1.测试变量的数据类型 ----   typeof

"undefined"            -----             如果这个值未定义

2.测试数字是否位于最大数和最小数之间     -----     isFinite()

var result = Number.MAX_VALUE + Number.MIN_VALUE;

alert( isFinite(result));   //false

3.判断一个参数是否"不是数值",它会努力尝试将这个参数转换为数值     ------  isNaN()

alert(isNaN(NaN));           //true

alert(isNaN(10));              //false

alert(isNaN("10"));           //false             

alert(isNaN("blue"));       //true

alert(isNaN(true));           //false

4.非数值转换为数值          ------        Number()   、 parseInt()   、parseFloat()

var num1 = Number("hello world");      //NaN

var num2 = Number(" ");                         //0

var num3 = Number("0000011");          //11

var num4 = Number(true);                      //1

var num5 = parseInt("0xA");                    //10(十六进制数)

var num6 = parseInt("070");                    //56(八进制数)

var num7 = parseInt("1234blue");          //1234

var num8 = parseInt(" ");                          //NaN

var num9 = parseInt(22.5);                      //22

十六进制指定基数可不带0x

var num10 = parseInt("AF" ,  16);            //175

var num11 = parseInt("AF");                     //NaN

指定基数

var num12 = parseInt("10" , 2);                     //2(按二进制进行解析)

var num13 = parseInt("10" , 8);                     //8(按八进制进行解析)

var num14 = parseInt("10" ,10);                    //10(按十进制进行解析)

var num15 = parseInt("10" , 16);                   //16(按十六进制进行解析)

var num16 = parseFloat("1234blue");         //1234整数

var num17 = parseFloat("0xA");   &nb