Exception handling in promises

Question

Sometimes constructing a promise there may be an exceptional condition such as database refused the connection or invalid host name parameter for database, for example:

In db.js

const mysql = require('mysql');

module.exports.connect = (options) => {
  return new Promise((resolve, reject) => {
    try {
      const connection = mysql.createConnection(getDBConfig());
      connection.connect();
      resolve(connection);
    } catch (err) {
      throw new Error('Error connecting database');
    }
  });
};

function getDBConfig() {
  const isProd = process.env.NODE_ENV === 'production';
  const config = {
    debug: !isProd,
    host: 'localhost',
    user: 'root',
    database: 'xxx'
  };

  if (isProd) {
    return Object.assign({
      password: 'root'
    }, config);
  } else {
    return Object.assign({
      password: 'xxx'
    }, config);
  }
}

In app.js

'use strict';

const database = require('./db');
const server = require('./server');

// log unhandled execpetions
process.on('uncaughtException', (err) => {
  console.error('Unhandled Exception', err.message);
});

process.on('uncaughtRejection', (err, promise) => {
  console.error('Unhandled Rejection', err);
});

database.connect({
})
.then((connection) => {
  console.log('Connected to database');
});

In the above case there is no instance of mysql running, the output that I get in the console is:

Connected to database
Unhandled Exception connect ECONNREFUSED 127.0.0.1:3306

Which is not as expected, I need to know what is the recommended approach and what I am doing wrong here?

Answer 1

You're not handling errors in the connect call. See lines with *** comments:

module.exports.connect = (options) => {
  return new Promise((resolve, reject) => {
    try {
      const connection = mysql.createConnection(getDBConfig());
      connection.connect(err => {                         // ***
        if (err) {                                        // ***
          reject(new Error('Error connecting database')); // ***
        } else {                                          // ***
          resolve(connection);                            // ***
        }                                                 // ***
      });                                                 // ***
    } catch (err) {
      throw new Error('Error connecting database');
    }
  });
};

The try / catch will catch synchronous errors that occur, but connect reports success/failure via callback. More in the npm mysql documentation .

And, of course, then handle the rejection with a catch handler at point of use, as trincot pointed out .