Summary: in this tutorial, you’ll learn about JavaScript variables and how to use variables to store values in the application.

A variable is a label that references a value like a number or string. Before using a variable, you need to declare it.

Declare a variable

To declare a variable, you use the var keyword followed by the variable name as follows:

Code language: JavaScript (javascript)

A variable name can be any valid identifier. By default, the message variable has a special value undefined if you have not assigned a value to it.

Variable names follow these rules:

• Variable names are case-sensitive. This means that the message and Message are different variables.
• Variable names can only contain letters, numbers, underscores, or dollar signs and cannot contain spaces. Also, variable names must begin with a letter, an underscore (_) or a dollar sign ($).
• Variable names cannot use the reserved words.

By convention, variable names use camelcase like message, yourAge, and myName.

JavaScript is a dynamically typed language. This means that you don’t need to specify the variable’s type in the declaration like other static typed languages such as Java or C#.

Starting in ES6, you can use the let keyword to declare a variable like this:

Code language: JavaScript (javascript)

It’s a good practice to use the let keyword to declare a variable. Later, you’ll learn the differences between var and let keywords. And you should not worry about it for now.

Initialize a variable

Once you have declared a variable, you can initialize it with a value. To initialize a variable, you specify the variable name, followed by an equals sign (=) and a value.

For example, The following declares the message variable and initializes it with a literal string "Hello":

Code language: JavaScript (javascript)

To declare and initialize a variable at the same time, you use the following syntax:

Code language: JavaScript (javascript)

For example, the following statement declares the message variable and initializes it with the literal string "Hello":

Code language: JavaScript (javascript)

JavaScript allows you to declare two or more variables using a single statement. To separate two variable declarations, you use a comma (,) like this:

Code language: JavaScript (javascript)

Since JavaScript is a dynamically typed language, you can assign a value of a different type to a variable. Although, it is not recommended. For example:

Code language: JavaScript (javascript)

Change a variable

Once you initialize a variable, you can change its value by assigning a different value. For example:

Code language: JavaScript (javascript)

Undefined vs. undeclared variables

It’s important to distinguish between undefined and undeclared variables.

An undefined variable is a variable that has been declared but has not been initialized with a value. For example:

Code language: JavaScript (javascript)

In this example, the message variable is declared but not initialized. Therefore, the message variable is undefined.

In contrast, an undeclared variable is a variable that has not been declared. For example:

Code language: JavaScript (javascript)

Output:

Code language: JavaScript (javascript)

In this example, the counter variable has not been declared. Hence, accessing it causes a ReferenceError.

Constants

A constant holds a value that doesn’t change. To declare a constant, you use the const keyword. When defining a constant, you need to initialize it with a value. For example:

Code language: JavaScript (javascript)

Once defining a constant, you cannot change its value.

The following example attempts to change the value of the workday constant to 4 and causes an error:

Error:

Code language: JavaScript (javascript)

Later, you’ll learn that the const keyword actually defines a read-only reference to a value in the constants tutorial.

Summary

• A variable is a label that references a value.
• Use the let keyword to declare a variable.
• An undefined variable is a variable that has been declared but not initialized while an undeclared variable is a variable that has not been declared.
• Use the const keyword to define a read-only reference to a value.