flash - 无类型变量相对于对象有什么好处？ null 和 undefined 有什么区别？

Question

据此:http://help.adobe.com/en_US/ActionScript/3.0_ProgrammingAS3/WS5b3ccc516d4fbf351e63e3d118a9b90204-7f9f.html引用:

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.

但是，当我将其测试为:

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 允许赋值给 undefined？ (不是大问题，因为它仍然打印为空)

如果我们将 null 与 undefined 进行比较，结果为 true(即使 null 存储在对象中)。如果 null 和 undefined 相等，那么区分它们有什么意义？

Answer 1

Flash 有类型转换来转换某些类型。

一些示例:

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

或者:

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

所以当你分配 undefined至 Object实例 Flash 将其转换为 null以同样的方式。

您的比较在评估之前对不兼容的类型应用了类型转换 Boolean .

您可以使用严格比较来获得 false :

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