promise的实现原理

	function myPromise(fn) {
      this.cbs = [];
      const resolve = (value) => {
        setTimeout(() => {
          this.data = value;
          this.cbs.forEach((cb) => cb(value));
        })
      }
      fn(resolve);
    }
    myPromise.prototype.then = function (onResolved) {
      return new myPromise((resolve) => {
        this.cbs.push(() => {
          const res = onResolved(this.data);
          if (res instanceof myPromise) {
            res.then(resolve);
          } else {
            resolve(res);
          }
        });
      });
    }


    let a = new myPromise(function (resolve) {
      setTimeout(function () {
        resolve('123')
      }, 1000)
    })
    a.then((e) => {
      console.log(e)
    })