Variables

Variables may be defined as Global, or Local.

Variables declared outside a function with or without var are global

Variables declared inside a function without var are global
Variables declared inside a function with var are local to that function

A global and a local variable can have the same name, although not advisable.
The global variable cannot be used in the same function as the local variable with the same name.

Click a function name to see example

var count=0 Count is set both as a global variable
function one(){
var count=1
 
count++
 
}
and a local variable in function one.
The local count value will always be as defined in function one

irrespective of the value of the global count

function two(){

 
} 		Function two will always show the current value of the global count.

function three(){
count++

 
} 		Function three will change the value of the global count each time it is run.

Variables Previous Value

Sometimes it is required that a variables previous value is remembered by the script.
For this example I am using links to show how this works.
Clicking a link below will tell you which link has been selected and which was the last link to be selected.

ONE

TWO

THREE

FOUR

Here you can see the script at work when you press a link above.

<script type="text/javascript">
<!--
a =0
function test(n){
// value of "a" before statement is executed =
alert("link number = " +n+"\n\value_a = "+a)
a = n // value of "a" after statement is executed
}
// -->
</script>

<a href="#" onclick="test(1)">ONE</a>
<a href="#" onclick="test(2)">TWO</a>
<a href="#" onclick="test(3)">THREE</a>
<a href="#" onclick="test(4)">FOUR</a>

A value is passed to the function when it is called.
Variable "a" is not changed until after the statement which means that if variable "a" is used before the statement it will hold the previous value

The above script does not serve any purpose other than to show the principles behind its function.

Script Breakdown

a = 0
Sets the variable a to zero.

function test(n){
The value of argument n is set by the onclick event calling function test(), n being set to the value in the parenthises.

<a href="#" onclick="test(1)">ONE</a>
<a href="#" onclick="test(2)">TWO</a>
<a href="#" onclick="test(3)">THREE</a>
<a href="#" onclick="test(4)">FOUR</a>

alert("link number = " +n+"\n\n a = "+a)
Action, Displays the result, this is purely to show the script at work.

a = n
Sets variable a to equal argument n.

When the script first loads variable a, being outside the function, is set to zero.
Argument n is given a value when the function is called by the onclick event
Variable a is then given the same value as argument n at the end of the function after the functions action.

This means that the value of a can be used in the functions action to determine a previous value of n.

Note:
If you look at the script in the source of this document you might get a bit confused as I have incorporated other functions and actions in order to get the display effects for the examples above.