upload_madness.js

  const folderName = filenamify(req.body.folderName);

  const createSize = (file) => new Promise((resolve, reject) => {
    // Returns a promise
    console.log("Inside resize", file);
    return sharp(file.buffer).resize(file.w, file.w, {
      fit: sharp.fit.inside,
      withoutEnlargement: true,
    })
    .toFormat('jpeg')
    .toBuffer()
    .then(resizedBuffer => {
      console.log("resizedBuffer", resizedBuffer);
      file.resizedBuffer = resizedBuffer;
      resolve(file);
    });
  });

  const resize = (file) => {
    const outputSizes = [{ name: "large", w: 1200 },
    { name: "medium", w: 600 },
    { name: "small", w: 300 } ];
    const buffer = file.buffer;
    const originalName = file.originalname;

    return outputSizes.map(outputSize => {
      outputSize.buffer = buffer;
      outputSize.originalName = originalName;
      return outputSize;
    });
  };

  const upload = (file) => new Promise((resolve, reject) => {
    const fileName = folderName + "/" + file.name + "/" + new Date().getTime() + "_" + file.originalName;
    const params = {
      Bucket: bucketPictures,
      Key: fileName,
      Body: file.resizedBuffer, };

    console.log("file: ", file);
    console.log("params: ", params);

    return s3.upload(params, (err, data) => {
      if(err) {
        console.error(err);
        return reject(err);
      }
      resolve(data);
    });
  });

  Promise.all(req.files.map(resize)).then( files => {
    const resizedFlat = _.flatten(files);
    return Promise.all(resizedFlat.map(createSize)).then( resizedFiles => {
      return Promise.all(resizedFlat.map(upload)).then( uploadResults => {
        res.status(201).json({ message: uploadResults });
      });
    });
  }).catch(err => res.status(500).json({ err: err }));