Promise execution

with-wait.js

async function main () {
  const actions = [...Array(5)].map((_, i) => new Promise((resolve) => {
    console.log(`Promise ${i} has started already...`)
    console.time(`Promise ${i}`)
    setTimeout(() => {
      console.log(`Promise ${i} has finished!`)
      console.timeEnd(`Promise ${i}`)
      resolve()
    })
  }))

  // chain the promises
  return Promise.all(actions)
}

main().then(() => console.log('After main')).catch(err => {
  console.error(err)
  process.exit(1)
})

/*
$ node with-wait.js
Promise 0 has started already...
Promise 1 has started already...
Promise 2 has started already...
Promise 3 has started already...
Promise 4 has started already...
Promise 0 has finished!
Promise 0: 1.777ms
Promise 1 has finished!
Promise 1: 1.511ms
Promise 2 has finished!
Promise 2: 1.519ms
Promise 3 has finished!
Promise 3: 1.540ms
Promise 4 has finished!
Promise 4: 1.576ms
After main
*/

without-wait.js

async function main () {
  const actions = [...Array(5)].map((_, i) => new Promise((resolve) => {
    console.log(`Promise ${i} has started already...`)
    console.time(`Promise ${i}`)
    setTimeout(() => {
      console.log(`Promise ${i} has finished!`)
      console.timeEnd(`Promise ${i}`)
      resolve()
    })
  }))
  
  // note that none of the promises created above have a `.then` anywhere!
}

main().then(() => console.log('After main')).catch(err => {
  console.error(err)
  process.exit(1)
})

/*
$ node without-wait.js
Promise 0 has started already...
Promise 1 has started already...
Promise 2 has started already...
Promise 3 has started already...
Promise 4 has started already...
After main
Promise 0 has finished!
Promise 0: 4.878ms
Promise 1 has finished!
Promise 1: 4.656ms
Promise 2 has finished!
Promise 2: 4.680ms
Promise 3 has finished!
Promise 3: 4.694ms
Promise 4 has finished!
Promise 4: 4.740ms
*/