Use stream.PassThrough to count file size

pass-the-fun-node-v14-and-under.js

// node pass-the-fun-node-v14-and-under.js inputfile
// this will create a clone of inputfile with inputfile.bak
// and count the bytes written using stream.PassThrough
// note the simplified contructor form (as illustrated in 
// pass-the-fun-node-v15.js does not work as expected in
// node v14

const fs = require('fs')
const { pipeline, PassThrough } = require('stream')

const input = fs.createReadStream(process.argv[2])
const out = fs.createWriteStream(process.argv[2] + '.bak')

class Counter extends PassThrough {
  constructor (options) {
    super(options)
    this.length = 0
  }

  _transform (chunk, encoding, callback) {
    this.length += chunk.length
    console.log(`this.length=${this.length}, read ${chunk.length} bytes`)
    callback(null, chunk)
  }
}
const counter = new Counter()

pipeline(input, counter, out, err => {
  if (err) {
    console.error(err)
  } else {
    console.log('done')
    console.log(counter.length)
  }
})

pass-the-fun-node-v15.js

const fs = require('fs')
const { pipeline, PassThrough } = require('stream')

const input = fs.createReadStream(process.argv[2])
const out = fs.createWriteStream(process.argv[2] + '.bak')

const counter = new PassThrough({
  construct (callback) {
    this.length = 0
    callback()
  },
  transform (chunk, encoding, callback) {
    this.length += chunk.length
    console.log(`this.length=${this.length}, read ${chunk.length} bytes`)
    callback(null, chunk)
  }
})

pipeline(input, counter, out, err => {
  if (err) {
    console.error(err)
  } else {
    console.log('done')
    console.log(counter.length)
  }
})