If you have the same issue, just check wether you use this guide:
https://www.geeksforgeeks.org/upload-and-retrieve-image-on-mongodb-using-mongoose/
if so, just do the same as here:

function toBase64(arr) {
   arr = new Uint8Array(arr) if it's an ArrayBuffer
   return btoa(
      arr.reduce((data, byte) => data + String.fromCharCode(byte), '')
   );
}

I store images im mongoDB like this:

img: {
  contentType: "image/png"
  data: {
    data: Array(135239),
    type: "Buffer"
  }
}

Then i got them on frontend and trying to use in html img tag, but there are no pictures
I have tried to transform the data to

let blob = new Blob([img.data], {type : img.contentType})
let img = document.createElement('img').setAttribute('src', URL.createObjectURL(blob))
let container = document.getElementById('container')
container.appendChild(img)

or

let img = document.createElement('img').setAttribute('src', `data:${img.contentType};base64,${img.data}`)
let container = document.getElementById('container')
container.appendChild(img)

but none of it does not seem to work

P.S. In the guide was written i should transform img.data.toString(‘base64’), but it does not work too