Javascript基础

alert(typeof(NaN));        //Number
alert(typeof(Infinity));   //Number,Infinity 属性用于存放表示正无穷大的数值。
alert(typeof(null));       //object
alert(typeof(undefined));  //undefined

alert(parseInt("123abc"));            //123
alert(new String("abc") == "abc");    //true
alert(new String("abc") === "abc");   //false

alert("abc" == "abc");                //true
alert("abc" === "abc");               //true

var a = "123abc";
alert(typeof(a++));  //string
alert(a);            //Nan

var a = "123abc";
a.valueOf = function(){
alert("***");   //不会执行到此函数
return parseInt(a);
}
alert(++a); 		//NaN
alert(a-0); 		//NaN

var a = new String("123abc");
a.valueOf = function(){
alert("***");   //会执行到此函数
return parseInt(a);
}
alert(++a); 		//124
alert(a-0); 		//124

var a = new Object();
a.toString = function(){
alert("toString");
return "123abc";
}
a.valueOf = function(){
alert("valueOf");
return parseInt(a);
}
alert(++a);
alert(a-0);

//执行结果：valueOf  toString  124  124
//先进入valueOf方法 再进入toString方法

var s = new String("abc");
alert(typeof(s) == typeof("abc"));  //false   因为 typeof(s) 为object类型   typeof("abc"为string类型
alert(s === "abc");                 //false
alert(s.toString() == s);

var a = new Object();
a.toString = function(){return "a"};
var b = new Object();
b.toString = function(){return "b"};
alert(a>b);      //false
a.valueOf = function(){return 1};
b.valueOf = function(){return 0};
alert(a>b);     //true