JavaScript如何利用Promise控制并发请求个数

var p = function () {
  return new Promise(function (resolve, reject) {
    setTimeout(() => {
      console.log("1000");
      resolve();
    }, 1000);
  });
};
var p1 = function () {
  return new Promise(function (resolve, reject) {
    setTimeout(() => {
      console.log("2000");
      resolve();
    }, 2000);
  });
};
var p2 = function () {
  return new Promise(function (resolve, reject) {
    setTimeout(() => {
      console.log("3000");
      resolve();
    }, 3000);
  });
};

p()
  .then(() => {
    return p1();
  })
  .then(() => {
    return p2();
  })
  .then(() => {
    console.log("end");
  });

Promise.all((promises: [])).then((fun: function));

var promises = function () {
  return [1000, 2000, 3000].map((current) => {
    return new Promise(function (resolve, reject) {
      setTimeout(() => {
        console.log(current);
      }, current);
    });
  });
};

Promise.all(promises()).then(() => {
  console.log("end");
});

function multiRequest(urls, maxNum) {
 const len = urls.length; // 请求总数量
 const res = new Array(len).fill(0); // 请求结果数组
 let sendCount = 0; // 已发送的请求数量
 let finishCount = 0; // 已完成的请求数量
 return new Promise((resolve, reject) => {
     // 首先发送 maxNum 个请求，注意：请求数可能小于 maxNum，所以也要满足条件2
     // 同步的 创建maxNum个next并行请求 然后才去执行异步的fetch 所以一上来就有5个next并行执行
     while (sendCount < maxNum && sendCount < len) { 
         next();
     }
     function next () {
         let current = sendCount ++; // 当前发送的请求数量，后加一 保存当前请求url的位置
         // 递归出口
         if (finishCount >= len) { 
         // 如果所有请求完成，则解决掉 Promise，终止递归
             resolve(res);
             return;
         }
         const url = urls[current];
         fetch(url).then(result => {
             finishCount ++;
             res[current] = result;
             if (current < len) { // 如果请求没有发送完，继续发送请求
                 next();
             }
         }, err => {
             finishCount ++;
             res[current] = err;
             if (current < len) { // 如果请求没有发送完，继续发送请求
                 next();
             }
         });
     }
 });
}