无类型变量对对象有什么好处？null和undefined有什么区别？

Question

根据这个:http://help.adobe.com/en_US/ActionScript/3.0_ProgrammingAS3/WS5b3ccc516d4fbf351e63e3d118a9b90204-7f9f.html Quote:

无类型变量与Object类型的变量不同.关键的区别在于,无类型变量可以保持特殊值undefined,而Object类型的变量不能保存该值.

但是,当我测试它时:

            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?");

两个问题:

• 为什么obj允许赋值为undefined？(不是一个大问题,因为它仍然打印为null)
• 如果我们将null与undefined进行比较,结果为true(即使null存储在Object中).如果它们相等,那么在null和undefined之间做出区别的重点是什么？

Answer 1

• Flash具有类型转换以转换某些类型.

一些样本:

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

要么:

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

因此,当您分配undefined给Object实例Flash时,将其转换为null相同的方式.

• 您的比较在评估之前对不兼容类型应用了类型转换Boolean.

你可以使用严格的比较来false:

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