Trying out some concepts of Promise and async/await in Javascript.

async_await.js

function waiter(str) {
  return new Promise((resolve, reject) => {
    return setTimeout(() => resolve(str), 1000)
  })
}

await waiter('one').then((res) => waiter(res + 'two'))
// 'onetwo'

await waiter('one').then((res) => { waiter(res + 'two') })
// undefiend

await waiter('one').then(async (res) => (await waiter(res + 'two')))
// 'onetwo'

await waiter('done')
// 'done'

await waiter('done').then(val => console.log(val))
// 'done'
// undefined 

await waiter('done').then(val => (val + 2))
// 'done2' [after one sec]

await waiter('done').then(val => waiter(val + 2))
// 'done2' [after two secs]

for (let i=0; i<5; i++) {
    await waiter('wait: '+i)
    console.log('log: '+i)
}
// log: 0
// log: 1
// log: 2
// log: 3
// log: 4

for (let i=0; i<5; i++) {
    await waiter('wait: '+i).then(console.log)
    console.log('log: '+i)
}
// wait: 0
// log: 0
// wait: 1
// log: 1
// wait: 2
// log: 2
// wait: 3
// log: 3
// wait: 4
// log: 4

function waiterTimed(str) {
  const startTime = Date.now()
  console.log("log start:", str)
  return new Promise((resolve, reject) => {
    return setTimeout(() => {
      const endTime = Date.now()
      console.log("log end:", str, endTime - startTime)
      resolve(str)
    }, 1000)
  })
}

await waiterTimed('one').then((res) => waiterTimed(res + 'two'))
// 'onetwo'

for (let i=0; i<5; i++) {
    await waiterTimed('wait: '+i).then(console.log)
    console.log('log: '+i)
}