Postal.js is an in-memory message bus - very loosely inspired by AMQP - written in JavaScript. Postal.js runs in the browser, or on the server-side using Node.js. It takes the familiar "eventing-style" paradigm (of which most JavaScript developers are familiar) and extends it by providing "broker" and subscriber implementations which are more sophisticated than what you typically find in simple event delegation.

Using a local message bus can enable to you de-couple your web application's components in a way not possible with other 'eventing' approaches. In addition, strategically adopting messaging at the 'seams' of your application (e.g. - between modules, at entry/exit points for browser data and storage) can not only help enforce better overall architectural design, but also insulate you from the risks of tightly coupling your application to 3rd party libraries. For example:

• If you're using a client-side binding framework, and either don't have - or don't like - the request/communication abstractions provided, then grab a library like amplify.js or reqwest. Then, instead of tightly coupling them to your application, have the request success/error callbacks publish messages with the appropriate data and any subscribers you've wired up can handle applying the data to the specific objects/elements they're concerned with.
• Do you need two view models to communicate, but you don't want them to need to know about each other? Have them subscribe to the topics about which they are interested in receiving messages. From there, whenever a view model needs to alert any listeners of specific data/events, just publish a message to the bus. If the other view model is present, it will receive the notification.
• Want to wire up your own binding framework? Want to control the number of times subscription callbacks get invoked within a given time frame? Want to keep subscriptions from being fired until after data stops arriving? Want to keep events from being acted upon until the UI event loop is done processing other events? Postal.js gives you the control you need in these kinds of scenarios via the options available on the SubscriptionDefinition object.
• postal.js is extensible. Plugins like postal.when can be included to provide even more targeted functionality to subscribers. Postal.federation provides the core bits needed to federate postal instances running in different environments (currently the only federation plugin available is postal.xframe for federating between windows in the browser, but more plugins are in the works). These - and more - are all things Postal can do for you.

Postal.js is in good company - there are many options for <airquotes>pub/sub</airquotes> in the browser. However, I grew frustrated with most of them because they often closely followed an event-delegation-paradigm, instead of providing a structured in-memory message bus. Central to postal.js are four concepts:

• channels should be provided to allow for logical partitioning of "topics"
• topics should be hierarchical and allow plain string or wildcard bindings
• messages should include envelope metadata
• subscriber callbacks should get a consistent method signature

A channel is a logical partition of topics. Conceptually, it's like a dedicated highway for a specific set of communication. At first glance it might seem like that's overkill for an environment that runs in an event loop, but it actually proves to be quite useful. Every library has architectural opinions that it either imposes or nudges you toward. Channel-oriented messaging nudges you to separate your communication by bounded context, and enables the kind of fine-tuned visibility you need into the interactions between components as your application grows.

In my experience, seeing publish and subscribe calls all over application logic is usually a strong code smell. Ideally, the majority of message-bus integration should be concealed within application infrastructure. Having a hierarchical-wildcard-bindable topic system makes it very easy to keep things concise (especially subscribe calls!). For example, if you have a module that needs to listen to every message published on the ShoppingCart channel, you'd simply subscribe to "#", and never have to worry about additional subscribes on that channel again - even if you add new messages in the future. If you need to capture all messages with ".validation" at the end of the topic, you'd simply subscribe to "#.validation". If you needed to target all messages with topics that started with "Customer.", ended with ".validation" and had only one period-delimited segment in between, you'd subscribe to "Customer.*.validation" (thus your subscription would capture Customer.address.validation and Customer.email.validation").

Here are four examples of using Postal. All of these examples - AND MORE! - can run live here. (Please bear in mind this fiddle is pulling the postal lib from github, so running these in IE will not work due to the mime type mismatch.) Be sure to check out the wiki for API documentation and conceptual walk-throughs.

// This gets you a handle to the default postal channel...
// For grins, you can get a named channel instead like this:
// var channel = postal.channel( 'DoctorWho' );
var channel = postal.channel();

// subscribe to 'name.change' topics
var subscription = channel.subscribe( "name.change", function ( data ) {
	$( "#example1" ).html( "Name: " + data.name );
} );

// And someone publishes a name change:
channel.publish( "name.change", { name : "Dr. Who" } );

// To unsubscribe, you:
subscription.unsubscribe();

// postal also provides a top-level ability to subscribe/publish
// used primarily when you don't need to hang onto a channel instance:
var anotherSub = postal.subscribe({
	channel  : "MyChannel",
	topic    : "name.change",
	callback : function(data, envelope) {
		$( "#example1" ).html( "Name: " + data.name );    
	}
});

postal.publish({
	channel : "MyChannel",
	topic   : "name.change",
	data    : {
	    name : "Dr. Who"
	}
});

The * symbol represents "one word" in a topic (i.e - the text between two periods of a topic). By subscribing to "*.changed", the binding will match name.changed & location.changed but not changed.companion.

var chgSubscription = channel.subscribe( "*.changed", function ( data ) {
	$( "<li>" + data.type + " changed: " + data.value + "</li>" ).appendTo( "#example2" );
} );
channel.publish( "name.changed",     { type : "Name",     value : "John Smith" } );
channel.publish( "location.changed", { type : "Location", value : "Early 20th Century England" } );
chgSubscription.unsubscribe();

The # symbol represents 0-n number of characters/words in a topic string. By subscribing to "DrWho.#.Changed", the binding will match DrWho.NinthDoctor.Companion.Changed & DrWho.Location.Changed but not Changed.

var starSubscription = channel.subscribe( "DrWho.#.Changed", function ( data ) {
	$( "<li>" + data.type + " Changed: " + data.value + "</li>" ).appendTo( "#example3" );
} );
channel.publish( "DrWho.NinthDoctor.Companion.Changed", { type : "Companion Name", value : "Rose"   } );
channel.publish( "DrWho.TenthDoctor.Companion.Changed", { type : "Companion Name", value : "Martha" } );
channel.publish( "DrWho.Eleventh.Companion.Changed",    { type : "Companion Name", value : "Amy"    } );
channel.publish( "DrWho.Location.Changed",              { type : "Location",       value : "The Library" } );
channel.publish( "TheMaster.DrumBeat.Changed",          { type : "DrumBeat",       value : "This won't trigger any subscriptions" } );
channel.publish( "Changed",                             { type : "Useless",        value : "This won't trigger any subscriptions either" } );
starSubscription.unsubscribe();

var dupChannel = postal.channel( "Blink" ),
    dupSubscription = dupChannel.subscribe( "WeepingAngel.#", function( data ) {
                          $( '<li>' + data.value + '</li>' ).appendTo( "#example4" );
                      }).distinctUntilChanged();
// demonstrating multiple channels per topic being used
// You can do it this way if you like, but the example above has nicer syntax (and *much* less overhead)
dupChannel.publish( "WeepingAngel.DontBlink", { value:"Don't Blink" } );
dupChannel.publish( "WeepingAngel.DontBlink", { value:"Don't Blink" } );
dupChannel.publish( "WeepingAngel.DontEvenBlink", { value:"Don't Even Blink" } );
dupChannel.publish( "WeepingAngel.DontBlink", { value:"Don't Close Your Eyes" } );
dupChannel.publish( "WeepingAngel.DontBlink", { value:"Don't Blink" } );
dupChannel.publish( "WeepingAngel.DontBlink", { value:"Don't Blink" } );
dupSubscription.unsubscribe();

Please visit the postal.js wiki for API documentation, discussion of concepts and links to blogs/articles on postal.js.

There are four main ways you can extend Postal:

• Write a plugin. Need more complex behavior that the built-in SubscriptionDefinition doesn't offer? Write a plugin that you can attach to the global postal object. See postal.when for an example of how to do this.
• Write a custom federation plugin, to federate instances of postal across a transport of your choice.
• You can write an entirely new bus implementation if you wanted. The postal subscribe, publish and addWiretap calls all simply wrap a concrete implementation provided by the postal.configuration.bus object. For example, if you wanted a bus that stored message history in local storage and pushed a dump of past messages to a new subscriber, you'd simply write your implementation and then swap the default one out by calling: postal.configuration.bus = myWayBetterBusImplementation.
• You can also change how the bindingResolver matches subscriptions to message topics being published. You may not care for the AMQP-style bindings functionality. No problem! Write your own resolver object that implements a compare and reset method and swap the core version out with your implementation by calling: postal.configuration.resolver = myWayBetterResolver.

It's also possible to extend the monitoring of messages passing through Postal by adding a "wire tap". A wire tap is a callback that will get invoked for any published message (even if no actual subscriptions would bind to the message's topic). Wire taps should not be used in lieu of an actual subscription - but instead should be used for diagnostics, logging, forwarding (to a websocket publisher or a local storage wrapper, for example) or other concerns that fall along those lines. This repository used to include a console logging wiretap called postal.diagnostics.js - you can now find it here in it's own repo. This diagnostics wiretap can be configured with filters to limit the firehose of message data to specific channels/topics and more.

Please - by all means! While I hope the API is relatively stable, I'm open to pull requests. (Hint - if you want a feature implemented, a pull request gives it a much higher probability of being included than simply asking me.) As I said, pull requests are most certainly welcome - but please include tests for your additions. Otherwise, it will disappear into the ether.

Here's where Postal is headed:

• Add-ons to enable message capture and replay are in the works and should be ready soon.
• The SubscriptionDefinition object will be given the ability to pause (skip) responding to subscriptions
• We'll be working on experimental "lightweight" builds of postal, providing a basic SubscriptionDefinition prototype, but removing the more advanced options if you don't need them. Reduced size builds would be offered alongside full builds, giving you page-weight-sensitive options.
• What else would you like to see?