Question

So I realize that in no way do I want to do:

Element.protoype.myfunc = function () {}

But, is this the same or not and is this a good practice?

var e = document.querySelector(q);
    e.html = function (html) {
        this.innerHTML = html;
    }
e.html("Am I in trouble?");

Answer 1

Extending Element will not work in all browsers (notably IE<8). See also this SO question

Extending single elements may result in memory leaks: if such elements are deleted, the method can still exist, containing a link to the non existent element. See this link (it’s about handler methods, but it can also apply to extension methods afaik).