I am defining two prototypes for array and one prototype needs to call other like that:

Array.prototype.swap = function (x, y) {
      var b = this[x];
      this[x] = this[y];
      this[y] = b;
      return this;
    };
Array.prototype.rotate = function () {
      for (idx in this) {
        nextIdx = idx + 1 < this.length ? idx + 1 : 0;
        this.swap(idx, nextIdx);
      }
      return this;
    };

When I run [1,2,3,4].rotate() i get an error

> TypeError: this.swap is not a function
>     at Array.rotate (/home/user/array.js:11:10)

I think it's impossible to use an user-defined prototype inside another, but I want to understand why, and if I'm wrong (it's possible), how can I use? Thanks!