A function
is a group of reusable code which can be called anywhere in your program. This
eliminates the need of writing the same code again and again. It helps
programmers in writing modular codes. Functions allow a programmer to divide a
big program into a number of small and manageable functions.
Like any
other advanced programming language, JavaScript also supports all the features
necessary to write modular code using functions. You must have seen functions
like alert() and write() in the earlier chapters. We
were using these functions again and again, but they had been written in core
JavaScript only once.
JavaScript
allows us to write our own functions as well. This section explains how to
write your own functions in JavaScript.
Function
Definition
Before we
use a function, we need to define it. The most common way to define a function
in JavaScript is by using the function
keyword, followed by a unique function name, a list of parameters (that might
be empty), and a statement block surrounded by curly braces.
Syntax
The basic syntax is shown here.
<script
type="text/javascript">
<!--
function
functionname(parameter-list)
{
statements
}
//-->
</script>
Example
Try the following
example. It defines a function called sayHello that takes no parameters:
<script
type="text/javascript">
<!--
function
sayHello()
{
alert("Hello
there");
}
//-->
</script>
Calling a Function
To invoke a function somewhere later in the
script, you would simply need to write the name of that function as shown in
the following code.
<html>
<head>
<script type="text/javascript">
function
sayHello()
{
document.write
("Hello there!");
}
</script>
</head>
<body>
<p>Click
the following button to call the function</p> <form>
<input
type="button" onclick="sayHello()" value="Say
Hello"> </form>
<p>Use different
text in write method and then try...</p>
</body>
</html>
Output
Click the
following button to call the function
Say Hello
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