promise.js

const a = () => (
  new Promise((res) => {
    console.log('-- a start');
    setTimeout(() => {
      console.log('a');
      res('-- a finish');
    }, 1000)
  })
)
const b = () => (
  new Promise((res) => {
    console.log('-- b start');
    setTimeout(() => {
      console.log('b');
      res('-- b finish');
    }, 500)
  })
)

// [a, b].forEach((f) => f());

const solution_a = function () {
  a()
    .then((r) => {
      console.log(r);
      return b();
    })
    .then((r) => {
      console.log(r);
    });
};
const solution_b = function () {
  [a, b].reduce((arr, f) => {
    return arr.then((r) => f())
  }, Promise.resolve());
}
async function solution_c() {
  var result = await a();
  console.log(result);
  var result = await b();
  console.log(result);
};

solution_a();
solution_b();
solution_c();