node/test/parallel/test-child-process-stdout-flush-exit.js

'use strict';
const common = require('../common');
const assert = require('assert');

// if child process output to console and exit
if (process.argv[2] === 'child') {
  console.log('hello');
  for (let i = 0; i < 200; i++) {
    console.log('filler');
  }
  console.log('goodbye');
  process.exit(0);
} else {
  // parent process
  const spawn = require('child_process').spawn;

  // spawn self as child
  const child = spawn(process.argv[0], [process.argv[1], 'child']);

  let stdout = '';

  child.stderr.setEncoding('utf8');
  child.stderr.on('data', function(data) {
    console.log('parent stderr: ' + data);
    assert.ok(false);
  });

  // check if we receive both 'hello' at start and 'goodbye' at end
  child.stdout.setEncoding('utf8');
  child.stdout.on('data', function(data) {
    stdout += data;
  });

  child.on('close', common.mustCall(function() {
    assert.strictEqual(stdout.slice(0, 6), 'hello\n');
    assert.strictEqual(stdout.slice(stdout.length - 8), 'goodbye\n');
  }));
}