JavaScript: Promise.all returning undefined

Question

I'm trying to create a user account creation script with a focus on unique usernames - a prefix and a suffix from a pool, a list of existing usernames, and a list of reserved usernames.

That's just the start of it (no saving yet!), and already that would require three connections, so I just decided to see if I can code a function that would handle them all.

Here's my code so far - and it's on AWS Lambda, and tested via API Gateway, if that means anything:

const dbConnMysql = require('./dbController');
var methods = {
    createUser: function() {
        let getPrefixSuffixList = new Promise((resolve, reject) => {
            let connection = dbConnMysql.createConnection();
            dbConnMysql.startConnection(connection)
            .then((fulfilled) => {
                let table = 'userNamePool';
                return dbConnMysql.selectFrom(connection, table, '*', null);
            })
            .then((fulfilled) => {
                console.log(fulfilled);
                return dbConnMysql.closeConnection(connection)
                .then((fulfilled) => {
                    resolve(fulfilled);
                });
            })
            .catch((error) => {
                console.log(error);
                reject(error);
            });
        });

        let getTempUserNameList = new Promise((resolve, reject) => {
            // Same as getPrefixSuffixList, different table
        });

        let getRealUserNameList = new Promise((resolve, reject) => {
            // Same as getPrefixSuffixList, different table
        });

        return new Promise((resolve, reject) => {
            Promise.all([getPrefixSuffixList, getTempUserNameList, getRealUserNameList])
            .then((fulfilled) => {
                console.log(fulfilled[0]);
                console.log(fulfilled[1]);
                console.log(fulfilled[2]);
                let response = {
                    "statusCode": 200,
                    "headers": {"my_header": "my_value"},
                    "body": {"Prefix Suffix":fulfilled[0], "Temp UserName List":fulfilled[1], "Real UserName List":fulfilled[2]},
                    "isBase64Encoded": false
                };
                resolve(response);
            })
            .catch((error) => {
                let response = {
                    "statusCode": 404,
                    "headers": {"my_header": "my_value"},
                    "body": JSON.stringify(error),
                    "isBase64Encoded": false
                };
                reject(response);
            })
        });
    }
};    
module.exports = methods;

This function is called elsewhere, from index.js:

app.get('/createUserName', function (req, res) {
    var prom = Register.createUser();
    prom.then((message) => {
        res.status(201).json(message);
    })
    .catch((message) => {
        res.status(400).json(message);
    });
})

Now I'm not entirely sure if what I did with the Promise.All is correct, but from what little I know, if one promise fails, the Promise.All fails.

However, the individual promises do work just fine, and log out the respective results from the database. But inside the Promise.All, it all just logs out undefined.

Is there something I'm missing?

Answer 1

The cause of your problem is this. You need to run the functions, these then return the promise that will eventually resolve:

    Promise.all([getPrefixSuffixList(), getTempUserNameList(), getRealUserNameList()])

Here is some simpler code as well. In general there is no need for new Promise(). This code may fix other issues. Also, the undefined could be being printed from any part of the code, make sure it's being printed where you think it is.

// Dummy MySQL connector
const dbConnMysql = {
  createConnection: () => 'Connection',
  startConnection: conn => new Promise(resolve => setTimeout(resolve, 100)),
  selectFrom: (conn, t, q, n) =>
    new Promise(resolve =>
      setTimeout(() => {
        console.log(`${conn}: SELECT ${q} FROM ${t}`);
        resolve(`x ${t} RECORDS`);
      }, 100)
    ),
  closeConnection: conn => new Promise(resolve => setTimeout(resolve, 100)),
};

const methods = {
  createUser() {
    const getPrefixSuffixList = () => {
      const connection = dbConnMysql.createConnection();
      return dbConnMysql
        .startConnection(connection)
        .then(() => {
          const table = 'userNamePool';
          return dbConnMysql.selectFrom(connection, table, '*', null);
        })
        .then(fulfilled => {
          console.log(fulfilled);
          return dbConnMysql.closeConnection(connection).then(() => fulfilled);
        })
        .catch(error => {
          console.log(error);
          // Note: this catch will stop the error from propagating
          // higher, it could also be the cause of your problem.
          // It's okay to catch, but if you want the error to
          // propagate further throw a new error here. Like this:
          throw new Error(error);
        });
    };

    const getTempUserNameList = () => {
      // Same as getPrefixSuffixList, different table
    };

    const getRealUserNameList = () => {
      // Same as getPrefixSuffixList, different table
    };

    return Promise.all([getPrefixSuffixList(), getTempUserNameList(), getRealUserNameList()])
      .then(fulfilled => {
        console.log('fulfilled[0]: ', fulfilled[0]);
        console.log('fulfilled[1]: ', fulfilled[1]);
        console.log('fulfilled[2]: ', fulfilled[2]);
        return {
          statusCode: 200,
          headers: { my_header: 'my_value' },
          body: {
            'Prefix Suffix': fulfilled[0],
            'Temp UserName List': fulfilled[1],
            'Real UserName List': fulfilled[2],
          },
          isBase64Encoded: false,
        };
      })
      .catch(error => ({
        statusCode: 404,
        headers: { my_header: 'my_value' },
        body: JSON.stringify(error),
        isBase64Encoded: false,
      }));
  },
};

methods.createUser();