Accessing data from multiple callbacks

Question

I am developing an Express nodeJS web app and am trying to increase my knowledge of asynchronous programming and how to access the data that I need properly.

I have two callback functions which query an SQL database. My functions are declared like so:

function CustomerList(callback) {
  const db = require('../db');
  db.query('SELECT * FROM users WHERE role = (?)', ['User'], (error, results, fields) => {
    if (error) throw error;
    callback(results);

});

}

function AdminList(callback) {
   const db = require('../db');
   db.query('SELECT * FROM admins WHERE role = (?)', ['Moderator'], (error, results, fields) => {
    if (error) throw error;
    callback(results);

});

}

I use the data retrieved from both callbacks and send it to my view engine template to be displayed in the browser like so:

 CustomerList((userData) => {  

      AdminList((adminData) => {

          res.render('adminhome', {
              title: 'Admin - Home',
              results: 'test',
              customers: userData,
              admins: adminData
          });

      });

This works as expected and I can manipulate the data however I please on my view template. But... This seems... 'clunky', and I feel like this method I am using is going to cause me trouble in the future if I create additional functions to retrieve more data, and then keep nesting additional callbacks to pass the data to my view template.

Essentially, I am struggling to find a suitable approach to retrieve the UserData results and AdminData results and pass them to my res.render API call.

What are better alternatives to my current approach?

Use Promise.all or leverage async/await.

zero298
– zero298

2018-07-16 14:36:01 +00:00
Commented Jul 16, 2018 at 14:36 — zero298
– zero298, Commented Jul 16, 2018 at 14:36

Rahul Sharma · Accepted Answer · 2018-07-16 14:37:45Z

1

Yes, it'll create callback hell if you call multiple callbacks inside the callback, use promises(async/await)

const util = require('util');
const db = require('../db');
const query = util.promisify(db.query);

const getData = async () => {
    try {
        const CustomerList = await query('SELECT * FROM users WHERE role = (?)', ['User']);
        const AdminList = await query('SELECT * FROM admins WHERE role = (?)', ['Moderator']);

        // OR Promise.all

        const [CustomerList, AdminList] = await Promise.all([
            query('SELECT * FROM users WHERE role = (?)', ['User']),
            query('SELECT * FROM admins WHERE role = (?)', ['Moderator'])
        ]);
    } catch (error) {
        console.log(error);
    }
}

answered Jul 16, 2018 at 14:37

Rahul Sharma

10.1k1 gold badge20 silver badges41 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Fuller Over a year ago

This does look like the solution to my problem, however, I am getting: "TypeError: Cannot read property 'typeCast' of undefined". I am researching this error now, but any ideas?

Fuller Over a year ago

After researching, i changed my db.js file to use a connectionPool instead of just a connection, this allowed your code snippet to work perfectly. Thank you!! :)

Collectives™ on Stack Overflow

Accessing data from multiple callbacks

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related