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First week of JS study

2022-07-02 08:37:00 【Zhangzanhao】

Catalog

Output input box

Variable

data type

Output input box

The output and input boxes are the pages that will pop up after you open the web page

The input box is that you can answer the questions asked by the program ape in the input field

The output box will only pop up a paragraph , You can only confirm or cancel, not enter

And one of the console.log Is to open your own web page , Press down F12 The buttons , Click the console to see

<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<script type="text/javascript">
			prompt(' This is an input box ')
			alert(' This is an output box ')
			console.log(' This is what programmers can see ');
		</script>
	</head>
	<body>
	</body>
</html>

Variable

One , Variables are boxes for storing data
Two , Variables occupy memory space , Like opening a new space in memory
3、 ... and , The variable name is the room number of each space , So variables have spaces and variable names
Four , Declaration of non assignment has undefind If you do not declare or assign a value, an error will be reported Assignment without declaration can be used

How to declare variables ？ How to assign a value ？ Look at the code below

<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<script>
            var age;
			//  Declare variables , Open up space 
			age = 18;
			//  assignment , Bring the data to the room 
			
			var mynane = ' Kakasi ';
			var age = 30;
			var email = '[email protected]';
			var gz = 2000;
			
			//  Simple declaration and assignment 
			var youname = prompt(' Please enter your name ')
			
			//  Assign multiple values at one time 
			var hisname = 'kkk',
				hername = 'lll';
			
			//  The name of the variable   Cannot start with a number   You can't use keywords   Strictly distinguish case   Hump nomenclature  myFirstName name It has a special meaning   Only $ and _ Two symbols begin 
			
			var a = 1,
				b = 2;
			//  Exchange data of two variable names 
			var c = 0;
			c = a
			a = b
			b = c
        </script>
	</head>
	<body>
		<div></div>
	</body>
</html>

data type

In fact, the data type is similar to python The data types are similar , Not much here

For more details, see the following code

<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<script>
			var num; // there num We don't know what data type it is 
			var num = 10; //num It belongs to digital type 
			
			// js It's dynamic language , The data types of variables can vary 
			var numo = 10;
			numo = ' character string String'
			//  This is wrong in other languages , But here you can 
			
			//  Simple data type   Digital （ Integers and floating point numbers ）  Boolean type   String type  Undefined（ No assignment ） Null（ Null value ）
			
			//  Digital number
			//  Digital   Table octal   Add... Before the number 0    Hexadecimal   Add... Before the number 0x
			// Infinity -Infinity NaN 
			// isNaN()  Judge whether it is a number   If it's a number, go back False   Not just back Ture
			console.log(isNaN(12))
			
			//  character string String
			//  String escape character  \t Indent  \n Line break 
			console.log(' I am a \n Grosvenor LTD handsome ')
			//  Output string length  length
			var myname = 'his is my name';
			console.log(myname.length);
			//  String splicing   As long as there is a character type, no matter what is behind it, you can connect 
			console.log( 12 + 'pink')  // '12pink'
			//  String concatenation variables 
			var age = 18;
			console.log('pink' + age + ' year ')  // 'pink18 year '  '+age+' Abbreviation 
			
			var age = prompt(' How old are you this year ')
			var str = ' You have been ' + age + ' Years old! ';
			alert(str);
			
			//  Boolean type boolean
			//  When Boole True Participate in the operation when 1 Look if 
			// Undefined  That is, the unassigned variables and strings are added together , Add it to the number and it becomes NaN
			// Null  That is, the variable is Null Add it to the string and connect it , Adding to a number is equivalent to 0, Nothing 
			//  Verification type 
			var num = 10
			console.log(typeof num);  //number
			// prompt  The retrieved value is a string 
			
			
			//  Data type conversion 
			//  One . Number type changes to character type 
			var age = 18.2;
			var str = age.toString();
			console.log(typeof str)
			
			console.log(String(age))
			console.log(typeof age)+.3
			
			console.log(age + '')  //  Turned into 'age'   Implicit variant 
			
			//  Two . Character type changes to number type 
			var num = parseInt(age)  //  It's round 3.14 change 3  If you start with a number, you will remove the following letters   No, the beginning of the number will become NaN
			console.log(num)
			
			var num = parseFloat(age)  //  Will change decimal , Floating point numbers   If you start with a number, you will remove the following letters   No, the beginning of the number will become NaN
			console.log(num)
			
			console.log(age - 0)  //  by 18.2     Implicit conversion 
			console.log('123'-'100') // 23  The plus sign closes , The minus sign will go to the string and then subtract 
		</script>
	</head>
	<body>
		<div></div>
	</body>
</html>

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