How to check if a stream is base64 encoded?

Question

I'm trying to understand the node js streams and I'm experimenting a bit, so this may be a dumb question.

In the example I'm using a Duplex stream to check if the stream is fully encoded as base64:

const { Duplex } = require("stream");
const fs = require("fs");

class IsBase64Encoded extends Duplex {
  constructor(callback) {
    super();
    this.isBase64 = true;
    this.isBase64Callback = callback;
  }

  _read() {}

  _write(chunk, encoding, callback) {
    if (
      this.isBase64 &&
      Buffer.from(chunk.toString(), "base64").toString("base64") !==
        chunk.toString()
    ) {
      this.isBase64 = false;
    }

    this.push(chunk);
    callback();
  }

  _final() {
    this.isBase64Callback(this.isBase64);
    this.push(null);
  }
}

const inputStream = fs.createReadStream(`${__dirname}/sample`);

const callback = (isBase64) => {
  console.log("IS STREAM BASE64 ENCODED: ", isBase64);
};
inputStream.pipe(new IsBase64Encoded(callback));

Now, is this an actual legit way to do such a thing? I'd guess it is, since it seems to work, but I want to understand if there are any side effects.