code: function diff(a, b) {
    var diff = {};
    var key;
    for (key in b) { if (b.hasOwnProperty(key) && b[key] !== a[key]) { diff[key] = b[key]; }}
    for (key in a) { if (a.hasOwnProperty(key) && !(key in b)) { diff[key] = undefined; }}
    return diff;
  }
docs: return 'diff' object with the props from object b which are different (or missing) in object a includes tombstone value === undefined for props which exist in object a but not in object b,