gistfile1.js

User.findOne({"email": req.body.email", function (err, doc) {
  if (err) {
    // The query returned an error and we should either ignore
    // it or do like a 500 error or something.
  } else {
    if (doc) {
      // email exists
    } else {
      // email doesn't exist
    }
  }
});