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 14-10 shows a function
that creates one variable with global scope and two with 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 isn’t one, so let’s make our own—it’s sure to come in handy in
the future—with Example 14-11.
<script> function ...
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