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    An implementation of the mediator pattern for asynchronous events in Javascript
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    Build Status

    Version 0.11.0

    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,
        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

    <script src="/js/Mediator.min.js"></script>
      var Mediator = require("mediator-js").Mediator,
          mediator = new Mediator();
      mediator.subscribe("wat", function(){ console.log(arguments); });
      mediator.publish("wat", 7, "hi", { one: 1 });


    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>)


    • subscribe: is alias for on and bind
    • publish: is alias for trigger and emit
    • off: is an alias for remove
    • once: can be used 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 application: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 0.11.0

    Version 0.10.1

    • Accepted a PR that fixed some AMD require issues, and allowed upwards- recursing events to fire on events even where the most-specific event is not fired (a listener on "application:chat:message" will fire on an event like "application:chat:message:jack", even if there's no matching listenener on "jack".)

    Version 0.9.8

    • Accepted a ton of PRs from tbusser that fixed some issues and improved performance.

    Version 0.9.7

    • Fixed bug where subscribers that failed predicates were decrementing calls.

    Version 0.9.6

    • Fixed AMD-style export; export constructor, not instance

    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. (Previously used Mediator.js.)

    Version 0.9.1

    • Fixed AMD / define syntax
    • Exposed Mediator.version

    Version 0.9.0

    • Reversed order of recursion: now calls parents instead of children channels
    • Lowercase methods
    • Aliases: on and bind are aliased to subscribe, and trigger and emit are bound to publish. off is an alias for remove.
    • Moved tests to mocha from jasmine
    • Supports AMD, requirejs, and browser loading
    • Lots of cleanup around extra variables, and jslinted
    • Published to NPM under "mediator-js"
    • Added travis-ci build

    Version 0.6.1

    • Cleaned up some typos
    • Save pointer to channel within subscription
    • Save namespace in channel
    • Fixed bugs in SetPriority

    Version 0.6.0

    • Added ability to stop the chain of calls using c.stopPropagation()

    Version 0.5.0

    • Added ability to access and update subscribing objects
      • Subscribers now have a unique ID and can be queried by id or by function
      • Subscriber class can have its function, context, or options updated
      • Subscriber priority can be updated post-addition
      • Channels made public by Mediator.GetChannel
      • Added a little performance test

    Version 0.4.2

    • Added Priority to calls, allowing you to set callback index

    Version 0.4.1

    • Minor internal updates

    Version 0.4.0

    • Predicate no longer acts as a channel and is moved to an options object at the end of the subcription call.
    • Signatures changed; context moved to the end of subscriptions
    • Namespacing for subscription binding


    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).

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