Help with todo app function

Hey guys, just creating a simple todo app for some practice. However I cannot seem to get my .toggleItemState() function to work. I know “this” is undefined right now but I don’t know why or how i would bind it to the dblclicked “li” element properly. I also cannot figure out how to get the index of that listItem ill be using as a parameter (so I can change it’s “completed” property).

Any other feedback would also be greatly appreciated.