JavaScript variables

Just like algebra

x=5
y=6
z=x+y

In algebra , we use letters (such as x) to hold values (such as 5).

Through the above expression z=x+y, we can calculate the value of z to be 11.

In JavaScript, these letters are called variables.

Note: You can think of variables as containers for storing data.

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JavaScript Variables

Like algebra, JavaScript variables can be used to store Values (such as x=5) and expressions (such as z=x+y).

Variables can use short names (such as x and y) or more descriptive names (such as age, sum, totalvolume).

• Variables must start with a letter

• Variables can also start with $ and _ symbols (but we do not recommend this)

• Variable names are case-sensitive (y and Y are different variables)

Note: JavaScript statements and JavaScript Variables are case-sensitive.

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Declaring (creating) JavaScript variables

Creating variables in JavaScript is usually called "declaring" a variable.

We use thevarkeyword to declare variables:

var carname;

Variable declaration After that, the variable is empty (it has no value).

To assign a value to a variable, use the equal sign:

carname="Volvo";

However, you can also declare a variable when Assign a value to it:

var carname="Volvo";

In the following example, we create a variable named carname and assign it the value "Volvo" ", and then put it into the HTML paragraph with id="demo":

     
  
点击这里来创建变量，并显示结果。
 点击这里 
   

Run and try it

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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";

Statement It can also span multiple lines:

var lastname="Doe",
age=30,
job="carpenter";

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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;