What do I name these things

wat.js

function derp(expansions) {
  var o = {};

  // parse a period-delimited string
  // and use the resulting array to
  // add keys to an object.
  function f1(acc, item) {
    f2(acc, item.split('.'));
    return acc;
  }

  // drills into an object, adding properties
  // along the way. mutates the original object.
  function f2(o, xs) {
    if (xs.length === 0) {
      return;
    }
    else {
      var x = xs.shift();
      o[x] = o[x] || {};
      f2(o[x], xs);
    }
  }

  return _.reduce(expansions.split(','), f1, o);
};

derp('Foo');

/*

{
  'Foo': {}
}

*/

derp('Foo.Bar.Baz,Foo,Qux,Ding.Dong')

/*
{ 
  'Qux': {}, 
  'Ding': { 
    'Dong': {}
  }, 
  'Foo': { 
    'Bar': { 
      'Baz': {}
    }
  }
}
*/

• reconstitute