How to replace last occurrence of characters in a string using javascript

I'm having a problem finding out how to replace the last ', ' in a string with ' and ':

Having this string: test1, test2, test3

and I want to end out with: test1, test2 and test3

I'm trying something like this:

var dialog = 'test1, test2, test3';    
dialog = dialog.replace(new RegExp(', /g').lastIndex, ' and ');

but it's not working