metaprogramming - Is it possible to implement dynamic getters/setters in JavaScript?

Question

I am aware of how to create getters and setters for properties whose names one already knows, by doing something like this:

// A trivial example:
function MyObject(val){
    this.count = 0;
    this.value = val;
}
MyObject.prototype = {
    get value(){
        return this.count < 2 ? "Go away" : this._value;
    },
    set value(val){
        this._value = val + (++this.count);
    }
};
var a = new MyObject('foo');

alert(a.value); // --> "Go away"
a.value = 'bar';
alert(a.value); // --> "bar2"

Now, my question is, is it possible to define sort of catch-all getters and setters like these? I.e., create getters and setters for any property name which isn't already defined.

The concept is possible in PHP using the __get() and __set() magic methods (see the PHP documentation for information on these), so I'm really asking is there a JavaScript equivalent to these?

Needless to say, I'd ideally like a solution that is cross-browser compatible.

Answer 1

2013 and 2015 Update (see below for the original answer from 2011):

This changed as of the ES2015 (aka "ES6") specification: JavaScript now has proxies. Proxies let you create objects that are true proxies for (facades on) other objects. Here's a simple example that turns any property values that are strings to all caps on retrieval:

"use strict";
if (typeof Proxy == "undefined") {
    throw new Error("This browser doesn't support Proxy");
}
let original = {
    "foo": "bar"
};
let proxy = new Proxy(original, {
    get(target, name, receiver) {
        let rv = Reflect.get(target, name, receiver);
        if (typeof rv === "string") {
            rv = rv.toUpperCase();
        }
        return rv;
      }
});
console.log(`original.foo = ${original.foo}`); // "original.foo = bar"
console.log(`proxy.foo = ${proxy.foo}`);       // "proxy.foo = BAR"