Subj : Re: Strange Rhino NativeString behavior
To   : Matt McShane
From : Igor Bukanov
Date : Sun Oct 24 2004 12:45 pm

Matt McShane wrote:
> Hello,
> 
> I am having trouble understanding some Rhino behavior, hopefully someone 
> here can explain for me.
> 
> I have added a function to Object.protoype, let's call it myFunc. Here 
> is the Java implementation:
> 
> public static Object myFunc(Context cx, Scriptable thisObj,
>                               Object[] args, Function funOb)
> {
> Object someAttributeValue = thisObj.get("someAttribute",thisObj);
> System.out.println(someAttributeValue.toString());
> return Context.getUndefinedValue();
> }
> 
> I am simply assuming that the object has a property called 
> "someAttribute" and printing it out (I took out null checks, etc. for 
> clarity)
> 
> This works for the first two of the following tests but not the third.
> 
> var obj = new Object()
> obj.someAttribute = "xyz"
> obj.myFunc() //prints xyz
> 
> var date = new Date()
> date.someAttribute = "xyz"
> date.myFunc() //prints xyz
> 
> 
> var msg = "Hello, world."
> msg.someAttribute = "xyz"
> msg.myFunc() //prints o.m.j.UniqueTag@1027b4d: NOT_FOUND
> 
> I understand why the someAttribute property in the NativeString test 
> isn't found - it's because the instance of NativeString on which the 
> property is "put" is different than the instance passed as thisObj to my 
> myFunc. In other words, Rhino is creating a different instance of 
> NativeString with new NativeString("Hello, world.") before passing it to 
> myFunc. But I don't know why this is. There may be a good reason to use 
> different instances - but why aren't properties copied over?

You hit here JavaScript discrepancy about primitive type != object. 
ECMAScript standard effectively states that x.y should be implemented 
via converting x to object and then accessing its properties. If x is 
string, it is converted using new String(x) so your last example is 
equivalent to:

var msg = "Hello, world."
(new String(msg)).someAttribute = "xyz"
(new String(msg)).myFunc() //prints o.m.j.UniqueTag@1027b4d: NOT_FOUND

Thus any property assignment to primitive string is useless operation 
since the temporary created String object will be lost.

Regards, Igor

.