JavaScript Functions

JavaScript Functions

A function will be executed by an event or by a call to the function.To keep the browser from executing a script when the page loads, you can put your script into a function. A function contains code that will be executed by an event or by a call to the function. You may call a function from anywhere within a page (or even from other pages if the function is embedded in an external .js file).

Functions can be defined both in the <head> and in the <body> section of a document. However, to assure that a function is read/loaded by the browser before it is called, it could be wise to put functions in the <head> section

Syntax

function functionname(var1,var2,…,varX)
{
some code
}

The parameters var1, var2, etc. are variables or values passed into the function. The { and the } defines the start and end of the function.

Note: A function with no parameters must include the parentheses () after the function name.

Note: Do not forget about the importance of capitals in JavaScript! The word function must be written in lowercase letters, otherwise a JavaScript error occurs! Also note that you must call a function with the exact same capitals as in the function name.

Example

<html>
<head>
<script type=”text/javascript”>
function displaymessage()
{
alert(“Hello World!”);
}
</script>
</head>

<body>
<form>
<input type=”button” value=”Click me!” onclick=”displaymessage()” />
</form>
</body>
</html>

 

If the line: alert(“Hello world!!”) in the example above had not been put within a function, it would have been executed as soon as the page was loaded. Now, the script is not executed before a user hits the input button. The function displaymessage() will be executed if the input button is clicked.

You will learn more about JavaScript events in the JS Events chapter.


The return Statement

The return statement is used to specify the value that is returned from the function.

So, functions that are going to return a value must use the return statement.

The example below returns the product of two numbers (a and b):

Example

<html>
<head>
<script type=”text/javascript”>
function product(a,b)
{
return a*b;
}
</script>
</head>

<body>
<script type=”text/javascript”>
document.write(product(4,3));
</script>

</body>
</html>

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