A wrapper around a plain old JavaScript object that fires events when properties/constituents are modified.

var foo = { bar: 1, baz: ["a", "b", "c"], blah: function() { this.bar = 5; }}
var model = new Proxy(foo);
model.subscribe(function() { … }, model);
model.bar = 6; // fires change event
model.baz.set(1, "B"); // fires change
model.blah(55); // fires change