join.js

function join() {
  var j = {}, stuff = [];
  
  j.join = function(newstuff) {
    var enter = newstuff.filter(function(n) {
    	return stuff.indexOf(n) === -1;
    });
    var gotit = newstuff.filter(function(n) {
    	return stuff.indexOf(n) !== -1;
    });
    var exit = stuff.filter(function(n) {
    	return newstuff.indexOf(n) === -1;
    });
    stuff = newstuff;
    return {
    	enter: enter,
      gotit: gotit,
      exit: exit
  	};
  };
  
  return j;
}

// first: construct a join
var myjoin = join();
var j = myjoin.join(['cats']);
j.enter;
j.gotit;
j.exit;

// new let's add dogs and kittens
var k = myjoin.join(['cats', 'dogs', 'kittens']);
k.enter;
k.gotit;
k.exit;

// kittens grow up
var l = myjoin.join(['cats', 'dogs']);
l.enter;
l.gotit;
l.exit;