just show about how to use promise and async await in same file

async-await-promise-test.js

// need to use babel 
// npm install babel-cli
// babel-node file.js

const a = (x) => {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(x)
    }, 3000)
  })
}
const b = (x) => {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(x)
    }, 2000)
  })
}

const c = (x) => {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(x)
    }, 1000)
  })
}

const d = (x) => {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(x)
    }, 500)
  })
}

const foo = async () => {
  console.log(await a(3))
  console.log(await b(4))
}

foo()
c(1)
.then(val => console.log(val))
.then(() => d(2))
.then(val => console.log(val))

// output: 1 2 3 4