Difference between revisions of "Understanding JavaScript Functions"
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Revision as of 18:36, 24 April 2007
JavaScript function provide a way to organize scripts and promote reuse. Rather than create longs sections of script scattered throughout an HTML page the script is instead organized into logical groups. JavaScript functions are simple to use, in fact you only need to do two things with a function, declare it and call it.
Declaring and Calling a JavaScript Function
The syntax of a JavaScript function is as follows:
function ''name'' ( ''argument1'', ''argument2'', ''argument3'' ... )
{
''statements''
return ''value''
}
The name specifies how we will refer to this function when we call it. The arguments specify values that are passed through to the function to be processed. The statements section represents the body of the function that performs the processing. The optional return statement allows a value to be returned the section of scripted that called the function (for example to return a status or the result of a calculation.
Arguments and return values are not mandatory. the following simple example shows a function defined in the <head> section of an HTML page and a call to this function in the <body>:
<html>
<head>
<title>A Simple JavaScript Function Example</title>
<script language="JavaScript" type="text/javascript">
function sayHello()
{
alert("Hello!");
}
</script>
</head>
<body>
Calling a function
<script language="JavaScript" type="text/javascript"> sayHello( "24th", "July" ); sayHello ( "1st", "August" ); sayHello ( "24th", "May" ); </script>
</body> </html>
</pre>
In this example the function is called multiple times, passing through arguments that are then used in the function to construct a string to bne displayed in the alert dialog. To achieve this without functions it would ne necessary to repeat the script in the function 3 times over in the body. Clearly placing the script ina function and re-using it over and over is a much more efficient approach.
Next we need to look at how a function might return a value.
Calling a function
<script language="JavaScript" type="text/javascript"> sayHello( "24th", "July" ); sayHello ( "1st", "August" ); sayHello ( "24th", "May" ); </script>
</body> </html>
</pre>
Calling a function
<script language="JavaScript" type="text/javascript"> sayHello( "24th", "July" ); sayHello ( "1st", "August" ); sayHello ( "24th", "May" ); </script>
</body> </html>
</pre>
In this example the function is called multiple times, passing through arguments that are then used in the function to construct a string to bne displayed in the alert dialog. To achieve this without functions it would ne necessary to repeat the script in the function 3 times over in the body. Clearly placing the script ina function and re-using it over and over is a much more efficient approach.
Next we need to look at how a function might return a value.
Returning a Value from a Function
Calling a function
<script language="JavaScript" type="text/javascript"> sayHello( "24th", "July" ); sayHello ( "1st", "August" ); sayHello ( "24th", "May" ); </script>
</body> </html>
</pre>
In this example the function is called multiple times, passing through arguments that are then used in the function to construct a string to bne displayed in the alert dialog. To achieve this without functions it would ne necessary to repeat the script in the function 3 times over in the body. Clearly placing the script ina function and re-using it over and over is a much more efficient approach.
Next we need to look at how a function might return a value.
