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.

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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

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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

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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".

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