Question

I’m using Google Chrome 16.0.912.63 and have the following code:

__defineGetter__('x', function(){ 
    console.log('In Getter: x'); return 1; 
});

__defineGetter__('undefined', function(){ 
    console.log('In Getter: undefined'); return 2; 
});

console.log(x);
console.log(undefined);

The getter function for x is entered correctly, but the getter for undefined is never entered. The output logged to my console is:

In Getter: x
1
undefined

I was always under the impression that undefined behaves the same as any other global variable. I know that statements such as

undefined = 10;
undefined = null;
undefined = 'defined';

Are all valid, so what is different about undefined that makes it unable to have a getter (at least in my browser).

JSFiddle

Answer 1

It doesn’t behave like any normal variable, it just pretends to, at least on Firefox and Chrome. Try it:

var x;
undefined = "Hello!";
x === undefined; // true; undefined doesn't change

It’s more like a readonly property but doesn’t throw an error when it’s written to.