code: function makeMap (
    str,
    expectsLowerCase
  ) {
    const map = Object.create(null);
    const list = str.split(',');
    for (let i = 0; i < list.length; i++) {
      map[list[i]] = true;
    }
    return expectsLowerCase
      ? val => map[val.toLowerCase()]
      : val => map[val]
  }
docs: Make a map and return a function for checking if a key
  is in that map.