call a javascript method on click event of buttons in CKEditor toolbar

i have a requirement to call a javascript method on the click event of the Undo button of CKEditor toolbar.

i've used the bellow code but its not working.

CKEDITOR.on('Undo', function(){
test();
});

function test(){
var flagA = 0;
}

i need a solution for : in which file and where i've to call that method ?