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 XHR is implemented correctly per spec, then it will not leak:

An XMLHttpRequest object must not be garbage collected if its state is OPENED and the send() flag is set, its state is HEADERS_RECEIVED, or its state is LOADING, and one of the following is true:

It has one or more event listeners registered whose type is readystatechange, progress, abort, error, load, timeout, or loadend.

The upload complete flag is unset and the associated XMLHttpRequestUpload object has one or more event listeners registered whose type is progress, abort, error, load, timeout, or loadend.

If an XMLHttpRequest object is garbage collected while its connection is still open, the user agent must cancel any instance of the fetch algorithm opened by this object, discarding any tasks queued for them, and discarding any further data received from the network for them.

So after you hit .send() the XHR object (and anything it references) becomes immune to GC. However, any error or success will put the XHR into DONE state and it becomes subject to GC again. It wouldn't matter at all if the XHR object is sync or async. In case of a long sync request again it doesn't matter because you would just be stuck on the send statement until the server responds.

However, according to this slide it was not implemented correctly at least in Chrome/Chromium in 2012. Per spec, there would be no need to call .abort() since the DONE state means that the XHR object should already be normally GCd.

I cannot find even slightest evidence to back up the MDN statement and I have contacted the author through twitter.