Introduction to the concept of JavaScript variables

The concept of variable

A variable is a quantity that changes.
A variable can be seen as a container that holds data
A variable can be seen as an "unknown number". x = 10
The variable can be regarded as a "symbol" and "code".
Variables generally refer to program data.
Variables exist and operate in memory.
Variables are temporary data. We can think of computer memory as "small grids" one by one. Each "small grid" can store the name of a variable and the value of the variable.

Declaration of variables

The declaration of variables is equivalent to booking a "room" in a hotel.
Syntax format: var variable name = variable value
Variables are declared using the system keyword var.
Example:

var name; //Declare a variable
var name,like,age; //Declare multiple variables at the same time, separated by commas in English
var name = "Xiao Ming"; //On one side Declaration side assignment

Naming rules for variables

Variables The name can contain letters, numbers, and underscores.
Variable names cannot start with numbers, but can start with letters or underscores. Such as: var _name; (correct) var 3abc; (wrong)
The variable name cannot be a system keyword. Such as: var, switch, for, try, case, else, while, etc.
Variable names in JS are case-sensitive. For example: name and Name are two variables
The names of variables in JS must be meaningful.
If the variable name consists of multiple words, how should it be expressed?

"Camel case naming": the first word is all lowercase, and the first letter of each subsequent word is capitalized. For example: var getUserName
"Underline naming": all words are lowercase and connected with underscores in the middle. For example: var get_user_name

Assign a value to the variable

Variable assignment is to put things into "space".
Use the assignment sign "=" to assign a value to a variable.

Syntax: var variable name = variable value

Example: var name = "Xiao Ming";

Note: "＝" understand

Assign the "operation result" on the right side of "=" to the variable name on the left.
The right side of "=" should be operated first, and then the result of the operation is assigned to the variable on the left.
The left side of "=" can only be a variable name, not an "operational expression".

<!DOCTYPE HTML>
<html>
    <head>
        <meta charset="utf-8">
        <title>php.cn</title>
        <script>
            //声明变量x，然后把8赋值给x
            var x = 8;
            //声明变量y，然后把x+80的结果赋值给y
            var y = x+80;
            //分别输出x，y
            document.write(x);
            document.write(y);
        </script>
    </head>
    <body>
    </body>
</html>

Note:

Every statement in JS usually ends with a semicolon (;) in English. This semicolon is not required. In order to be compatible with PHP, it is best to add a semicolon.
Operators, variables, and operations can be separated by spaces, making such a program easier to read.

var a = 100 and var a=100 are the same

Continuing Learning

new file

<!DOCTYPE HTML> <html> <head> <meta charset="utf-8"> <title>php.cn</title> <script> //声明变量x，然后把8赋值给x var x = 8; //声明变量y，然后把x+80的结果赋值给y var y = x+80; //分别输出x，y document.write(x); document.write(y); </script> </head> <body> </body> </html>

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Introduction to the concept of JavaScript variables

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