distinct.js

Array.prototype.filterNot = function (callback) {
  var result = [];
  for (e of this) {
    if (!callback(e)) {
      result.push(e);
    }
  };
  return result;
}

Array.prototype.distinct = function (callback) {
  var result = [];
  var rec = function(next) {
    if (next.length === 0) {
      return result;
    } else {
      var head = next[0];
      var left = next.filterNot(callback.bind(null, head));
      result.push(head)
      return rec(left)
    }
  };

  return rec(this);
};