register.js

function doesUserExist() { 
  pg.connect(conString, function (err, client, done) {
    client.query('SELECT user_id FROM clients.users where user_id=$1', [req.body.user], function(err, result) {        
      if (err) {
        console.log('error fetching client from pool', err);
        return;
      }
      doNext(result.rowCount == 1);
    });
  });
}

function doNext(userExists) {
  ...
}