code: function mergeDiff(a, diff) {
    var key, val;
    if (a && diff) {
      for (key in diff) { if (diff.hasOwnProperty(key)) {
        val = diff[key];
        if (val === undefined) {
          delete a[key];
        } else {
          a[key] = val;
        }
      }}
    }
    return a;
  }
docs: just like _.merge but use a diff object to overwrite or delete properties of object a deletes properties whose value in the diff === undefined (apply the tombstone)