JavaScript语法之类型转换

柔情只为你懂 2022-07-15 16:44 213阅读 0赞

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