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Functions
A function is a block of code that will be executed from a call
or when that function is executed. It is a set of JavaScript statements, each
statement separated by a semicolon (;). You can call on the same function
several times from the same script. There are two different types of functions,
user-defined and pre-defined. They are defined in the head of a webpage inside
the script tags. You can then call on them throughout that page. If you want to
use a certain function on more than one webpage, you need to link externally to
the function in the header of each document that uses that function. The
introduction in the course notes tells you how to do this.
Defining a Function
You declare a function by stating function, then the name, then a list of
arguments, followed by the block of code inside the brackets {}. Below in
Figure 7.1 you can see a function named createdON defined that alerts a user
that the webpage was created by Aeolus.
| Figure 7.1 |
<script
language="JavaScript" type="text/javascript">
function createdON()
{
alert("This webpage was created by Aeolus."
}
</script> |
The function above contains no argument set, however a blank set of
parenthesis is still required. The argument set refers to the values that were
passed to the function by a function call from a form or other location within
the document. By placing a function in the head of a document you are ensuring
that all the script will be loading up before the function call takes place.
Return Statement
Sometimes you will want a function to return a value after executing a block of
code. As on a form when a user needs to enter their name, you want the function
to look at the name and determine if the name is valid, and then return true if
it is. Making sure the name has no numbers or erroneous characters in it. Below
in Figure 7.2 is a function that looks at rank to determine if the candidate
holds a valid rank for a commander, and returns true if it is or false if it
isn't.
| Figure 7.2 |
<script
language="JavaScript" type="text/javascript">
function cmdrRankCheck(rank)
{
if (rank == CM || rank == CPT || rank == MAJ || rank == LC || rank ==
COL)
{
return true;
}
else ()
{
return false;
}
}
</script> |
You don't always have to return true or false, you can also return values. If
you were to make a calculator using JavaScript on the web you would want to
return the answer to whatever the user keyed in which would be any series of
numbers.
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