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2. JS variable type conversion, automatic conversion, manual conversion, what is the difference between parseint(), parsefloat(), number()?

2022-07-24 18:21:00 【yingxingyf】

2.js Variable type conversion 、 Automatic conversion 、 Manual switching 、 Excuse me, parseInt(),parseFloat(),Number() The difference between ？

1） Implicit （ Automatically ） transformation , When different data types participate in the expression operation process, they will be converted to the same type for operation 
 Implicit conversion rules for string and numeric type operations ：
	1. String plus number , The number will be converted into a string .
	2. Number minus string , Turn a string into a number . If the string is not a pure number, it will be converted to NaN. String minus number is the same . two 
	 The subtraction of a string is first converted to a number .
	3. ride , except , Greater than , The same is true for the conversion of less than and minus .
	
2） Show （ Manual ） transformation 
	 String to value ：
	parseInt()、parseFloat()、Number()
	 Value to string :
	toString()
 notes ：NaN It's not a number , But it is still a numeric type , not a number,NaN yes Number type .

<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
		
	</head>
	<body>
		
	</body>
</html>
<script type="text/javascript">
//js Data type conversion :  When operations occur between different data types , You need to convert data types first , And then calculate .
//1、 Automatic conversion 
//1）、 +
/*
var str=" Age is ："+ 20;// The first 20 Convert to string type , And then we'll do the stitching 
alert(str);
*/
//2）、-
//var str="25" - 20;// The first "25" Turn to number type , Then subtract ;
//var str="ab"-20; // The first "ab" Turn to number type , Then subtract ; because "ab" Can't be converted to numbers , That's it NaN(not a number)
//alert(str);

//3）、 ride , except , Greater than , The same is true for the conversion of less than and minus .

//2、 Manual switching 
//1)、 Turn a string into a number 
// parseInt();// Convert a string to an integer 
// parseFloat();// Convert string to floating point （ decimal ）
// number();// Turn a string into a number 

//parseInt():
// Conversion rules ： Start with the first character of the string ,
// If the first character is non numeric , So the result is NaN;
//  If the first character is a number , Then keep looking back , Until you encounter the first non number or the end . Turn all the previous contents into numbers .
/*
//var str="250";//250
//var str="250.61.8";//250
//var str="250.612a.8";//250
//var str="a250.612a.8";//NaN

var num = parseInt(str);// hold str To integer ;
alert(typeof num);
alert(num);
*/

//parseFloat():
// Conversion rules ： Follow parseInt Close to ;
// Start with the first character of the string ,
// If the first character is non numeric , So the result is NaN;
//  If the first character is a number , Then keep looking back ,
//  Encounter the first decimal point , Then normal conversion ,
//  Meet the second decimal point or other non numeric or ending . Turn all the previous contents into numbers .

//var str="250.6";//250.6
//var str="250.61.8";//250.61
//var str="250.612a.8";//250.612

//var str="a250.612a.8";//NaN
/*
var num = parseFloat(str);
alert(typeof num);
alert(num);
*/

//Number():
//Number() The rules of transformation ：
//Number() Look at the whole , As long as the content in the string is not a legal number , It would be NaN;

//var str="250.6";
//var str="250";   
//var str="250.61.8";//NaN
//var str="250.612a.8";//NaN
//var str="a250.612a.8";//NaN

/*
var num = Number(str);
alert(typeof num);
alert(num);
*/

//2)、toString()

var age = 12;
var str = age.toString();
alert(typeof str);


/*----------------------------------- Interview questions --------------------------------------------*/
/*
// Interview questions ： Excuse me, parseInt(),parseFloat(),Number() The difference between ？
// answer ：
  1、parseInt() String to integer ,parseFloat() Convert string to floating point ,Number() Convert string to numeric ;
  2、
  Number(): Look at the whole , As long as the content in the string is not a legal number , The result is NaN; otherwise , It will be converted to numeric type normally ( Integer or floating point ).
  parseInt() and parseFloat() The conversion rules of are similar （ similar ）;
   Front to back , If the first character is not a number , that , The result is NaN; If the first character is a number （ Keep looking back ）：
   1）、parseInt()： If you encounter a decimal point or other non numeric characters or endings , Then convert the previous content into numbers 
   2）、parseFloat()： If you encounter the second decimal point or other non numeric characters or endings , Then convert the previous content into numbers 

  */ 


</script>
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