flush-stdout.js

#!/usr/bin/env node

const doFlush = process.argv.includes('flush')

const child = () => {
  const N = 1e6

  for (let i = 0; i < N; i++) {
    const arr = [i,i,i,i,i,i,i,i,i,i]
    const a = arr.toString() + ' ' + arr.length
    console.log(a)
  }

  if (doFlush)
    process.stdout.write('', () => process.exit())
  else
    process.exit()
}

const parent = () => {
  const { spawn } = require('child_process')
  const child = spawn(process.execPath, [__filename, 'child', doFlush ? 'flush' : 'noflush'])
  let lastChunk = null
  child.stdout.on('data', c => lastChunk = c.toString())
  child.stdout.on('close', () => console.log(lastChunk.slice(-100)))
}

if (process.argv.includes('child'))
  child()
else
  parent()

/*
$ node flush-stdout.js
287,3287,3287,3287,3287,3287,3287,3287,3287 10
3288,3288,3288,3288,3288,3288,3288,3288,3288,3288 10

$ node flush-stdout.js flush
999997,999997,999997,999997,999997,999997 10
999998,999998,999998,999998,999998,999998,999998,999998,999998,999998 10
999999,999999,999999,999999,999999,999999,999999,999999,999999,999999 10
*/