Edit in JSFiddle

  // this doesn't create a new user but rather updates an existing one by the user name
  // a request looks like this: `https://nodeserver.com/username23?pass=12345` plus the associated JSON data sent in the body of the req
  app.put("/:user", (req, res) => {
    // get the user, compare the hashed + salted password sent in the query to the one in the database, if they match, then update the data
    collection.find({ user: req.params.user }).toArray((err, docs) => {
      if (err) {
        res.send("An error occured in getting the user info." );
      } else {
        // there were matches (there are users with that username)
        if (docs.length > 0) {
          // get the first users password (each user should have a unique username)
          let hashedPasswordInDatabase = docs[0].pass;
          
          // get the salt
          const salt = docs[0].salt;

          // if there was no password sent in the query of the url (after the `?`)
          if (!req.query.pass) {
            res.send("There was no password associated with the GET req.");
          }

          // The password sent in the GET req.
          // Enclose in template to make sure it is a string (so if a password is a number, it treats it as a string).
          // Prepend the salt 
          const hashedQueryPassword = hash(`${salt}${req.query.pass}`); 

          // if the password that was sent with the get request matches the user's password
          if (hashedPasswordInDatabase === hashedQueryPassword) {
            // update user info in database

            // hash the password for storing
            req.body.pass = hash(req.body.pass);
            collection.updateOne(
              { user: req.params.user }, // if the username is the same, update the user
              { $set: { ...req.body, user: req.params.user, salt } }, // make sure to include the salt in the update of info
              (err, r) => {
                if (err) {
                  console.log("An error occurred in updating information");
                }
              }
            );
            res.send("All went well");
          } else {
            // otherwise, the password was wrong. Generic error message sent back
            res.send("Something went wrong in user authentication.");
          }
        } else {
          res.send("There were no users with that name found. ");
        }
      }
    });
  });