fun-of-promise.js

const _ = require('lodash');

const task = (n) => {
  return new Promise((resolve, reject)=> {
    setTimeout(() => {
      console.log("t"+n+" done");
      resolve(n);
    }, 1000*n);
  });
};
const check = (message) => {
  return (res) => {
    console.log(res, 'received');
    return new Promise((resolve, reject) => {
      if(message) console.log(message);
      resolve();
    });
  };
};

const seq = () => {
  return task(1)
    .then(() => task(2))
    .then(() => task(3))
    .then(check('completed'));
};

const par = () => {
  return Promise.all([task(1),task(2),task(3)])
    .then(check())
    .then((xs) => task(4))
    .then(check());
};

seq().then(par);
//par();