Question

Is it possible to access the original property value from a Javascript getter?

If not, are there other “modern” javascript techniques for achieving something similar?

That is, if I have code like this

o = {}
o.foo = "bar"
o.__defineGetter__("foo", function(){
    //can I access the value "bar" here?
    return "some other value";
});

Is it possible to access the value “bar” from within my getter function? Or does using __defineGetter__ blow away the property?

(Context: not a newbie developer, but I’ve been ignoring non-cross-browser Javascript for the past 4/5 years and I’m looking to catch up)

Answer 1

ECMAscript 262 edition 5 introduced the option for having getters and setters on objects officially defined by spec. You can setup those either directly on an object literal

var foo = {
    get bar() {
        return "foo";
    }
};

or using Object.defineProperty()

var foo = { };
Object.defineProperty(foo, 'bar', {
    get: function() {
        return "foo";
    }
});

However, the problem remains the same. Overwritting a property will irrepealable overwrite it. What you can do, is use the objects prototype chain to gleam properties.

---

var foo = { };

foo.bar = 42;

Object.defineProperty(Object.getPrototypeOf( foo ), 'bar', {
    get: function() {
        return 'overwritten';
    },
    configurable: true
});

console.log( foo.bar );   // 42
delete foo.bar;
console.log( foo.bar );   // "overwritten"