JavaScript Multi-Line Comments

Multi line comments start with /* and end with */.
The following example uses a multi line comment to explain the code:

Example <script type="text/javascript"> /* The code below will write one heading and two paragraphs */ document.write("<h1>This is a heading</h1>"); document.write("<p>This is a paragraph.</p>"); document.write("<p>This is another paragraph.</p>"); </script>

                                          

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Using Comments to Prevent Execution

In the following example the comment is used to prevent the execution of a single code line (can be suitable for debugging):

Example <script type="text/javascript"> //document.write("<h1>This is a heading</h1>"); document.write("<p>This is a paragraph.</p>"); document.write("<p>This is another paragraph.</p>"); </script>

In the following example the comment is used to prevent the execution of a code block (can be suitable for debugging):

Example <script type="text/javascript"> /* document.write("<h1>This is a heading</h1>"); document.write("<p>This is a paragraph.</p>"); document.write("<p>This is another paragraph.</p>"); */ </script>

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Using Comments at the End of a Line

In the following example the comment is placed at the end of a code line:

Example <script type="text/javascript"> document.write("Hello"); // Write "Hello" document.write(" Dolly!"); // Write " Dolly!" </script>

JavaScript Variables

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Variables are "containers" for storing information.

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Do You Remember Algebra From School?

Do you remember algebra from school? x=5, y=6, z=x+y
Do you remember that a letter (like x) could be used to hold a value (like 5), and that you could use the information above to calculate the value of z to be 11?
These letters are called variables, and variables can be used to hold values (x=5) or expressions (z=x+y).

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JavaScript Variables

As with algebra, JavaScript variables are used to hold values or expressions.
A variable can have a short name, like x, or a more descriptive name, like carname.
Rules for JavaScript variable names:

• Variable names are case sensitive (y and Y are two different variables)
• Variable names must begin with a letter or the underscore character

Note: Because JavaScript is case-sensitive, variable names are case-sensitive.

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Example

A variable's value can change during the execution of a script. You can refer to a variable by its name to display or change its value.
This example will show you how

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Declaring (Creating) JavaScript Variables

Creating variables in JavaScript is most often referred to as "declaring" variables.
You can declare JavaScript variables with the var statement:

var x; var carname;

After the declaration shown above, the variables are empty (they have no values yet).
However, you can also assign values to the variables when you declare them:

var x=5; var carname="Volvo";

After the execution of the statements above, the variable x will hold the value 5, and carname will hold the value Volvo.
Note: When you assign a text value to a variable, use quotes around the value.

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Assigning Values to Undeclared JavaScript Variables

If you assign values to variables that have not yet been declared, the variables will automatically be declared.
These statements:

x=5; carname="Volvo";

have the same effect as:

var x=5; var carname="Volvo";

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Redeclaring JavaScript Variables

If you redeclare a JavaScript variable, it will not lose its original value.

var x=5; var x;

After the execution of the statements above, the variable x will still have the value of 5. The value of x is not reset (or cleared) when you redeclare it.

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JavaScript Arithmetic

As with algebra, you can do arithmetic operations with JavaScript variables:

y=x-5; z=y+5;

You will learn more about the operators that can be used in the next chapter of this tutorial.