const { Readable, Transform, pipeline } = require('stream')

const createCounterReader = () => {
  let count = 0;
  return new Readable({
    objectMode: true,
    read() {
      count += 1;
      console.log('read', count)
      this.push({count});
    },
  });
};

const sleep = (delay) => {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve()
    }, delay)
  })
}


async function main () {
  const readable = createCounterReader();
  let counter = 0;
  
  const transform = new Transform({
    objectMode: true,
    highWaterMark: 10,
    transform: async (chunk, encoding, callback) => {
      const { count } = chunk
      console.log('transform:', count);
      await sleep(1000)
      console.log('transformEnd:', count);
      ++counter
      callback(null, `${counter}. ${JSON.stringify(chunk)} \n`)
    }
  })

  // pipeline stream executes sequential (downstream ↓)
  readable.pipe(transform)
}

main().catch((error) => console.error(error.toString()))