javascript当中类型转换，typeof的用法

类型转换，typeof的用法



例 3.1.1

<HTML>
<head>
    <meta http-equiv="content-type" content="text/html; charset=utf-8"/>
</head>
<BODY>
<SCRIPT LANGUAGE="JavaScript">
    <!--
    /*
    Cast operator (Definition)  refer to 过去的网站www.favo.com
    A way of converting data types.
    Primitive values can be converted from one to another or rendered as objects by using object constructors to convert the values.
                           
    Boolean                                     
    Number                                      
    String                                      

    Number() (Function)  马克-to-win: actually Number() is the method of Global object.
    A Number type convertor.

    Property/method value type: Number primitive
    JavaScript syntax: - Number()
    - Number(aValue)
    Argument list: aValue A value to be converted to a number.

    When the Number() constructor is called as a function, it will perform a type conversion.
    The following values are yielded as a result of calling Number() as a function:

    Value                            Result
    Number                            No conversion, the input value is returned unchanged.
    Non numeric string                NaN

    window.Number("23");在favo中查不出来， 但Idea中可以打点打出来。
    */
    var a = 9;
/*下句话如果放在ie8中执行， 必须打开debug工具*/
//    console.log(typeof(a));
    document.writeln(typeof(a));
    var as = String(a);
    //String是Global的方法
    document.writeln("typeof(as) is " + typeof(as));
    var x = window.Number("23");
    document.writeln("typeof(x) is " + typeof(x));
    var age2 = Number("56");
    document.writeln(typeof(age2) + "is typeof(age2)");
    var age3 = new Number(56);
    document.writeln(typeof(age3) + "is typeof(age3)");
    var y = Number("23xyz");
    document.writeln("x=" + x + ",y=" + y + "typeof y is " + typeof(y));
    //-->
</SCRIPT>
</BODY>
</HTML>


输出结果：
number typeof(as) is string typeof(x) is number numberis typeof(age2) objectis typeof(age3) x=23,y=NaNtypeof y is number




例 3.1.2

<head>
    <meta http-equiv="content-type" content="text/html; charset=utf-8"/>
</head>
<script>
    /*javascript support type automatic conversion.the same value is equal */
    document.writeln(6 == "6");
    /*the following requires that value and type should be the same.*/
    document.writeln(6 === "6");
        /*When the Global object is created, it always has at least the following properties:
       Object object       Function object       Array object       String object
       Boolean object       Number object       Date object       Math object
       Value properties
   */
    var obj1 = new window.Object();
    var obj2 = new Object();
    /*Object references must refer to the same object instance.otherwise return false.*/
    document.writeln(obj1 == obj2);
    document.writeln(typeof(obj1) === typeof(obj2));
</script>


输出结果：
true false false true