How to use static variables in a Javascript function

There may be times when a static variable is needed in a Javascript function; static variables maintain their value between function calls and are tidier than using a global variable because they cannot be modified outside of the function.

Example

The easiest way to explain how to do this is with an example. The following example function is called "foo" and it has a static variable called "counter". Each time it is called the variable is incremented and written to the document.

function foo() {

    if( typeof foo.counter == 'undefined' ) {
        foo.counter = 0;
    }
    foo.counter++;
    document.write(foo.counter+"<br />");
}

When foo() is first called, foo.counter has not yet been defined. The first line in the example function above checks to see if it’s not yet defined and if not initialises it with a value.

Calling foo() multiple times like so:

foo();
foo();
foo();
foo();

would write this to the document:

1
2
3
4