gistfile1.js

function Set() {
  this.data = {};
  this.length = 0;
  for (var i = 0; i !== arguments.length; i++) {
    this.add(arguments[i]);
  }
}
(function(proto) {

  function eachProp(obj, fn) {
    for (var prop in obj) {
      if (obj.hasOwnProperty(prop)) {
        fn(prop);
      }
    }
  }

  proto.add = function(obj) {
    var $this = this;
    if (obj instanceof Set) {
      eachProp(obj.data, function(e) { add($this, e); });
    } else if (obj instanceof Array) {
      eachProp(obj.data, function(e) { add($this, obj[e]); });
    } else {
      add(this, obj);
    }
  };

  function add(ctx, obj) {
    ctx.data[obj] = true;
    ctx.length++;
  }

  proto.remove = function(obj) {
    var $this = this;
    if (obj instanceof Set) {
      eachProp(obj.data, function(e) { remove($this, e); });
    } else if (obj instanceof Array) {
      eachProp(obj.data, function(e) { remove($this, obj[e]); });
    } else {
      remove(this, obj);
    }
  };

  function remove(ctx, obj) {
    delete ctx.data[obj];
    ctx.length--;
  }

  proto.contains = function(obj) {
    return !!(this.get(obj));
  };

  proto.intersection = function(set) {
    var res = new Set();
    eachProp(this.data, function(e) {
      if (set.data.hasOwnProperty(e)) {
        res.add(e);
      }
    });
    return res;
  };

  proto.union = function(set) {
    var res = new Set();
    res.add(this);
    res.add(set);
    return res;
  };

  proto.difference = function(set) {
    var res = new Set();
    res.add(this);
    res.remove(set);
    return res;
  };

  proto.toArray = function() {
    var ret = [];
    eachProp(this.data, function(e) { ret.push(e) });
    return ret;
  };

  proto.toString = function() {
    return this.toArray().join(",");
  };
})(Set.prototype);

var a = new Set(1,3,5), b = new Set(1,2,3);

a.add(7);
print(a.toString());

b.add(4);
print(b.toString());

print(a.union(b).toString());
print(a.intersection(b).toString());
print(a.difference(b).toString());