var str = "";
alert(typeof (str));//string
str = ;
alert(typeof (str));//number
var sum = str + ; //+编程两个数字相加
alert(sum);// 

var sum = "" + "";
alert(typeof (sum)); //string
alert(sum);// 
var sum = "" + ;
alert(typeof (sum)); //string
alert(sum);//
　　　　　
　　　　　var sum = + "";
　　　　　alert(typeof (sum));//string
　　　　　alert(sum);
var area = "" * "";
alert(typeof (area)); //number
alert(area);//
var sub = "" - "";
alert(typeof (sub)); //number
alert(sub);//
var div = "" / "";
alert(typeof (div));//number
alert(div);//. 

var area = "" * "a";
alert(typeof (area));//number
alert(area);//NaN
var sub = "a" - "";
alert(typeof (sub));//number
alert(sub);//NaN
sub = "a" - ;
alert(typeof (sub));//number
alert(sub);//NaN
var div = "" / "a";
alert(typeof (div));//number
alert(div);//NaN
div = "a" / ;
alert(typeof (div));//number
alert(div);//NaN 

var a = true;
alert(typeof(a));//boolean
var b = "true";
var ab = a + b;
alert(typeof (ab));//string
alert(ab); //truetrue
alert(a == b);//false 

var c = "";
alert(typeof(c));//string
var d = ;
alert(typeof (d));//number
alert(c == d);//true 

var a = null;
alert(typeof (a));//object
var b = "hello";
var ab = a + b;
alert(typeof (ab));//string
alert(ab); //nullhello
var c = ;
var ac = a * c;
alert(typeof (ac)); //number
alert(ac);//
if (a ) //false
{
} else
{
alert("false");
}
var u;
alert(a == u); //true