A light utility class to help implement the Mediator pattern

Source files:

  1. mediator.js


Build Status

Version 0.9.5

A light utility class to help implement the Mediator pattern for easy eventing

Mediator is a simple class that allows you to register, unregister, and call subscriber methods to help event-based, asyncronous programming. Its purpose is to make the usage of WebSockets, Ajax calls, DOM events, or any other asynchronous operations easy to maintain and test.

Mediator has no dependencies on any other libraries.

1.12kb, minifed and gzipped


My specific use case: bind elements easily for WebSocket callbacks. But, you may find usage in it for all kinds of things: as an event management system, to decouple calls between javascript functions, Ajax request callbacks, and more. There's an excellent online book that talks about Mediators more in detail by Addy Osmani.


Using in Node

The package is in NPM as mediator-js. Include it in your project like so:

var Mediator = require("mediator-js"),
    mediator = new Mediator();

mediator.subscribe("wat", function(){ console.log(arguments); });
mediator.publish("wat", 7, "hi", { one: 1 });

Using in the Browser

Mediator.js is compatible with browser module-loading solutions, including but not limited to Browserify, Almond.js, Require.js, and others.

Note: if using AMD / Almond module loading, use the NPM package name: require("mediator-js").Mediator


You can register events with the mediator two ways using channels. You can add a predicate to perform more complex matching. Instantiate a new mediator, and then you can being subscribing, removing, and publishing.

To use it in the browser, include mediator.min.js from the root here, or the unminified version at lib/mediator.js.

Subscription signature: var mediator = new Mediator();

mediator.subscribe(channel, callback, <options>, <context>);
mediator.publish(channel, <data, data, ... >)
mediator.remove(channel, <identifier>)

Additionally, on and bind are aliased to subscribe, and trigger and emit are bound to publish. off is an alias for remove. You can use once to subscribe to an event that should only be fired once.

Subscriber signature:

function(<data, data ...>, channel);

The channel is always returned as the last argument to subscriber functions.

Mediator.subscribe options (all are optional; default is empty):

  predicate: function(*args){ ... }
  priority: 0|1|... 
  calls: 1|2|...

Predicates return a boolean and are run using whatever args are passed in by the publishing class. If the boolean is true, the subscriber is run.

Priority marks the order in which a subscriber is called.

calls allows you to specify how many times the subscriber is called before it is automatically removed. This is decremented each time it is called until it reaches 0 and is removed. If it has a predicate and the predicate does not match, calls is not decremented.

A Subscriber object is returned when calling Mediator.subscribe. It allows you to update options on a given subscriber, or to reference it by an id for easy removal later.

  id, // guid
  fn, // function
  options, // options
  context, // context for fn to be called within
  channel, // provides a pointer back to its channel
  update(options){ ...} // update the subscriber ({ fn, options, context })


var mediator = new Mediator();

// Alert data when the "message" channel is published to
// Subscribe returns a "Subscriber" object
mediator.subscribe("message", function(data){ alert(data); });
mediator.publish("message", "Hello, world");

// Alert the "message" property of the object called when the predicate function returns true (The "From" property is equal to "Jack")
var predicate = function(data){ return data.From === "Jack" };
mediator.subscribe("channel", function(data){ alert(data.Message); }, { predicate: predicate });
mediator.publish("channel", { Message: "Hey!", From: "Jack" }); //alerts
mediator.publish("channel", { Message: "Hey!", From: "Audrey" }); //doesn't alert

You can remove events by passing in a channel, or a channel and the function to remove or subscriber id. If you only pass in a channel, all subscribers are removed.

// removes all methods bound directly to a channel, but not subchannels

// unregisters *only* MethodFN, a named function, from "channel"
mediator.remove("channel", MethodFN);

You can call the registered functions with the Publish method, which accepts an args array:

mediator.publish("channel", "argument", "another one", { etc: true });

You can namespace your subscribing / removing / publishing as such:

mediator.subscribe("application:chat:receiveMessage", function(data){ ... });

// will call parents of the appllication:chat:receiveMessage namespace
// (that is, next it will call all subscribers of application:chat, and then
// application). It will not recursively call subchannels - only direct subscribers.
mediator.publish("application:chat:receiveMessage", "Jack Lawson", "Hey");

You can update Subscriber priority:

var sub = mediator.subscribe("application:chat", function(data){ ... });
var sub2 = mediator.subscribe("application:chat", function(data){ ... });

// have sub2 executed first
mediator.getChannel("application:chat").setPriority(, 0);

You can update Subscriber callback, context, and/or options:

sub.update({ fn: ..., context: { }, options: { ... });

You can stop the chain of execution by calling channel.stopPropagation():

// for example, let's not post the message if the from and to are the same
mediator.subscribe("application:chat", function(data, channel){
  alert("Don't send messages to yourself!");
}, options: {
  predicate: function(data){ return data.From == data.To },
  priority: 0


Version 0.9.5 * Fixed issue with requring from node

Version 0.9.4 * Fixed issue with auto-removing subscribers after a maximum amount of calls

Version 0.9.3 * Make AMD name match npm package name (mediator-js). Used Mediator.js previously.

Version 0.9.1 * Fixed AMD / define syntax * Exposed Mediator.version

Version 0.9.0

Version 0.6.1

Version 0.6.0

Version 0.5.0

Version 0.4.2

Version 0.4.1

Version 0.4.0


This class and its accompanying README and are MIT licensed.

In Closing

Have fun, and please submit suggestions and improvements! You can leave any issues here, or contact me at (@ajacksified).