app.js

function Button() {
    this.open = function() {
        // Developer decided that this event is semantically okay to be propagated.
        this.fire( 'open', { propagate: true } );
    };
}
Button.events = { open: 'open' };

// This constructor implement a collection interface.
function Toolbar() {}
Toolbar.events = { open: 'open' };

var toolbar = new Toolbar();

// Here is a default - straightforward behaviour with on WTF.
// Fourth argument is not provided which means that only `open` events fired on toolbar will call a callback.
toolbar.on( Toolbar.events.open, function( e ) {
    console.log( e.target instanceof Toolbar ); // Will always be `true`.
} );

// Here we introduce a new, fourth parameter which filter out targets.
toolbar.on( Button.events.open, function( e ) {
    console.log( e.target instanceof Button ); // Will always be `true`.
    console.log( 'Hell, yeach button opened!' );
}, toolbar, 1337, /* Filtered by target - */Button );

toolbar.add( new Button() );
toolbar.add( new Button() );