JavaScript variable declaration
JavaScript variables are used to store characters, numbers, arrays, and even object resources, etc., so that they can be used where we need them.
Declare (create) a variable through the var keyword:
var variable name;
JavaScript variable naming rules
Variable names start with letters (a-z, A-Z) or underscores _, and can be followed by any letters or numbers and underscores, but not spaces. Additionally, variable names are case-sensitive.
The following variable names are valid:
var_char
varChar
_varChar
char5
Tip
# Regarding variable naming, we recommend that the variable name should be a combination that indicates its actual semantics, such as my_name or myName format.
JavaScript variable assignment
You can assign a value to a variable while declaring it:
var my_name = "Jack"; // Text character variable, enclosed in double quotes
var number = 2; // Numeric variable
In fact, JavaScript also supports directly assigning variables Assign a value without declaring the variable in advance:
my_name = "Jack";
number = 2;
The assigned variable will be automatically declared, But declaring a variable in advance is a good programming habit.
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>php中文网(php.cn)</title>
<script type="text/javascript">
var a = 3;
var b = 4+a;
window.alert(b);
</script>
</head>
<body>
</body>
</html>
One statement, multiple variables
You can declare many variables in one statement. The statement starts with var and uses commas to separate variables:
var lastname="Doe", age=30, job="carpenter";
The statement can also span multiple lines ：
var lastname="Doe",
age=30,
job="carpenter";
Value = undefined
In computer programs, variables without value are often declared. The value of a variable declared without a value is actually undefined.
After executing the following statement, the value of variable carname will be undefined:
var carname;
JavaScript arithmetic
You can do arithmetic with JavaScript variables, using operators like = and +:
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>php中文网(php.cn)</title>
<script type="text/javascript">
var a = 3;
var b = 4+a;
window.alert("b =" + b);
</script>
</head>
<body>
</body>
</html>
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