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

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 paramter 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)
    });

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 lets 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));

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();

const util = require('util');

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

async function getMxRecord() {

const addresses = await dnsResolveAsync('arjunphp.com', 'MX');

console.log(addresses);

}

getMxRecord();

I hope you like this Post, Please feel free to comment below, your suggestion and problems if you face - we are here to solve your problems.

Arjun

I am Arjun from Hyderabad (India). I have been working as a software engineer from last 7+ years, and its my passion to learn new things and implement them as a practice. Aside from work, I likes gardening and spending time with pets.

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

Example