How to use Private Methods in JavaScript

Interestingly, just defining initializeSomethingElse within Cell only results in ~10% slowdown, and that yields a truly private method. E.g.

    function Cell(x, y) {
      var self = this;
      self.x = x;
      self.y = y;
      self.things = [1, 2, 3];

      // private -unreachable- method
      initializeSomethingElse();

      // Private methods here
      function initializeSomethingElse() {
        self.dir = Math.atan2(self.y, self.x);
        self.total = 0;
        self.things.forEach(function(thing) {
          self.total += thing;
        });
      }
    }