Upload to S3 using multer without a middleware function

I am using multer to upload media to my s3 bucket. I am using multer-s3 as a middleware to upload media like:

const upload = multer({
  storage: multerS3({
    s3: s3,
    bucket: myBucket,
    key: function (req, file, cb) {
      cb(null, new Date().getTime() + '_' + file.originalname)
    }
  })
});

and calling it in route as:

router.post("/media",upload.single("media"))

This works great. But a scenario that is not working for me is: Let say i upload one image and i want to store multiple version of it by resizing it before uploading. I am not able to call the upload function like a normal one. I want to do something like:

let thumbnail = myFunctionToReturnImageFile(req.file);
upload(thumbnail);

I understand i need to send some multi-part/form-part but i am unable to find a solution. If you suggest me a something that will be great.

Option 1 - Use multer-s3-transform middleware to handle S3 upload with file transformations, like image resizing using sharp.

const multer = require('multer')
const multerS3 = require('multer-s3-transform')
const sharp = require('sharp')
const AWS = require('aws-sdk')
const S3 = new AWS.S3({
    accessKeyId: ...,
    secretAccessKey: ...
})

const upload = multer({
  storage: multerS3({
    s3: S3,
    bucket: ...,
    shouldTransform: true,
    transforms: [
      {
        id: 'original',
        key: (req, file, cb) => cb(null, new Date().getTime() + '_' + req.file.originalname),
        transform: (req, file, cb) => cb(null, sharp().jpg())
      },
      {
        id: 'large',
        key: (req, file, cb) => cb(null, new Date().getTime() + '_large_' + req.file.originalname),
        transform: (req, file, cb) => cb(null, sharp().resize(1200, 900).jpg())
      },
      {
        id: 'small',
        key: (req, file, cb) => cb(null, new Date().getTime() + '_small_' + req.file.originalname),
        transform: (req, file, cb) => cb(null, sharp().resize(400, 300).jpg())
      }
    ]
  })
})

router.post('/media', upload.single('media'))

Option 2 - If you insist on doing things manually, you may use plain multer to handle the incoming file, resize it with sharp and upload each resized file to S3 using AWS-SDK.

const multer = require('multer')
const sharp = require('sharp')
const AWS = require('aws-sdk')
const S3 = new AWS.S3({
    accessKeyId: ...,
    secretAccessKey: ...
})

router.post('/media', multer().single('media'), (req, res) => {
  // req.file represents the uploaded file
  let time = new Date().getTime()
  
  Promise.all([
    // original file
    S3.upload({
      Bucket: ...,
      Key: time + '_' + req.file.originalname,
      Body: req.file.buffer
    }),
    
    // large preview
    sharp(req.file)
      .resize(1200, 900)
      .toBuffer()
      .then(resized => S3.upload({
        Bucket: ...,
        Key: time + '_large_' + req.file.originalname,
        Body: resized
      }).promise()),
    
    // small thumbnail
    sharp(req.file)
      .resize(400, 300)
      .toBuffer()
      .then(resized => S3.upload({
        Bucket: ...,
        Key: time + '_small_' + req.file.originalname,
        Body: resized
      }).promise()),
  ])
  .then(() => res.send("Images uploaded"))
  .catch(e => {
    console.warn(e) // debug this error
    res.status(500).send("Unable to upload images")
  })
})