a()
b()

async function a() {
  for (n=1; n<100; n++) {
    console.log('a', n)
    await sleep(1)
  }
}

async function b() {
  for (n=1; n<100; n++) {
    console.log('b', n)
    await sleep(3)
  }
}

function sleep(ms) {
  return new Promise((resolve) => {
    setTimeout(resolve, ms);
  })
}