stilldontunderstand.js

const Promise = require("bluebird");
const fs = Promise.promisifyAll(require('fs'))

function getNumbers() {
    var fileNumbers = [];
    for(var n = 0; n < 10; n++){
        fileNumbers.push(n);
    }
    return fileNumbers;
}

Promise.all(getNumbers())
    .mapSeries(function (number) {
         //I'm using setTimeout to emulate the processing time of the real scrip
        return Promise.delay(3000).then(function() {
            return writeMyFile(number);
        });
    });

function writeMyFile(fn){
    //I'm using writeFile here but Jimp uses writeFileStream and writes a buffer to it... 
    //the buffer doesn't return which is the same issue here
    return fs.writeFileAsync("set\\" + fn + ".txt", "Hey there!")
        .then(function () {
            console.log("The file was saved!"))
        })
        .catch(function (err) {
            console.log(err)
        });
}