encode.js

// setup a new encoder with a write callback
const enc = Module.create_encoder(writeEncodedData);

// do some encoding
// ...

// dispose encoder
Module.free_encoder(enc);

// The write callback
function writeEncodedData (pointer, size) {
  // Get the encoded data array
  const data = Module.HEAPU8.slice(pointer, pointer + size);
  // And write it to a stream
  myStream.write(data);
  return 0; // Write status code 0 = success
}