Question

I wish to extend the Object and Array classes so that I will be able to listen to any changes made to their instances after construction.

The best I can do now is to add custom get and set functions for fields given at initialization/construction of an Instance:

var myObj = new CustomObject({name:'foo'});

myObj.name = 'bar'; // this will log "'name' in 'myObj' updated:'bar'"

//however:

myObj.age = 85; // this mutation will slip by unnoticed, 
                // since the 'age' field was never specified 
                // at initialization meaning no custom set / get 
                // functions where attached.

Is there a way to achieve the following functionality?

var myObj = new CustomObject();

myObj.name = 'foo';// this should log something like:
                   // "A new field 'name' was created for myObj with value 'foo'"

Note:I’m looking for a solution that doesn’t involve polling.

What I have now (resulting in the functionality shown in the first codeblock):

function CustomObject(data) {

    var that = this;

    for(var field in data){         
        prepField(field);                                   
    }

    function prepField(field){                  
        Object.defineProperty( that, field, {
            set: function(val){         
                data[field] = val;
                console.log(field,'in',that,'updated:',val);
            },
            get: function(){
                console.log(field,'in',that,'requested');
                return data[field];                 
            },
            enumerable:true
        });         
    }

    return this;

}

Thanks in advance!

Answer 1

Firefox (mozilla) has something called Proxy that can do this.

MDN Proxy docs

This is a non-standard feature currently, but appears as though it (or something similar) may become part of ECMAScript 6.

From http://wiki.ecmascript.org/doku.php?id=harmony:proposals :