for-await.js

'use strict'

const { Readable, finished } = require('stream')
const { promisify } = require(util)

async function run (origin, dest) {
  try {
    const write = buildWrite(dest)
    // This is an async iterator
    for await (let chunk of origin) {
      await write(chunk.toString().toUpperCase())
    }
    await promisify(finished)(dest)
  } catch (err) {
    origin.destroy(err)
    dest.destroy(err)
  }
}


function buildWrite (stream) {
  // This is a good way of wrapping stream.write into a Promise.
  // We are waiting for a drain event to resolve, and we are wrapping
  // the error event. A consumer should probably use finished to
  // know if the stream has completed.
  const streamError = null
  stream.on('error', function (err) {
    streamError = err
  })

  return write

  function write (chunk) {
    if (streamError) {
      return Promise.reject(streamError)
    }

    return new Promise(function (resolve, reject) {
      const res = stream.write(chunk)
      if (res) {
        resolve()
      } else {
        stream.once('drain', resolve)
      }
    })
  }
}

// startup and run the pipeline
const origin = new Readable({
  read (n) {
    this.push('hello')
    this.push(' ')
    this.push('world')
    this.push(null)
  }
})
run(origin, process.stdout).catch(console.log)