promise-chain.js

const g = n => n + 1;
const f = n => n * 2;

const wait = time => new Promise(
  (resolve, reject) => setTimeout(
    resolve,
    time
  )
);
wait(300)
  .then(() => 20)
  .then(g)
  .then(f)
  .then(value => console.log(value)) // 42
;


const res = time => new Promise(
  (resolve, reject) => resolve(30)
);
res(300)
  .then(g)
  .then(f)
  .then(value => console.log(value)) // 42
;