It's not a polyfill

[rexblack]

This is actually not a polyfill but a ponyfill which then is misleading in the way how to use it.

From a polyfill you would expect to import it like this and all is done:

import 'node-contains';

Instead you will need you use it like this:

import contains from 'node-contains';

if (contains(node, anotherNode)) {
  /* ... */
}

So, please either make it a polyfill by patching Node.prototype or call it a ponyfill.

Of course, best would be to create some kind of documentation... ;-)