pipe demo

add.js

#!/usr/bin/env node

let data = '';

const args = process.argv.slice(2);

if (args.length) {
  console.log(sum(args));
  process.exit(0);
}

process.stdin.resume();
process.stdin.setEncoding('utf-8');

process.stdin.on('data', (chunk) => data = data + chunk);
process.stdin.on('end', () => {
  console.log(sum(data.split(' ')));
  process.exit(0);
})

function sum(arr) {
  return arr
    .map(n => parseInt(n, 10))
    .reduce((acc, n) => acc + n, 0);
}