Node Js – convert a callback-based function to a Promise-based one?

Last updated on November 21, 2022

In this post, you will learn how to convert a callback-based function to a promise-based one using util.promisify() function.As a part of Node Js version 8, promisify() function added to util module.

In simple worlds util.promisify() function converts a regular function into a promise. The final parameter of the function passed to promisify must be a callback and it should follow Node’s callback style. i.e. taking a (err, value) => ... callback as the last argument, and returns a version that returns promises.

Example

  const dns = require('dns');
    dns.resolve('arjunphp.com', 'MX', function(err, addresses) {    
        if(err) throw new Error(err);
        console.log(addresses)
    });

In this above example, you can see that we are getting response in callback style. Now let’s convert it to Promise

const util = require('util');
const dnsResolveAsync = util.promisify(dns.resolve);
dnsResolveAsync('arjunphp.com', 'MX')
    .then(addresses => console.log(addresses))
    .catch(err => console.log(err));

Or, equivalently using async functions:

const util = require('util');
const dnsResolveAsync = util.promisify(dns.resolve);

async function getMxRecord() {
    const addresses = await dnsResolveAsync('arjunphp.com', 'MX');
    console.log(addresses);
}

getMxRecord();