JavaScript variable

JavaScript variable

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A variable is a "container" for storing information.

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It's like alge

x=5
y=6
z=x+y

In algege while using letters (such as x) to save values (such as 5).

From the above expression z-x-y, we can calculate that the value of z is 11.

In JavaScript, these letters are called variables.

	You can think of variables as containers for storing data.

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JavaScript variable

As with algetory, JavaScript variables can be used to hold values (such as x-5) and expressions (such as z-x-y).

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

• The variable must begin with a letter
• Variables can also start with the $ and s symbols (although we don't recommend this)
• Variable names are case sensitive (y and Y are different variables)

	Both JavaScript statements and JavaScript variables are case sensitive.

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JavaScript data type

JavaScript variables can also hold other data types, such as text values (name "Bill Gates").

In JavaScript, a text like "Bill Gates" is called a string.

There are many types of JavaScript variables, but for now, we're just looking at numbers and strings.

When you assign a text value to a variable, you should surround it with double or single quotes.

Do not use quotation marks when you assign a value to a variable that is a value. If you surround a value with quotation marks, the value is treated as text.

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Declare (create) JavaScript variables

Creating a variable in JavaScript is often referred to as a "declaration" variable.

We use var keywords to declare variables:

After the variable is declared, the variable is empty (it has no value).

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

However, you can also assign variables when they are declared:

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

Tip: You can practice declaring JavaScript variables in the JavaScript programming practice section of this site.

	A good programming habit is to declare the required variables uniformly at the beginning of the code.

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One statement, multiple variables

You can declare many variables in a statement. The statement starts with var and separates the variables with commas:

Declarations can also span multiple lines:

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Value = undefined

In computer programs, valueless variables are often declared. A variable that is not declared with a value, and its value is actually undefined.

The value of the variable carname will be undefined after the following statement has been executed:

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Re-declarate the JavaScript variable

If you re-declare a JavaScript variable, the value of the variable is not lost:

After the following two statements are executed, the value of the variable carname remains "Volvo":

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JavaScript arithmes

You can do the calculation by using javaScript variables, using operators such as s and s:

You'll learn more about JavaScript operators later in this tutorial.

You can learn about JavaScript arithmes in the JavaScript Programming Practices section.

JavaScript variable learning diagram