Obfuscate javascript properties?

Question

I've recently tested UglifyJS and YUI Compressor and noticed something odd.
Both minifiers don't seem to change the names of object properties, only the names of variables and functions.

for instance if I have the following code:

var objName = {first:2, second:4};
alert(objName.first + " " + objName.second);

the names first and second remain unchanged in the minified version.
Why is that?

Answer 1

Since in javascript a new scope is created in a function, you can scope your code in an immediately invoked function.

   // scoped
(function() {
    var objName = {first:2, second:4};
    alert(objName.first + " " + objName.second);
})();

Then using Google's Closure Compiler, if you turn on the "Advanced" optimization it will see that the properties are only used locally, and will obfuscate them.

   // result
var a={a:2,b:4};alert(a.a+" "+a.b);

Answer 2

It's because it doesn't know where the object is going to be used. It could be used externally by other code and you wouldn't want your other code to have to change whenever you obfuscate it.

Edit So basically, it's like that to prevent obfuscation from breaking external/internal references to properties that may not be possible to figure out while obfuscating.