Call a function when a property gets set on an object

If you really insist on keeping the test.hello = "world" syntax to detect changes for existing properties, then you'll have to wait a few years for Object.watch to become part of the next EcmaScript standard.

Luckily, you can do the same in EcmaScript 5 using Object.defineProperty. Eli Grey made a nice Object.watch polyfill which you can call like this:

var test = {};
test.watch("hello", function(propertyName, oldValue, newValue) {
    console.log(propertyName + " has been set to " + newValue);
});
test.hello = "world"; // triggers the watch handler

You could modify his code to trigger a different handler inside the getter as well, so you can detect property accesses.

Unfortunately, browser support is limited to modern browsers including Internet Explorer 9, Firefox 4, Chrome, Opera 12 and Safari 5.

If you want to trigger a handler when a new property is set, you'll have even more trouble. The best you could do is wrapping your object inside a proxy and placing a set trap. You can then detect whether the property already existed by testing if this.getOwnPropertyDescriptor(name) returns a 'truthy' value. The Proxy API is very experimental though and only a few browsers provide a prototype implementation to play with. You'll probably have to wait quite a while to get a completed API with decent browser support.

try this example in chrome (as mentioned in previous comments it uses ES6 Proxy):

var p = new Proxy(
    {},
    {
        get: function(obj, name) {
            console.log('read request to ' + name + ' property');
            if (name == 'test_test') return 1234;
            else return 'Meh';
        },
        set: function(obj, name, value) {
            console.log('write request to ' + name + ' property with ' + value + ' value');
        },
    }
);

console.log(p.test_test);
console.log(p.test);
p.qqq = 'test';

result:

read request to test_test property
1234
read request to test property
Meh
write request to qqq property with test value

var test = {};

Object.defineProperty(test, "hello", {
    get : function () {
        return this._hello;
    },
    set : function (val) {
        alert(val);
        this._hello = val;
    }
});

test.hello = "world";

Something like that. But it will not work on old browsers.

You can find more options here: http://robertnyman.com/javascript/javascript-getters-setters.html