lodash - using promises in node.js to create and compare two arrays

I needed to compare two arrays the first one a couple of filenames from a database, the second one a list of files I already downloaded to my client. The Idea was to load whatever files are missing on the client.
As the reading via fswas two slow, I tried using Promises to wait for one function to finish before the next starts. But somehow I got lost...
My code so far:

let filesIneed = [];
let filesIhave = [];
let filesToFetch = [];
getLocalFiles().then(getFilesIneed).then(getfilesToRetreive);

function getLocalFiles() {

    fs.readdir(localPath, (err, files) => {
        files.forEach(file => {
                filesIhave.push(file)
        });
    })
    return Promise.all(filesIhave);
}

function getFilesIneed () {
    for (let x of docs) {//this is my JSON

        filesIneed.push(y.NameOfFileIShouldHave);
        }
    }
    return Promise.all(filesIneed);
}

function getfilesToRetreive() {
    filesToFetch = _.difference(filesIneed, filesIhave);
    return Promise.all(filesToFetch);
}



console.log(filesToFetch);

I do get the first and second array ("filesIneed" and "filesIhave"), but difference is always empty. So maybe I just mangled up the Promises, as this concept is completely new to me and I'm aware I only understood half of it.