as function arguments to f.

stream.skipDuplicates(isEqual) drops consecutive equal elements. So, from [1, 2, 2, 1] you'd get [1, 2, 1]. Uses the === operator for equality checking by default. If the isEqual argument is supplied, checks by calling isEqual(oldValue, newValue). For instance, to do a deep comparison,you can use the isEqual function from underscore.js like stream.skipDuplicates(_.isEqual).

stream.concat(otherStream) concatenates two streams into one stream so that it will deliver events from stream until it ends and then deliver events from otherStream. This means too that events from stream2, occurring before the end of stream will not be included in the result stream.

stream.merge(otherStream) merges two streams into one stream that delivers events from both

stream.startWith(value) adds a starting value to the stream, i.e. concats a single-element stream contains value with this stream.

stream.skipWhile(f) skips elements while given predicate function holds true. The Function Construction rules below apply here.

stream.skipWhile(property) skips elements while the value of the given Property is true.

stream.skipUntil(stream2) skips elements from stream until a Next event appears in stream2. In other words, starts delivering values from stream after first event appears in stream2.

stream.bufferWithTime(delay) buffers stream events with given delay. The buffer is flushed at most once in the given delay. So, if your input contains [1,2,3,4,5,6,7], then you might get two events containing [1,2,3,4] and [5,6,7] respectively, given that the flush occurs between numbers 4 and 5.

stream.bufferWithTime(f) works with a given "defer-function" instead of a delay. Here's a simple example, which is equivalent to stream.bufferWithTime(10):

stream.bufferWithTime(function(f) { setTimeout(f, 10) })

stream.bufferWithCount(count) buffers stream events with given count. The buffer is flushed when it contains the given number of elements. So, if you buffer a stream of [1, 2, 3, 4, 5] with count 2, you'll get output events with values [1, 2], [3, 4] and [5].

stream.bufferWithTimeOrCount(delay, count) buffers stream events and flushes when either the buffer contains the given number elements or the given amount of milliseconds has passed since last buffered event.

stream.toProperty() creates a Property based on the EventStream. Without arguments, you'll get a Property without an initial value. The Property will get its first actual value from the stream, and after that it'll always have a current value.

stream.toProperty(initialValue) creates a Property based on the EventStream with the given initial value that will be used as the current value until the first value comes from the stream.

Property

Bacon.Property a reactive property. Has the concept of "current value". You can create a Property from an EventStream by using either toProperty or scan method. Note depending on how a Property is created, it may or may not have an initial value.

Bacon.constant(x) creates a constant property with value x.

property.subscribe(f) subscribes a handler function to property. If there's a current value, an Initial event will be pushed immediately. Next event will be pushed on updates and an Bacon.End event in case the source EventStream ends. Returns a function that you call to unsubscribe.

property.onValue(f) similar to stream.onValue, except that also pushes the initial value of the property, in case there is one. See Function Construction rules below for different forms of calling this method. Just like subscribe, this method returns a function for unsubscribing.

property.onValues(f) like onValue, but splits the value (assuming its an array) as function arguments to f

property.assign(obj, method [, param...]) calls the method of the given object with each value of this Property. You can optionally supply arguments which will be used as the first arguments of the method call. For instance, if you want to assign your Property to the "disabled" attribute of a JQuery object, you can do this:

myProperty.assign($("#my-button"), "attr", "disabled")

A simpler example would be to toggle the visibility of an element based on a Property:

myProperty.assign($("#my-button"), "toggle")

Note that the assign method is actually just a synonym for onValue and the function construction rules below apply to both.

property.sample(interval) creates an EventStream by sampling the property value at given interval (in milliseconds)

property.sampledBy(stream) creates an EventStream by sampling the property value at each event from the given stream. The result EventStream will contain the property value at each event in the source stream.

property.sampledBy(property) creates a Property by sampling the property value at each event from the given property. The result Property will contain the property value at each event in the source property.

property.sampledBy(streamOrProperty, f) samples the property on stream events. The result values will be formed using the given function f(propertyValue, samplerValue). You can use a method name (such as ".concat") instead of a function too.

property.skipDuplicates(isEqual) drops consecutive equal elements. So, from [1, 2, 2, 1] you'd get [1, 2, 1]. Uses the === operator for equality checking by default. If the isEqual argument is supplied, checks by calling isEqual(oldValue, newValue). The old name for this method was distinctUntilChanged.

property.changes() returns an EventStream of property value changes. Returns exactly the same events as the property itself, except any Initial events. Note that property.changes() does NOT skip duplicate values, use .skipDuplicates() for that.

property.and(other) combines properties with the && operator.

property.or(other) combines properties with the || operator.

property.startWith(value) adds an initial "default" value for the Property. If the Property doesn't have an initial value of it's own, the given value will be used as the initial value. If the property has an initial value of its own, the given value will be ignored.

Combining multiple streams and properties

Bacon.combineAsArray(streams) combines Properties, EventStreams and constant values so that the result Property will have an array of all property values as its value. The input array may contain both Properties and EventStreams. In the latter case, the stream is first converted into a Property and then combined with the other properties.

Bacon.combineAsArray(s1, s2...) just like above, but with streams provided as a list of arguments as opposed to a single array.

property = Bacon.constant(1)
stream = Bacon.once(2)
constant = 3
Bacon.combineAsArray(property, stream, constant)
# produces the value [1,2,3]

Bacon.combineWith(f, stream1, stream2...) combines given n Properties, EventStreams and constant values using the given n-ary function f(v1, v2 ...). To calculate the current sum of three numeric Properties, you can do

function sum3(x,y,z) { return x + y + z }
Bacon.combineWith(sum3, p1, p2, p3)

Bacon.combineTemplate(template) combines Properties, EventStreams and constant values using a template object. For instance, assuming you've got streams or properties named password, username, firstname and lastname, you can do

var password, username, firstname, lastname; // <- properties or streams
var loginInfo = Bacon.combineTemplate({
    magicNumber: 3,
    userid: username,
    passwd: password,
    name: { first: firstname, last: lastname }})

.. and your new loginInfo property will combine values from all these streams using that template, whenever any of the streams/properties get a new value. For instance, it could yield a value such as

{ magicNumber: 3,
  userid: "juha",
  passwd: "easy",
  name : { first: "juha", last: "paananen" }}

In addition to combining data from streams, you can include constant values in your templates.

Note that all Bacon.combine* methods produce a Property instead of an EventStream. If you need the result as an EventStream you might want to use property.changes()

Bacon.combineWith(function(v1,v2) { .. }, stream1, stream2).changes()

Bacon.mergeAll(streams) merges given array of EventStreams. Bacon.mergeAll(stream1, stream2 ...) merges given EventStreams.

Bacon.zipAsArray(streams) zips the array of stream in to a new EventStream that will have an array of values from each source stream as its value. Zipping means that events from each stream are combine pairwise so that the 1st event from each stream is published first, then the 2nd event from each. The results will be published as soon as there is a value from each source stream.

Be careful not to have too much "drift" between streams. If one stream produces many more values than some other excessive buffering will occur inside the zipped observable.

Example:

x = Bacon.fromArray([1,2,3])
y = Bacon.fromArray([10, 20, 30])
z = Bacon.fromArray([100, 200, 300])
Bacon.zipAsArray(x, y, z)

# produces values 111, 222, 333

Bacon.zipAsArray(stream1, stream2...) just like above, but with streams provided as a list of arguments as opposed to a single array.

Bacon.zipWith(streams, f) like zipAsArray but uses the given n-ary function to combine the n values from n streams, instead of returning them in an Array.

Bacon.zipWith(f, stream1, stream1...) just like above, but with streams provided as a list of arguments as opposed to a single array.

Bacon.onValues(a, b [, c...], f) is a shorthand for combining multiple sources (streams, properties, constants) as array and assigning the side-effect function f for the values. The following example would log the number 3.

function f(a, b) { console.log(a + b) }
Bacon.onValues(Bacon.constant(1), Bacon.constant(2), f)

Function Construction rules

Many methods in Bacon have a single function as their argument. Many of these actually accept a wider range of different arguments that they use for constructing the function.

Here are the different forms you can use, with examples. The basic form would be

stream.map(f) maps values using the function f(x)

As an extension to the basic form, you can use partial application:

stream.map(f, "bacon") maps values using the function f(x, y), using "bacon" as the first argument, and stream value as the second argument.

stream.map(f, "pow", "smack") maps values using the function f(x, y, z), using "pow" and "smack" as the first two arguments and stream value as the third argument.

Then, you can create method calls like this:

stream.onValue(object, method) calls the method having the given name, with stream value as the argument.

titleText.onValue($("#title"), "text") which would call the "text" method of the jQuery object matching to the HTML element with the id "title"

disableButton.onValue($("#send"), "attr", "disabled") which would call the attr method of the #send element, with "disabled" as the first argument. So if your property has the value true, it would call $("#send").attr("disabled", true)

You can call methods or return field values using a "property extractor" syntax. With this syntax, Bacon checks the type of the field and if it's indeed a method, it calls it. Otherwise it just returns field value. For example:

stream.map(".length") would return the value of the "length" field of stream values. Would make sense for a stream of arrays. So, you'd get 2 for ["cat", "dog"]

stream.map(".stuffs.length") would pick the length of the "stuffs" array that is a field in the stream value. For example, you'd get 2 for { stuffs : ["thing", "object"] }

stream.map(".dudes.1") would pick the second object from the nested "dudes" array. For example, you'd get "jack" for { dudes : ["john", "jack"] }.

stream.doAction(".preventDefault") would call the "preventDefault" method of stream values.

stream.filter(".attr", "disabled").not() would call .attr("disabled") on stream values and filter by the return value. This would practically inlude only disabled jQuery elements to the result stream.

If none of the above applies, Bacon will return a constant value. For instance:

mouseClicks.map({ isMouseClick: true }) would map all events to the object { isMouseClick: true }

Methods that support function construction include at least onValue, onError, onEnd, map, filter, assign, takeWhile, mapError and doAction.

Latest value of Property or EventStream

One of the common first questions people ask is "how do I get the latest value of a stream or a property". There is no getLatestValue method available and will not be either. You get the value by subscribing to the stream/property and handling the values in your callback. If you need the value of more than one source, use one of the combine methods.

Bus

Bus is an EventStream that allows you to push values into the stream. It also allows pluggin other streams into the Bus. The Bus practically merges all plugged-in streams and the values pushed using the push method.

new Bacon.Bus() returns a new Bus.

bus.push(x) pushes the given value to the stream.

bus.end() ends the stream. Sends an End event to all subscribers. After this call, there'll be no more events to the subscribers. Also, the bus.push and bus.plug methods have no effect.

bus.error(e) sends an Error with given message to all subscribers

bus.plug(stream) plugs the given stream to the Bus. All events from the given stream will be delivered to the subscribers of the Bus. Returns a function that can be used to unplug the same stream.

The plug method practically allows you to merge in other streams after the creation of the Bus. I've found Bus quite useful as an event broadcast mechanism in the Worzone game, for instance.

Event

Bacon.Event has subclasses Bacon.Next, Bacon.End, Bacon.Error and Bacon.Initial

Bacon.Next next value in an EventStream or a Property. Call isNext() to distinguish a Next event from other events.

Bacon.End an end-of-stream event of EventStream or Property. Call isEnd() to distinguish an End from other events.

Bacon.Error an error event. Call isError() to distinguish these events in your subscriber, or use onError to react to error events only. errorEvent.error returns the associated error object (usually string).

Bacon.Initial the initial (current) value of a Property. Call isInitial() to distinguish from other events. Only sent immediately after subscription to a Property.

Event properties and methods

event.value() returns the value associated with a Next or Initial event

event.hasValue() returns true for events of type Initial and Next

event.isNext() true for Next events

event.isInitial() true for Initial events

event.isEnd() true for End events

Errors

Bacon.Error events are always passed through all stream combinators. So, even if you filter all values out, the error events will pass though. If you use flatMap, the result stream will contain Error events from the source as well as all the spawned stream.

You can take action on errors by using the observable.onError(f) callback.

See documentation on onError, mapError, errors, skipErrors above.

In case you want to convert (some) value events into Error events, you may use flatMap like this:

stream = Bacon.fromArray([1,2,3,4]).flatMap(function(x) {
  if (x > 2)
    return new Bacon.Error("too big")
  else
    return x
})

Note also that Bacon.js combinators do not catch errors that are thrown. Especially map doesn't do so. If you want to map things and wrap caught errors into Error events, you can do the following:

var source, dangerousFunction // <- your stuff
wrapped = source.flatMap(function(x) {
  try
    return dangerousFunction(x)
  catch (e)
    return new Bacon.Error(e)
})

An Error does not terminate the stream. The method observable.endOnError() returns a stream/property that ends immediately after first error.

Bacon.js doesn't currently generate any Error events itself (except when converting errors using Bacon.fromPromise). Error events definitely would be generated by streams derived from IO sources such as AJAX calls.

Join Patterns

Join patterns are a generalization of the zip function. While zip synchronizes events from multiple streams pairwse, join patterns allow for implementation of more advanced synchronization patterns. Bacon.js uses the Bacon.when function to convert a list of synchronization patterns into a resulting eventstream.

Bacon.when Consider implementing a game with discrete time ticks. We want to handle key-events synchronized on tick-events, with at most one key event handled per tick. If there are no key events, we want to just process a tick.

  Bacon.when(
    [tick, keyEvent], function(_, k) { handleKeyEvent(k); handleTick },
    [tick], handleTick)

Order is important here. If the [tick] patterns had been written first, this would have been tried first, and preferred at each tick.

Join patterns are indeed a generalization of zip, and zip is equivalent to a single-rule join pattern. The following observables have the same output.

Bacon.zipWith(a,b,c, combine)
Bacon.when([a,b,c], combine)

Bacon.update creates a Property from an initial value and updates the value based on multiple inputs. The inputs are defined similarly to Bacon.when, like this:

var result = Bacon.update(
  initial,
  [x,y,z], function(previous,x,y,z) { ... },
  [x,y],   function(previous,x,y) { ... })

As input, each function above will get the previous value of the result Property, along with values from the listed Observables. The value returned by the function will be used as the next value of result.

Just like in Bacon.when, only EventStreams will trigger an update, while Properties will be just sampled. So, if you list a single EventStream and several Properties, the value will be updated only when an event occurs in the EventStream.

Here's a simple gaming example:

var scoreMultiplier = Bacon.constant(1)
var hitUfo = new Bacon.Bus()
var hitMotherShip = new Bacon.Bus()
var score = Bacon.update(
  0,
  [hitUfo, scoreMultiplier], function(score, _, multiplier) { return score + 100 * multiplier },
  [hitMotherShip], function(score, _) { return score + 2000 }
)

In the example, the score property is updated when either hitUfo or hitMotherShip occur. The scoreMultiplier Property is sampled to take multiplier into account when hitUfo occurs.

Join patterns as a "chemical machine"

A quick way to get some intuition for join patterns is to understand them through an analogy in terms of atoms and molecules. A join pattern can here be regarded as a recipe for a chemical reaction. Lets say we have observables oxygen, carbon and hydrogen, where an event in these spawns an 'atom' of that type into a mixture.

We can state reactions

make_water              = function(oxygen, hydrogen, hydrogen)  { /* ... consume oxygen and hydrogen ... */ }
make_carbon_monoxide    = function(oxygen, carbon)              { /* ... consume oxygen and carbon ... */ }

Bacon.when(
  [oxygen, hydrogen, hydrogen], make_water,
  [oxygen, carbon],             make_carbon_monoxide,
)

Now, every time a new 'atom' is spawned from one of the observables, this atom is added to the mixture. If at any time there are two oxygen atoms, and a hydrogen atom, the corresponding atoms are consumed, and output is produced via make_water.

The same semantics apply for the second rule to create carbon monoxide. The rules are tried at each point from top to bottom.

Join patterns and properties

Properties are not part of the synchronization pattern, but are instead just sampled. The following example take three input streams $price, $quantity and $total, e.g. coming from input fields, and defines mutally recursive behaviours in properties price, quantity and total such that

  • updating price sets total to price * quantity
  • updating quantity sets total to price * quantity
  • updating total sets price to total / quantity

```js var $price, $total, $quantity = ...

var quantity = $quantity.toProperty(1)

var price = Bacon.when( [$price], id, [$total, quantity], function(x,y) { return x/y }) .toProperty(0)

var total = Bacon.when( [$total], id, [$price, quantity], function(x,y) { return x*y }, [price, $quantity], function(x,y) { return

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