override existing onkeydown function

Question

My extensions go through every input entered on an on any website that is loaded.

What I do is I consider every onkeydown and manipulate it if it has some value.

Answer 1

Instead of document.onkeydown = returnKey;, use

document.addEventListener('keydown', returnKey, true);

The most important part of this line is the third argument. When the value of this parameter is true, the event listener is triggered at capturing phase, which cannot be prevented using event.preventDefault(); or event.stopPropagation();.