Introduction to variable data types
#Variables have data types. This type comes from the "value of the variable", in other words: value What type is the variable?
The types of variables in JS are:
Numeric type, character type, Boolean type, undefined, null, array, object, function
This Eight data types, divided into two major categories:
- ##Basic data types: numerical, character, Boolean Type, undefined type, empty type. Very notable feature: a variable name can only store one value.
Example: var a = 10;
- Composite data types: arrays, objects, functions. Salient feature: a variable name may store multiple values.
Example: var arr = [10,20,30,40]
In this section we introduce the basic data types, and the composite data types will be discussed later. The chapter introduces in detail
Numerical type: the
numerical value that the variable can perform mathematical operations on Types include: integer, floating point, NaN.
var a = 100;
var a = 0.9;
var a = 0;
Note: There is another very special value type The value is NaN. NaN (not a number) is not a number.
When converting other data types into numeric types and it cannot be converted, but the program cannot report an error, a NaN value will be returned.
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>php.cn</title>
<script>
//现在我们想让一个东西的长度变成原来的10倍
var length = "300m";
/*
一个字符串，是不能转换成有意义的数值的，只能转换成NaN
一个含纯数字的字符串，可以转成有意义的数值，大家可以修改length为纯数字的字符串，输出查看结果
*/
length = length*10;
document.write(length);
</script>
</head>
<body>
</body>
</html>
Character type: a string enclosed in single quotes or double quotes.
var a = "This is a string";
var b = "This is also a string";
var c = “”;
Single quotes and double quotes can be nested inside each other
- Only double quotes can be nested within single quotes;
- Only single quotes can be nested within double quotes.
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>php.cn</title>
<script type="text/javascript">
var name = "小明";
//加号为字符串连接符，我们之后会介绍
var str = "我的名字叫做'" +name+"'"
document.write(str)
</script>
</head>
<body>
</body>
</html>
If you want to nest double quotes within double quotes, the double quotes inside must be escaped (\").
In JS The escape character is backslash (\).
Commonly used escape characters are: \', \”, \\, \r, \n, etc.
That is, when the browser encounters a backslash (\), it will treat the character after it as a normal character. The so-called "normal" characters are a, b, c, &, etc.
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>php.cn</title>
<script type="text/javascript">
var name = "小明";
//加号为字符串连接符，我们之后会介绍
var str = "我的名字叫做\"" +name+"\""
document.write(str)
</script>
</head>
<body>
</body>
</html>
Boolean type
Boolean type is also called logical type. There are only two values: true (true), false (false).
Boolean type has only two states. Such as: gender, marital status, light switch, whether it is blacklisted, etc.
var a = true;
var b = false;
Boolean type is commonly used in if conditional judgment statements (we will introduce conditional judgment statements in the following chapters, everyone first observes the results )
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>php.cn</title>
<script type="text/javascript">
var x = 10;
var y = 110;
//x>y比较出来的结果是布尔值
if(x>y){
document.write(x+"比"+y+"大些");
}else{
document.write(y+"比"+x+"大些");
}
</script>
</head>
<body>
</body>
</html>
Undefined type
When a variable is defined but not assigned a value, an undefined type will be returned. The value of an undefined type has only one undefined.
When the property of an object does not exist, undefined type is also returned.
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>php.cn</title>
<script type="text/javascript">
var x;
document.write(x);
</script>
</head>
<body>
</body>
</html>
##empty type
When an object does not exist, a null type will be returned, and the value of the null type is only null.
can also be understood as: a placeholder for an object.
If you want to clear the value of a variable, you can assign a null value.
var a = 100;
var a = null ; //Assign a null to a variable to clear its value
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>php.cn</title>
<script type="text/javascript">
var x = null;
document.write(x);
</script>
</head>
<body>
</body>
</html>
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