Question

According to this : http://help.adobe.com/en_US/ActionScript/3.0_ProgrammingAS3/WS5b3ccc516d4fbf351e63e3d118a9b90204-7f9f.html Quote:

An untyped variable is not the same as a variable of type Object. The key difference is that untyped variables can hold the special value undefined , while a variable of type Object cannot hold that value.

However when I test it as :

            var objTest:Object = 123;           
            var untypedTest:* = 123;

            objTest = undefined;
            untypedTest = undefined;            
            //This is understandable but why was the assignment even allowed?
            trace(objTest); // prints null
            trace(untypedTest); // prints undefined

            objTest=null;
            untypedTest = null;         
            //This is also understandable ... both can store null 
            trace(objTest); // prints null 
            trace(untypedTest); // prints null 

            //If they are null whey are they being equal to undefined? 
            if(objTest==undefined)
                trace("obj is undefined");
            if(untypedTest==undefined)
                trace("untyped is undefined");
            //Because null is same as undefined!
            if(null==undefined)
                trace("null is same as undefined?");

Two questions:

• Why is assignment to undefined allowed for obj? (not a big issue since it still prints as null)
• If we compare null with undefined the result true (even if null stored in an Object). What is the point of making a difference between null and undefined if they are equal?

Answer 1

• Flash has type conversion to convert some types.

Some samples of that:

var i:int = NaN;
trace (i); // 0

Or:

var b:Boolean = null;
trace(b); // false

So when you’re assigning undefined to Object instance Flash converts it to null the same way.

• Your comparison applied type conversion on incompatible types before evaluating Boolean.

You can use strict comparison to have false:

if(null === undefined)
    trace("Never traced: null is not the same as undefined!");