How could I refactor the waterfall method from async to use ES6 promises?

Question

I have a route that allows a user to reset their password by sending them an email. Standard procedure for most websites. In this route, I import the async npm module and use the waterfall method so that I can handle the asynchronous nature of multiple functions. I'm still having a bit of trouble understanding promises, but I'm trying to replace waterfall with a promise or promise chain.

How could I refactor this route with a promise? Here are the steps contained in this route that is currently split up into 4 functions with waterfall.

1. First the route creates a reset token
2. Search for user based on email 2.5. If user is found, save user, otherwise return 404
3. Send email to user containing a reset url

4. Return a status of 200.

   app.post('/forgotPassword', function(req, res, next) {
   
       waterfall([
           // generate reset token
           function(done) {
               crypto.randomBytes(20, function(err, buf) {
           var token = buf.toString('hex');
           done(err, token);
       });
           },
           function(token, done) {
               // search for user with the given email
               User.findOne({ email: req.body.email }, function(err, user) {
                   // check to see if the user exists
                   if (!user) {
                       // user doesn't exist in database
                       return res.status(404).send();
                   }
                   // user exists, assign token with expiration date
                   user.resetPasswordToken = token;
                   user.resetPasswordExpires = Date.now() + 3600000; // 1 hour from now
   
                   // save the user model with the newly added
                   // token and expiration date
                   user.save(function(err) {
                       done(err, token, user);
                   });
               });
       },
           function(token, user, done) {
               var smtpTransport = nodemailer.createTransport('SMTP', {
                 service: 'SendGrid',
                 auth: {
                   user: config.sendgridUser,
                   pass: config.sendgridPassword
                 }
               });
   
               var mailOptions = {
                 to: user.email,
                 from: '[email protected]',
                 subject: 'Password Reset',
                 text: `Hello etc etc`,
   
               smtpTransport.sendMail(mailOptions, function(err) {
                   done(err, 'done');
               });
           }],
               function(err) {
                   // handle error
                   if (err) return next(err);
                   res.status(200).send();
               });
       }); // end POST route '/forgotPassword'
   

Answer 1

Promise is a very powerful tool. It can be hard to understand it at the beginning, but totally worth the effort! Please let me know if you have any doubts :)

app.post('/forgotPassword', function(req, res, next) 
{
    new Promise((resolve, reject) => 
    {
        // generate reset token
        crypto.randomBytes(20, (err, buf) =>
        {
            if(err)
                return reject(err);

            const token = buf.toString('hex');
            resolve(token);
        });     
    })
    .then((token) => 
    {
        return new Promise((resolve, reject) => {
            // search for user with the given email
            User.findOne({ email: req.body.email }, (err, user) => 
            {                       
                if (!user)
                    return reject(404);

                // user exists, assign token with expiration date
                user.resetPasswordToken = token;
                user.resetPasswordExpires = Date.now() + 3600000; // 1 hour from now

                // save the user model with the newly added
                // token and expiration date
                user.save(function(err) {
                    if(err)
                        return reject(err);

                    resolve(user);
                });
            });                 
        });
    })
    .then((user) => 
    {
        return new Promise((resolve, reject) => 
        {
            const smtpTransport = nodemailer.createTransport('SMTP', 
            {
              service: 'SendGrid',
              auth: {
                user: config.sendgridUser,
                pass: config.sendgridPassword
              }
            });

            const mailOptions = {
              to: user.email,
              from: '[email protected]',
              subject: 'Password Reset',
              text: `Hello etc etc`
            };

            smtpTransport.sendMail(mailOptions, (err) => 
            {
                if(err)
                    return reject(err);

                resolve();
            });             
        }); 
    })
    .then(() => res.sendStatus(200))        
    .catch((err) => 
    {
        //check if the error is the one from the DB where the user was not found
        if(err == 404) {
            return res.sendStatus(404);
        }

        return res.status(500).send(err);
    });
});