whatever.js

// Based on https://web.archive.org/web/20160310120458/http://wiki.ecmascript.org/doku.php?id=strawman:scoped_object_extensions

// in whatever-extras.js
import Whatever from './whatever.js';

export extension Extensions = Whatever.protoype {
  somethingComplex: function() { ... }
}

// in user code
import Whatever from './whatever.js';
import extension Extensions from './whatever-extras.js'

const whatever = new Whatever();
whatever.commonThing(123);
whatever.rareAndComplexThing(321);