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A function is a reusable code-block that will
be executed by an event, or when the function is called.
JavaScript Functions
To keep the browser from executing a script as soon as the
page is loaded, you can write your script as a function.
A function contains some code that will be executed only by
an event or by a call to that function.
You may call a function from anywhere within the page (or
even from other pages if the function is embedded in an external
.js file).
Functions are defined at the beginning of a page, in the
<head> section.
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>
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If the line: alert("Hello world!!"), in the example above had
not been written within a function, it would have been executed
as soon as the line was loaded. Now, the script is not executed
before the user hits the button. We have added an onClick event
to the button that will execute the function displaymessage()
when the button is clicked.
You will learn more about JavaScript events in the JS Events
chapter.
How to Define a Function
The syntax for creating a function is:
function functionname(var1,var2,...,varX)
{
some code
}
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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:
function functionname()
{
some code
}
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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.
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.
Example
The function below should return the product of two numbers
(a and b):
function prod(a,b)
{
x=a*b
return x
}
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When you call the function above, you must pass along two
parameters:
The returned value from the prod() function is 6, and it will
be stored in the variable called product. |
