spawn child processes with a promise

cmd.js

const { spawn } = require('child_process')

const readToEnd = stream =>
  new Promise(((resolve, reject) => {
    const parts = []
    stream.on('error', reject)
    stream.on('data', part => parts.push(part))
    stream.on('end', () => resolve(Buffer.concat(parts)))
  }))

const cmd = (args, data) => {
  const instance = spawn(args[0], args.slice(1), { stdio: ['pipe', 'pipe', 'inherit'] })
  instance.stdin.end(data)

  const read = readToEnd(instance.stdout)
  const exit = new Promise(((resolve, reject) => {
    instance.on('error', reject)
    instance.on('exit', exitCode => {
      if (exitCode === 0) {
        resolve()
      } else {
        reject(new Error('process exited with non-zero code'))
      }
    })
  }))
  return exit
    .then(() => read)
    .then(b => b.toString('utf8'))
}

module.exports = cmd