how to use sharp to resize image while uploading to s3 nodejs

Question

I use multer to upload the images to s3. The user clicks on upload button in the frontend, and uploads the image, which is then uploaded to s3 with the helper multer backend. But the images uploaded aren't optimised i.e if the user uploads a 4mb image, the image is uploaded with any compression. That indeed led to slow website frontend. How do I optimise or compress the image using sharp ?

Code to upload to s3 (nodejs):

const s3 = new aws.S3({
  accessKeyId: process.env.AWS_ACCESS_KEY,
  secretAccessKey: process.env.AWS_SECERT_KEY,
});
//S3
const uploadS3 = multer({
  storage: multerS3({
    s3: s3,
    bucket: "testbucket",
    acl: "public-read",
    metadata: function (req, file, cb) {
      cb(null, { fieldName: file.fieldname });
    },
    key: function (req, file, cb) {
      cb(null, shortid.generate() + "-" + file.originalname);
    },
  }),
});


router.post("/poster/create", uploadS3.array("posterPictures"), createPoster);

CreatePoster is a function that returns the path of the image in s3:

exports.createPoster = (req, res) => {
  let posterPictures = [];

  if (req.files.length > 0) {
    posterPictures = req.files.map((file) => {
      return { img: file.location };
    });
  }

  const { name, id } = req.body;

  const poster = new Poster({
    name: name,
    slug: slugify(name),
    id,
    posterPictures,
  });

  poster.save((error, poster) => {
    if (error) return res.status(400).json(error);
    if (poster) {
      return res.status(201).json({ poster, files: req.files });
    }
  });
};