This gist is for people who are new to JS, and want to write libraries which support both callbacks and Promises architecture.

support-promise-and-callback.js

// This file provides an example of how you can expose both Promises and callback support to your functions

const fs = require('fs');
//This function can be called via callbacks, as well as Promises
const readFileAsArray = function(file, cb = () => {}) { //callback is made an empty function in case I am using Promises,
    // in which case I wont be passing this argument and it wouldn't throw and undefined variable error.
    return new Promise((resolve, reject) => { //create a new promise
        fs.readFile(file, (err, data) => {
            if (err) {
                reject(err); //To support promises 
                cb(err); //To support callback
            } else {
                const lines = data.toString().trim().split("\n");
                resolve(lines); //Resolve for promise
                cb(null, lines); //Err argument null for success
            }
        });
    });
};

/*Callback Style*/
readFileAsArray('./numbers.txt', (err, lines) => { //Please create this numbers.txt file for the above program to run. 
    if (err) {
        throw err;
    } else {
        const numbers = lines.map(Number);
        const oddNumberCount = numbers.filter((item) => {
            return item % 2 === 1;
        });
        console.log(`Odd Number callback count = ${oddNumberCount.length}`);
    }
});
/*Promise style*/
readFileAsArray('./numbers.txt') //Please create this numbers.txt file for the above program to run. 
    .then(lines => {
        const numbers = lines.map(Number);
        const oddNumberCount = numbers.filter((item) => {
            return item % 2 === 1;
        });
        console.log(`Odd Number promise count = ${oddNumberCount.length}`);
    })
    .catch(err => {
        console.log(err);
    })