Unique method for Array in Javascript

ArrayUnique.js

Array.prototype.unique = function () {
  var cached = [];
  return this.filter(function (index) {
    if (cached.indexOf(index) === -1) {
      cached.push(index);
      return index;
    }
  }.bind(this));
};


console.log([1, 2, 3, 4, 4, 'a', 'a', -1, -1].unique());