Answer 1

function multiRequest(urls, maxNum) {
  let i = 0
  let res = []
  return new Promise(r => {
    for(; i < maxNum; i++) {
      addTask()
    }
    function addTask() {
      res[i] = fetch(urls[i])
      res[i].then(res => { i >= urls.length && r();i < urls.length && addTask(); i++;  })
    }
  }).then((_) => {
    Promise.all(res).then(ans => {
      console.log(ans)
    })
  })
}
function fetch(url) {
  return new Promise((resolve) => {
    let start = new Date()
    setTimeout(() => {
      resolve(`start: ${start};end: ${new Date()}`)
    }, 10000 * Math.random());
  })
}
multiRequest([1,2,3,4,5,6,7,8,9], 3)