Callback and Wrapper promises

callback2.js

'use strict';

var FS = require('fs');

// Callback version
FS.readFile('file.txt', 'utf8', function (err, data) {
    if (err) throw err;
    console.log('File has been read:', data);
    console.log
});

console.log('After readFile.');

// Promise wrapper version
function readfile(filename) {
    return new Promise(function (resolve, reject) {
        FS.readFile(filename, function (err, data) {
            if (err) {
                reject(err);
            }

            resolve(String(data));
        });
    });
}

// Calling the promise wrapper version
readfile('file.txt')
    .then(function (data) {
        console.log(`File has been read: ${data}`);
        return (data);
    })
    .then(function (param) {
        console.log(`After passing to the next ${param}`);
    })
    .catch(function (error) {
        console.log(`Error has happened: ${error}`);
    });


// Promise and Callback Wrapper version
function readfile2(filename, callback) {
    var p = new Promise(function (resolve, reject) {

        FS.readFile(filename, function (err, data) {
            
            // If callback
            if (callback && typeof(callback) === 'function') {
                if ( err ){
                    return callback(err, data);
                }
                return callback (null, data);
            }

            // If promise
            if (err) {
                reject(err);
            }

            resolve(String(data));
        });

    });
    return p;
}

// Promise or Callback Wrapper
readfile2('file.txt', function (err, data) {
    console.log(`Callback version ${data}`);
});

readfile2('file.txt')
    .then(function (data) {
        console.log(`File has been read: ${data}`);
        return (data);
    })
    .then(function (param) {
        console.log(`After passing to the next ${param}`);
    })
    .catch(function (error) {
        console.log(`Error has happened: ${error}`);
    });