Helper for running a shell command and buffering up its output

run.js

const childProcess = require('child_process');

function consumeStream(stream) {
  return new Promise((resolve, reject) => {
    const buffers = [];
    stream
      .on('data', buffer => buffers.push(buffer))
      .on('end', () => resolve(Buffer.concat(buffers)))
      .on('error', reject);
  });
}

async function run(commandAndArgs, stdin) {
  if (typeof commandAndArgs !== 'undefined' && !Array.isArray(commandAndArgs)) {
    commandAndArgs = [ commandAndArgs ];
  }

  const proc = childProcess.spawn(commandAndArgs[0], commandAndArgs.slice(1));

  const promises = {
    exit: new Promise((resolve, reject) => {
      proc
        .on('error', reject)
        .on('exit', exitCode => {
          if (exitCode === 0) {
            resolve();
          } else {
            reject(new Error(`Child process exited with ${exitCode}`));
          }
        });
    }),
    stdin: new Promise((resolve, reject) => {
      proc.stdin
        .on('error', reject)
        .on('close', resolve);
    }),
    stdout: consumeStream(proc.stdout),
    stderr: consumeStream(proc.stderr)
  };

  if (typeof stdin === 'undefined') {
    proc.stdin.end();
  } else {
    proc.stdin.end(stdin);
  }

  try {
    await Promise.all(Object.values(promises));
    return [
      await promises.stdout,
      await promises.stderr
    ];
  } catch (err) {
    err.stdout = await promises.stdout;
    err.stderr = await promises.stderr;
    throw err;
  }
}

// Use it :)

(async () => {
  try {
    const [ stdout, stderr ] = await run(['python', 'foo', 'bar'], 'stuff for stdin');
    console.log(`yay, success, stdout ${stdout}`);
  } catch (e) {
    console.log(`oh dear, command exited with ${e.code} and stderr ${e.stderr} and stdout ${e.stdout}`);
  }
})();