async function fetchData (a)  {
    const data = await fetch('//127.0.0.1:3000/test')
    const d = await data.json();
    console.log(d);
    return d;
}
fetchData(2) // 編號(hào) 1
fetchData(2) // 2
fetchData(2) // 3
fetchData(2) // 4
fetchData(2) // 4
fetchData(2) // 5
fetchData(2)
fetchData(2)

const cachedAsync = function(fn) {
    const cache = Object.create(null);
    return async str => {
        const hit = cache[str];
        if (hit) {
            return hit;
        }
        // 只緩存成功的Promise, 失敗直接重新請(qǐng)求
        return (cache[str] = await fn(str));
    };
};
const fetch2 = cachedAsync(fetchData)
fetch2(2);
fetch2(2);
fetch2(2);

  const cacheAsync = (promiseGenerator, symbol) => {
    const cache = new Map();
    const never = Symbol();
    return async (params) => {
      return new Promise((resolve, reject) => {
      // 可以提供鍵值
        symbol = symbol || params;
        let cacheCfg = cache.get(symbol);
        if (!cacheCfg) {
          cacheCfg = {
            hit: never,
            exector: [{ resolve, reject }],
          };
          cache.set(symbol, cacheCfg);
        } else {
          // 命中緩存
          if (cacheCfg.hit !== never) {
            return resolve(cacheCfg.hit)
          }
          cacheCfg.exector.push({ resolve, reject });
        }
        const { exector } = cacheCfg;
        // 處理并發(fā)，在請(qǐng)求還處于pending過(guò)程中就發(fā)起了相同的請(qǐng)求
        // 拿第一個(gè)請(qǐng)求
        if (exector.length === 1) {
          const next = async () => {
            try {
              if (!exector.length) return;
              const response = await promiseGenerator(params);
              // 如果成功了，那么直接resolve掉剩余同樣的請(qǐng)求
              while (exector.length) { // 清空
                exector.shift().resolve(response); 
              }
              // 緩存結(jié)果
              cacheCfg.hit = response;
            } catch (error) {
              // 如果失敗了 那么這個(gè)promise的則為reject
              const { reject } = exector.shift();
              reject(error);
              next(); // 失敗重試，降級(jí)為串行
            }
          };
          next();
        }
      });
    };
  };

const koa = require("koa");
const app = new koa();
function sleep(seconds) {
 return new Promise((resolve, reject) => {
   setTimeout(resolve, seconds);
 });
}
app.use(async (ctx, next) => {
 if (ctx.url === "/test") {
   await sleep(200);
   const n = Math.random();
   // 隨機(jī)掛掉接口
   if (n > 0.8) {
       ctx.body = n;
   } else {
       ctx.status = 404
       ctx.body = ''
   }
   next();
 }
});
app.listen(3000, "127.0.0.1", () =>
 console.log("listening on 127.0.0.1:3000")
);

  var fetch2 = cacheAsync(fetchData, "test2");
  async function fetchData(a) {
    const data = await fetch("http://127.0.0.1:3000/test");
    const d = await data.json();
    console.log(d);
    return d;
  }
   // 并發(fā)6個(gè)相同的請(qǐng)求
  console.log(fetch2(2));
  console.log(fetch2(2));
  console.log(fetch2(2));
  console.log(fetch2(2));
  console.log(fetch2(2));
  console.log(fetch2(2));