Global Variables

Global variables are ones defined outside of any functions (or within functions, but defined without the var keyword). They can be defined in the following ways:

    a = 123               // Global scope
var b = 456               // Global scope
if (a == 123) var c = 789 // Global scope

Regardless of whether you are using the var keyword, as long as a variable is defined outside of a function, it is global in scope. This means that every part of a script can have access to it.

Local Variables

Parameters passed to a function automatically have local scope. That is, they can be referenced only from within that function. However, there is one exception. Arrays are passed to a function by reference, so if you modify any elements in an array parameter, the elements of the original array will be modified.

To define a local variable that has scope only within the current function and has not been passed as a parameter, use the var keyword. Example 13-6 shows a function that creates one variable with global scope and two with local scope.

Example 13-6. A function creating variables with global and local scope

<script>
function test()
{
        a = 123               // Global scope
    var b = 456               // Local scope
    if (a == 123) var c = 789 // Local scope
}
</script>

To test whether scope setting has worked in PHP, we can use the isset function. But in JavaScript there is no such function, so Example 13-7 makes use of the typeof operator, which returns the string “undefined” when a variable is not defined.

Example 13-7. Checking the scope of the variables defined in the function test

<script>
test()

if (typeof a != 'undefined') document.write('a = "' + a + '"<br />')
if (typeof b != 'undefined') document.write('b = "' + b + '"<br />')
if (typeof c != 'undefined') document.write('c = "' + c + '"<br />')

function test()
{
    a     = 123
    var b = 456

    if (a == 123) var c = 789
}
</script>

The output from this script is the following single line:

a = "123"

This shows that only the variable a was given global scope, which is exactly what we would expect, given that we gave the variables b and c local scope by prefacing them with the var keyword.

If your browser issues a warning about b being undefined, the warning is correct but can be ignored.