in place prototype

compact.js

Array.prototype._compact = function(iterator) {
  var position = 0;
  for (var index = 0, length = this.length; index < length; index++) {
    if (this[index] != null)
      this[position++] = this[index];
  }
  this.length = position;
  return this;
};

map.js

Array.prototype._map = function(iterator, context) {
  iterator = iterator ? iterator.bind(context) : Prototype.K;
  for(var index = 0, length = this.length; index < length; index++) {
    this[index] = iterator(this[index], index);
  }
  return this;
};

reject.js

Array.prototype._reject = function(iterator, context) {
  iterator = iterator ? iterator.bind(context) : Prototype.K;
  var position = 0;
  for (var index = 0, length = this.length; index < length; index++) {
    if (!iterator(this[index], index))
      this[position++] = this[index];
  }
  this.length = position;
  return this;
};

reject2.js

return this._reject(function(value) { return value == null; });