promise.reduce.ts

export const doThis = count => Array.prototype.map.bind(Array.from({ length: count }));

export const Timeout = (timeout, promise?: Promise<any>) =>
  new Promise((resolve, reject) => {
    const timer = setTimeout(() => {
      reject(new Error(`Timeout(${timeout})`));
    }, timeout);
    // 如果完成了得清理
    const clear = () => clearTimeout(timer);
    promise.then(clear, clear);
  });

export const Reduce = (promises, initialValue, timeout?: number) => {
  if (!Array.isArray(promises)) return Promise.resolve(new Error('promises is not Array'));
  const promise = new Promise((resolve, reject) => {
    function handle(task, prevValue) {
      return (timer ? Promise.race([timer, Promise.resolve(task(prevValue))]) : Promise.resolve(task(prevValue)))
        .then((data) => {
          if (promises.length) {
            return handle(promises.shift(), data);
          }
          return resolve(data);
        })
        .catch(reject);
    }
    return Promise.resolve().then(() => handle(promises.shift(), initialValue));
  });

  const timer = timeout && Timeout(timeout, promise);
  // 超时
  return timer ? Promise.race([timer, promise]) : promise;
};

Reduce(doThis(PING_COUNT)((_, index) => (prevData) => {
    return pingPromise(this.url).then(data => {
      const latencySum = prevData + data.end - data.start;
      this.realTimeLatency = latencySum / (index + 1);
      return latencySum;
    });
  }), 0, 1).then((latencySum: number) => {
    return latencySum / PING_COUNT;
  });
}