How synchronous AJAX call could cause memory leak?

Question

I understand this general advice given against the use of synchronous ajax calls, because the synchronous calls block the UI rendering.

The other reason generally given

Answer 1

If the synchronous call is interrupted (i.e. by a user event re-using the XMLHttpRequest object) before it completes, then the outstanding network query can be left hanging, unable to be garbage collected.

This is because, if the object that initiated the request does not exist when the request returns, the return cannot complete, but (if the browser is imperfect) remains in memory. You can easily cause this using setTimeout to delete the request object after the request has been made but before it returns.

I remember I had a big problem with this in IE, back around 2009, but I would hope that modern browsers are not susceptible to it. Certainly, modern libraries (i.e. JQuery) prevent the situations in which it might occur, allowing requests to be made without having to think about it.