当对象创建或修改 JavaScript 中的键/值时，有没有办法触发回调？ [复制]

我试图弄清楚如何在 JavaScript 中创建一个对象，该对象将在创建和更改任意键/值时触发事件。这是一些伪代码：

var User = function () {
    var user = {};

    function keyAdded() { /* trigger an event */ }
    function keyChanged() { /* trigger an event */ }

    return user;
},
u = new User();

u.test = "testing"; // I want this to trigger an event (keyAdded)
u.test = "more testing"; // I want this to trigger another event (keyChanged)

我被困住了。有任何想法吗？

function User() {
    this.assign = function(key, value) {
        if ( !this.hasOwnProperty(key) ) {
            this[key] = value;
            alert("key added");
        } else {
            this[key] = value;
            alert("key changed");
        }
    };
    return this;
}

u = new User();

u.assign("test", "testing"); // will alert "key added"
u.assign("test", "more testing"); // will alert "key changed"