run jquery function on element. what is wrong with this code?

Question

What is wrong with this code?

$(function() {
    function testfunction() { $(this).addClass('testing');}
    $('.tester').testfunction();
});

Answer 1

testfunction() is not added to the jQuery function stack.

If you want to be able to call it on an arbitrary object, you should add it to the jQuery function stack:

$.fn.testfunction = function() {
   this.addClass('testing');
};

$('.tester').testfunction(); // success!

You should take a look at jQuery's Plugins/Authoring page for more information on how to properly write plugins.