parse-json.js

export default function parseJSON() {
  return new TransformStream({
    transform(chunk, controller) {
      controller.enqueue(JSON.parse(chunk));
    }
  });
}

split-stream.js

export default function splitStream(splitOn) {
  let buffer = '';

  return new TransformStream({
    transform(chunk, controller) {
      buffer += chunk;
      const parts = buffer.split(splitOn);
      parts.slice(0, -1).forEach(part => controller.enqueue(part));
      buffer = parts[parts.length - 1];
    },
    flush(controller) {
      if (buffer) controller.enqueue(buffer);
    }
  });
}

return new TransformStream() <-- you're instantiating one of these https://streams.spec.whatwg.org/#ts-class? I can't find anything on the web about those... People write about streams, but I can't find any other examples of TransformStream or their current support.

Was reading this and came here to learn. Can you explain more?