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Variables in JavaScript

In the previous tutorial we have discussed about comments. In this tutorial we will discuss about variables.

After this tutorial you will be able to

      1. What are variables? How to use it JavaScript application?

      2. Rules for creating variables.

      3. Global variables and local variables.

Let us start with the tutorial. Variables are the containers for storing information. In JavaScript variables are declared using var keyword.

For example

var author_name;

Let us take the example

		   
    <html>
      <head>
        <title>Variable example</title>
        <script language="JavaScript">	
	          function ClickMe()
	          {
	              var alert_text = "Hi, this is the variable tutorial";
                alert(alert_text);
	          }				
        </script>
      </head>
      <body>
          <form>
            <input type="button" name="btnComments" value="Click Me" onclick="javascript:ClickMe();">
          </form>
      </body>
    </html>

In the method ClickMe() I have created a variable called alert_text for storing a string data.

After this example let us see what are the rules we should keep in mind while creating variables.

1. Variable name should not be a keyword. In the above example if we rewrite the function like

		   
    function ClickMe()
    {
       var new = "Hi, this is the variable tutorial";
       alert(new);
    }

As new is the keyword in JavaScript. If you execute this function you will get an error "Object Expected".

2. Variable name should always start with alphabet or underscore(_). Let us see what will happen if we change the variable name from alert_text to 00alert_text.

		   
    function ClickMe()
    {
        var 00alert_text = "Hi, this is the variable tutorial";
        alert(00alert_text);
    }

When you run the application you will again encounter the error "Object Expected". Now change the variable name to _alert_text and the program will run.

3. Variable names are case senstive so alert_text and Alert_text are 2 different variables.

Global and local variable

1. A local variable is visible within the function where it is defined.

2. A global variable is visible through out the page.

To explain it let us create an example.

		   
    var global_variable = "This is the global variable";
    function ClickMe()
    {
        var local_variable = "This is the local variable";
        alert(local_variable);
        ClickMeAgain();
    }
    function ClickMeAgain() 
    {
        alert(global_variable);
    }

As you can see in the above example the use of global and local variable. The output of this will be "This is the local variable" and "This is the global variable" alert.

Posted By -	Karan Gupta

Posted On -	Wednesday, December 01, 2010

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