Replace element contents with document fragment javascript

I'm trying to replace all contents of an element with a document fragment:

var frag = document.createDocumentFragment()

The document fragment is being created just fine. No problems there. I add elements to it just fine, no problems there either. I can append it using element.appendChild(frag). That works just fine too.

I'm trying to create a "replace" method similar to jQuery's HTML. I'm not worried about old-browser compatibility. Is there a magical function to replace all content of an element?

I have tried element.innerHTML = frag.cloneNode(true), (as per every'replace element content' wiki I could find), that doesn't work. It gives me <div>[object DocumentFragment]</div>.

No libraries, please, not even a jQuery solution.

For clarity, I'm looking for a "magic" solution, I know how to remove all the existing elements one at a time and then append my fragment.