1）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);
更多请见：https://blog.csdn.net/qq_44594249/article/details/99077297