lorem.js

  router.put('/user/:refCode', function(req, res, next) {
    if (!req.params.refCode) res.json({ result: 'error', msg: 'must send an referral code'});

    var newEmail = req.body.email ? req.body.email : '';

    mailchimp.get({
      path: 'lists/' + listID + '/members?unique_email_id=' + req.params.refCode,
    })
      .then(function(search) {
        const user = search.members[0];
        let newReferrals = user.merge_fields.REFERRALS;

        if (user.merge_fields.REFERRALS.indexOf(newEmail) < 0) {
          newReferrals = newReferrals === '' ? newEmail : user.merge_fields.REFERRALS + ',' + newEmail;
        }

        mailchimp.patch({
          path: 'lists/' + listID + '/members/' + md5(user.email_address.toLowerCase()),
          body: {
            merge_fields: {
              REFERRALS: newReferrals,
            }
          }
        })
      
      // ....

    }