parallel

function parallel() {
  let args = Array.prototype.slice.call(arguments);
  let length = args.length;
  let funcsComplete = 0;
  let resFunc;
  let results = [];

  function complete(res) {
    funcsComplete += 1;
    results.push(res);
    checkAllComplete();
  }

  function checkAllComplete(fn) {
    if (funcsComplete === length) {
      resFunc(results);
    }
  }

  return function(fn) {
    resFunc = fn;
    args.forEach((_fn) => {
      setTimeout(_fn.bind(null, complete), 0);
    });
    return checkAllComplete();
  };
}

function a(cb) {
  console.log('func a start');
  setTimeout(() => {
    console.log('func a stop');
    return cb(1);
  }, 1000);
}

function b(cb) {
  console.log('func b start');
  setTimeout(() => {
    console.log('func b stop');
    return cb(2);
  }, 2000);
}

function c(cb) {
  console.log('func c start');
  setTimeout(() => {
    console.log('func c stop');
    return cb(3);
  }, 1500);
}

function d(cb) {
  console.log('func d start');
  setTimeout(() => {
    console.log('func d stop');
    return cb(2);
  }, 200);
}

parallel(a, b, c, d)((results) => console.log('yes', results));