What js object should i use to store bytes in firestore using Admin / Web SDK?

[Vloz]

Already asked there without success

Js Blob and SDK Bytes return a serializing error.

Error: Value for argument "data" is not a valid Firestore document. Couldn't serialize object of type "Bytes" (found in field "binaryData"). Firestore doesn't support JavaScript objects with custom prototypes (i.e. objects that were created via the "new" operator).

Serializing works when i send it as Uint8Array but then it's displayed as a base64 string in firestore emulator..?
And when i try to retrieve it with Web SDK it return to me as byteString/binaryString object..?

What's the correct way to store bytes in firestore?

[MILLERMARRU]

The correct type depends on which SDK you're using. Here's the full picture:

Admin SDK (Node.js) — use Buffer

import { getFirestore } from 'firebase-admin/firestore';

const db = getFirestore();

// Storing
const buffer = Buffer.from('hello world', 'utf-8');
// From a file: const buffer = fs.readFileSync('./image.png');

await db.collection('files').doc('doc-id').set({
  data: buffer,  // Buffer is accepted directly by the Admin SDK
});

// Retrieving — comes back as Buffer on Node.js
const snap = await db.collection('files').doc('doc-id').get();
const retrieved: Buffer = snap.data()!.data;
console.log(retrieved.toString('utf-8')); // 'hello world'

Web SDK (browser) — use Bytes

import { getFirestore, doc, setDoc, getDoc, Bytes } from 'firebase/firestore';

const db = getFirestore();

// Storing — wrap your Uint8Array in Bytes
const uint8 = new TextEncoder().encode('hello world');

await setDoc(doc(db, 'files', 'doc-id'), {
  data: Bytes.fromUint8Array(uint8),
  // or from base64: Bytes.fromBase64String('aGVsbG8gd29ybGQ=')
});

// Retrieving
const snap = await getDoc(doc(db, 'files', 'doc-id'));
const bytes: Bytes = snap.data()!.data;
const decoded = new TextDecoder().decode(bytes.toUint8Array());
console.log(decoded); // 'hello world'

Why Blob and plain objects fail

Firestore only serializes its own special types plus native JS primitives. Blob has a custom prototype and is not on that list — hence the "Firestore doesn't support JavaScript objects with custom prototypes" error. Uint8Array works because the SDK internally converts it to the Firestore Bytes type.

About the emulator showing base64

That's expected. The Firestore emulator UI displays Bytes fields as base64 strings — it's a display choice, not the actual stored format. When you retrieve the document in code, you get Buffer (Admin SDK) or Bytes (Web SDK), not a string.

TL;DR: Buffer with Admin SDK, Bytes.fromUint8Array() with Web SDK.