get opening tag including attributes - outerHTML without innerHTML

Question

I would like to retrieve a certain tag element with its attributes from the DOM. For example, from

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Answer 1

For future Googlers, there is a way to do this without jQuery:

tag = elem.outerHTML.slice(0, elem.outerHTML.indexOf(elem.innerHTML));

Since outerHTML contains the opening tag followed by a mirror of what innerHTML contains, we can substring the outerHTML from 0 (the beginning of the opening tag) to where the innerHTML begins (end of opening tag), and since innerHTML is a mirror of outerHTML, except for the opening tag, only the opening tag will be left!

This one works with
tags, tags, and other empty tags:

tag = elem.innerHTML ? elem.outerHTML.slice(0,elem.outerHTML.indexOf(elem.innerHTML)) : elem.outerHTML;

Because innerHTML would be empty in self-closing tags, and indexOf('') always returns 0, the above modification checks for the presence of innerHTML first.