Using Functions

Using Functions
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Using built-in functions

The following lines illustrate the use of built-in functions:

	document.write( "Hello" );
	document.write( Math.sqr( 2 ) );
	document.write( "The bigger of 4 and 5 is : " + Math.bigger(4, 5) );

Using user-defined functions

You can define your own functions in the same file that they are invoked in, or in a different file which you can then load in a browser whenever you wish to use the function. Each of these situations are illustrated below.

Defining and invoking a function in the same file

The following code defines and invokes a function named displayHello:

	<HTML>
	<SCRIPT>
	/////////////////////////////
	/// define function here ///
	/////////////////////////////
	function displayHello()
	{
	document.write( "Hello" )
	}
	/////////////////////////////
	/// invoke function here ///
	/////////////////////////////
	displayHello();
	</SCRIPT>
	</HTML>

The browser output when this HTML file is loaded is as follows:

Invoking a file defined in a different file

Some functions prove very useful; in order to use them in multiple Web pages they can be stored in a separate file. In the example below, the function displayHello has been defined in the file helloFunction.js. The HTML below uses two <SCRIPT> tags, one to load the function definition from helloFunction.js, and the second to invoke the function:

	<SCRIPT SRC="helloFunction.js"></SCRIPT>
	<SCRIPT> <!--
	/// invoke function here ///
	displayHello();
	</SCRIPT> -->

The contents of the file helloFunction.js is simply the JavaScript definition of the function:

	/// define function here ///
	function displayHello()
	{
	document.write( "Hello" )
	}

Notice that helloFunction.js is not an HTML file and does not contain any HTML tags. This is signified by choosing an appropriate file extension — the convention is to use the two-character extension ".js" for JavaScript files.

Executing code using 'eval'

The eval operator expects a String containing JavaScript as an argument, and will execute the String as if it where a JavaScript statement. The code below creates a String named myStatements and then executes the String using eval:

	var myStatements = " var n = 10; alert( n ); n++; alert( n ) " ;
	eval( myStatements );

The result of executing this code is the two alert dialogs: