easy running / chaining of spawn for gulp

shell.js

const { spawn } = require('child_process');
const through = require('through2');
const Promise = require('bluebird');
const EventEmitter = require('events');

const logger = require('../debug').spawn('gulp:shell');

class NonZeroExit extends Error {
  constructor(code, info) {
    super();
    this.info = info;
    let infoMsg = '';
    if (info) {
      infoMsg = `\n\nInfo:\n${info.join('')}`;
    }
    this.message = `Non 0 exit code: ${code}${infoMsg}`;
  }
}

const childProcessToEmitter = (child) => {
  const retEmitter = new EventEmitter();

  child.once('error', (e) => retEmitter.emit('error', e));
  child.once('close', (code) => {
    if (code !== 0) {
      return retEmitter.emit('error', new NonZeroExit(code, child.$$extraInfo));
    }
    retEmitter.emit('close');
  });
  return retEmitter;
};

function enQueue(q, item, limit = 10) {
  if (q.length === limit) {
    q.shift();
  }
  q.push(item);
  return item;
}

const handleOutput = (child, limit) =>
  through((data, _, cb) => {
    const str = String(data);
    enQueue(child.$$extraInfo, str, limit);
    logger(() => str);
    cb();
  });

const shell = (cmd, limit) => {
  const child = spawn(cmd, { shell: true });
  child.$$extraInfo = []; // cache last 10 lines
  child.stdout.pipe(handleOutput(child, limit));
  child.stderr.pipe(handleOutput(child, limit));

  return childProcessToEmitter(child);
};

const shells = (commands, done) => {
  const promise = Promise.each(
    commands,
    (c) =>
      new Promise((resolve, reject) =>
        shell(c)
          .once('close', resolve)
          .once('error', reject)
      )
  );

  if (!done) return promise;

  promise.then(() => done()).catch(done);
};

module.exports = {
  NonZeroExit,
  shell,
  shells
};