gistfile1.js

function intersection(a, b) {
  var shorter = a, longer = b, hash = {};
  if (a.length > b.length) { shorter = b; longer = a; }
  for (var i in shorter) { var o = shorter[i]; hash[o] = 1; }
  var collisions = [];
  for (var i in longer) {
    var o = longer[i];
    if (hash[o] == 1) collisions.push(o);
  }
  return collisions;
}