javascript 类型转换语法

1、转字符串类型：String(v)或v.toString()

2、转数字：Number(v)

3、自动转换：连接符+的一方是字符串，则另一方也会被转成字符串；其它算术运算符会把另一方转成数字。

//number to string
var str=String(123);
var str2=String(100*2);
document.write(str,"
",str2,"
");
num=3.141592627;
str=num.toString();
document.write(str,"
");
str=num.toFixed(2);//3.14
document.write(str,"
");
str=num.toPrecision(4);//3.142
document.write(str,"
");
num/=100;
str=num.toExponential(3);//3.142e-2
document.write(str,"
");
 
//boolean to string
str=String(true);
document.write(str==="true","
");
str=false.toString();
document.write(str==="false","
");
 
//Date to string
str=String(Date());
document.write(str,"
");
str=Date().toString();
document.write(str,"
");
//get year
var date=new Date();
document.write(date.getFullYear(),"year
");
//get month
document.write(date.getMonth()+1,"month
");
//get date
document.write(date.getDate(),"date
");
//get hour
document.write(date.getHours(),"hour
");
//get minute
document.write(date.getMinutes(),"minute
");
//get second
document.write(date.getSeconds(),"second
");
//get millisecond
document.write(date.getMilliseconds(),"millisecond
");
//get all milliseconds
document.write(date.getTime(),"time
");
//get day in week
var day=date.getDay();
switch(day){
    case 0:
        document.write("sunday
");
        break;
    case 1:
        document.write("monday
");
        break;
    case 2:
        document.write("Tuesday
");
        break;
    case 3:
        document.write("Wednsday
");
        break;
    case 4:
        document.write("Thursday
");
        break;
    case 5:
        document.write("Friday
");
        break;
    case 6:
        document.write("Saturday
");
    default:
        document.write("error day
");
        break;
}
 
//string to number
num=Number("3.14");
document.write(typeof(num),num,"
");
num=Number(" ");
document.write(typeof(num),num,"
");//0
num=Number("hello");
document.write(typeof(num),num,"
");//NaN
num=+"3.14";
document.write(typeof(num),num,"
");//3.14
str="hello";
num=+str;
document.write(typeof(num),num,"
");//NaN
 
 
//boolean to number
document.write(Number(true),"
");
document.write(Number(false),"
");
 
//Date to number
document.write((new Date()).getTime(),"
");
document.write(Number(new Date()),"
");
 
//auto transform
document.write(5+null,"
");//null to 0
document.write("5"+null,"
");//null to "null"
document.write("5"+1,"
");//1 to "1"
document.write("5"*2,"
");//"5" to 5