To return a function, you simply... return a function!! :)

For example:

function MagicFunction(...a) {
  function toBeReturned(...a) {
    do {
      // this code...
    } while (somethingIsTrue);
    return something;
  }

  return toBeReturned;    // This is how to return a function..
  return toBeReturned();  // This would be, instead, returning a function invocation..
}