promise A+ 第三遍

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    <title>promise A+ 规范</title>
  </head>
  <body>
    <script>
      let concept = `1：每个异步操作都看作一个任务，任务都有两个阶段和三个状态，分别是 settled 已决阶段 和 unsettled 未决阶段，分别是 pending 等待态
                     fulfilled 成功态 rejected 失败态
                     2：状态之间不可逆转，只能由 pending 态转 fulfilled态 或者 pending转 rejected态；
                     3：任务由未决状态转已决状态，其中由pending态转fulfilled态的动作称之为 resolve，由pending态转rejected态的动作称之为 rejected态；
                     4：任务都提供一个then方法，这个方法有两个回调参数，分别是 onFulfilled onRejected`;
      const PENDING = 'pending';
      const FULFILLED = 'fulfilled';
      const REJECTED = 'rejected';
      class myPromise {
        constructor(executor) {
          this.status = PENDING;
          this.data = null;
          this.reason = null;
          this.dispatchers = [];
          try {
            executor(this.__resolve.bind(this), this.__reject.bind(this));
          } catch (err) {
            this.__reject(err);
          }
        }
        __resolve(data) {
          if (this.status !== PENDING) return;
          this.status = FULFILLED;
          this.data = data;
          this.__dispatchers();
        }
        __reject(reason) {
          if (this.status !== PENDING) return;
          this.status = REJECTED;
          this.reason = reason;
          this.__dispatchers();
        }
        __dispatchers() {
          if (this.status === PENDING) return;
          for (const dispatcherConf of this.dispatchers) {
            this.__dispatcher(dispatcherConf);
          }
        }
        __dispatcher({ status, dispatcher, resolve, reject }) {
          if (this.status !== status) return;
          if (typeof dispatcher !== 'function') {
            this.status === FULFILLED ? resolve(this.data) : reject(this.data);
            return;
          }
          try {
            const result = dispatcher(this.status === FULFILLED ? this.data : this.reason);
            resolve(result);
          } catch (err) {
            reject(err);
          }
        }
        then(onFulfilled, onRejected) {
          return new myPromise((resolve, reject) => {
            this.dispatchers.push(
              {
                status: FULFILLED,
                dispatcher: onFulfilled,
                resolve,
                reject
              },
              {
                status: REJECTED,
                dispatcher: onRejected,
                resolve,
                reject
              }
            );
            this.__dispatchers();
          });
        }
        catch(err) {
          this.then(null, err);
        }
      }
      const myPro = new myPromise((resolve, reject) => {
        setTimeout(function () {
          resolve(1);
        }, 2000);
      });
      myPro.then(r => {
        console.log(r);
      });
    </script>
  </body>
</html>
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原文地址：https://www.cnblogs.com/pengxiangchong/p/16245363.html