How to promisify node callbacks

#javascript #node #programming

Consider having a classic callback in NodeJS like this one:

const fs = require('fs');

function readMyFile(myfile, options) {
    fs.readFile(myfile, options, function(err, file) {
        if(err) {
            console.log('err', err);
        } else {
            console.log('file', file);
        }
    });
}

readMyFile('myfile', 'utf8');

What if you want to do something like this

...
readMyFile('myfile', 'utf8')
    .then(file => console.log('file', file)
    ...

You can manually tranform your callback into a promise like this:

const fs = require('fs');

function readMyFilePromise(myfile, options) {
    return new Promise((resolve, reject) => {
        fs.readFile(myfile, options, function(err, file) {
            if(err) {
                console.log('err', err);
                reject(err);
            } else {
                console.log('file', file);
                resolve(file);
            }
        });
    });
}

readMyFilePromise('myfile', 'utf8')
    .then(file => console.log('file', file))
    .catch(err => console.log('err', err));

The good news is you can use the util module to do that. Moreover Util is built in nodejs.

You just have to do this:

const util = require('util');
const fs = require('fs');

const readPromise = util.promisify(fs.readFile);
readPromise('myfile', 'utf8')
    .then(file => console.log('file', file))
    .catch(err => console.log('err', err));