How Asynchronous JavaScript Works?

1-fake-async.js

function print(func, wait) {
  console.log(`${func}: ${wait}`)
  setTimeout(() => { console.log(`${func}: ${wait}, waited`) }, wait)
}

async function func1() {
  await print(1, 1000)
  await print(1, 1500)
  await print(1, 2000)
}

async function func2() {
  await print(2, 2000)
  await print(2, 1500)
  await print(2, 1000)
}

async function func3() {
  await Promise.all([
    print(3, 1000),
    print(3, 1500),
    print(3, 2000),
  ])
}

async function func4() {
  await Promise.all([
    print(4, 2000),
    print(4, 1500),
    print(4, 1000),
  ])
}

console.log('\nStart func1')
func1()
console.log('\nStart func2')
func2()
console.log('\nStart func3')
func3()
console.log('\nStart func4')
func4()
console.log('')

1-output.txt

Start func1
1: 1000

Start func2
2: 2000

Start func3
3: 1000
3: 1500
3: 2000

Start func4
4: 2000
4: 1500
4: 1000

1: 1500
2: 1500
1: 2000
2: 1000
1: 1000, waited
3: 1000, waited
4: 1000, waited
2: 1000, waited
3: 1500, waited
4: 1500, waited
1: 1500, waited
2: 1500, waited
2: 2000, waited
3: 2000, waited
4: 2000, waited
1: 2000, waited

2-async.js

function print(str) {
  console.log(str)
}

async function work(func, wait) {
  return new Promise(resolve => {
    const str = `${func}: ${wait}`
    print(str)
    setTimeout(() => resolve(`${str}, waited`), wait)
  })
}

async function func1() {
  print(await work(1, 1000))
  print(await work(1, 1500))
  print(await work(1, 2000))
}

async function func2() {
  print(await work(2, 2000))
  print(await work(2, 1500))
  print(await work(2, 1000))
}

async function func3() {
  const r = await Promise.all([
    work(3, 1000),
    work(3, 1500),
    work(3, 2000),
  ])
  print(r)
}

async function func4() {
  const r = await Promise.all([
    work(4, 2000),
    work(4, 1500),
    work(4, 1000)
  ])
  print(r)
}

console.log('\nStart func1')
func1()
console.log('\nStart func2')
func2()
console.log('\nStart func3')
func3()
console.log('\nStart func4')
func4()
console.log('')

2-output.txt

Start func1
1: 1000

Start func2
2: 2000

Start func3
3: 1000
3: 1500
3: 2000

Start func4
4: 2000
4: 1500
4: 1000

1: 1000, waited
1: 1500
2: 2000, waited
2: 1500
[ '3: 1000, waited', '3: 1500, waited', '3: 2000, waited' ]
[ '4: 2000, waited', '4: 1500, waited', '4: 1000, waited' ]
1: 1500, waited
1: 2000
2: 1500, waited
2: 1000
2: 1000, waited
1: 2000, waited