Thanks for this - I appreciate your contribution. However, it's quite duplicative. It's possible to accomplish your edits to support jQuery >= 3 with less code, as follows:

1. For the addBack() vs. andSelf() thing, instead of repeating the entire blocks of code, you can just do:

$('.foo')[jquery_ver >= 3 ? 'addBack' : 'andSelf']()

2. For the .error() vs. .fail() part, just use .fail() - that's supported sine v.1.5
3. Also, please don't feel the need to add extra { and }. This is down to coding style, and mine is not to have unnecessary brackets, though I know many would disagree.