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));
Promise.all可以保證，promises數(shù)組中所有promise對(duì)象都達(dá)到resolve狀態(tài)，才執(zhí)行then回調(diào)。

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) {
  // 請(qǐng)求總數(shù)量
  const len = urls.length;
  // 根據(jù)請(qǐng)求數(shù)量創(chuàng)建一個(gè)數(shù)組來(lái)保存請(qǐng)求的結(jié)果
  const result = new Array(len).fill(false);
  // 當(dāng)前完成的數(shù)量
  let count = 0;

  return new Promise((resolve, reject) => {
    // 請(qǐng)求maxNum個(gè)
    while (count < maxNum) {
      next();
    }
    function next() {
      let current = count++;
      // 處理邊界條件
      if (current >= len) {
        // 請(qǐng)求全部完成就將promise置為成功狀態(tài), 然后將result作為promise值返回
        !result.includes(false) && resolve(result);
        return;
      }
      const url = urls[current];
      console.log(`開始 ${current}`, new Date().toLocaleString());
      fetch(url)
        .then((res) => {
          // 保存請(qǐng)求結(jié)果
          result[current] = res;
          console.log(`完成 ${current}`, new Date().toLocaleString());
          // 請(qǐng)求沒有全部完成, 就遞歸
          if (current < len) {
            next();
          }
        })
        .catch((err) => {
          console.log(`結(jié)束 ${current}`, new Date().toLocaleString());
          result[current] = err;
          // 請(qǐng)求沒有全部完成, 就遞歸
          if (current < len) {
            next();
          }
        });
    }
  });
}