Recursive QuickSort in Javascript

quicksort.js

Array.prototype.filter = function(callback) {
  var result = [];
  var length = this.length;
  
  for (var i = 0; i < length; i++) {
    if (callback(this[i])) {
      result.push(this[i]);
    }  
  }

  return result;
};

Array.prototype.sort = function() {
  if (this.length < 2) return this;

  var pivot   = this[Math.floor(this.length / 2)];
  var less    = this.filter(function(item) { return item < pivot; });
  var equal   = this.filter(function(item) { return item == pivot; });
  var greater = this.filter(function(item) { return item > pivot; });

  return less.sort().concat(equal, greater.sort());
};