class MyPromise {
    constructor(excutor) {}
    then(onfulfilled, onrejected) {}
    catch(onrejected) {}
}

var p = new Promise((resolve, reject) => {
    resolve(1)
})
p.then(console.log)
p.then(console.log)

1
1

所以说内部应该有必要存这个结果值，同时还有必要存一系列的回调函数，所以先来看看构造函数的实现

构造函数

constructor(excutor) {
    // 状态
    this._state = 'pending'
    // 成功回调
    this._callbacks_resolved = []
    // 失败回调
    this._callbacks_rejected = []
    // 执行
    excutor((data) => {
        // 避免重复执行
        if (this._state !== 'pending') {
            return
        }
        // 保存结果
        this._data = data
        // 状态变更
        this._state = "resolved"
        // 处理
        this._handle()
    }, (err) => {
        if (this._state !== 'pending') {
            return
        }
        // 错误信息
        this._err = err
        // 状态变更
        this._state = "rejected"
        this._handle()
    })
}

看看处理函数

也就是根据对应的状态执行对应的回调，当然不要忘了处理完之后要清空

_handle() {
    if (this._state === 'pending') {
        return;
    }
    // 为什么要延时，then的东西是不能马上执行的，可以看看micro-task和macro-task，这里只能模拟
    setTimeout(() => {
        if (this._state === 'resolved') {
            this._callbacks_resolved.forEach(cb => cb(this._data))
        } else {
            this._callbacks_rejected.forEach(cb => cb(this._err))
        }
        this._callbacks_resolved = []
        this._callbacks_rejected = []
    })
}

then

到了关键的一步，也就是核心的then
这个then的放回结果本身就是一个Promise，所以是可以无限的套的，也就像这样.then().then()
还支持值传递，上一个没有处理的，直接流到下一级，上一级把错误处理后，下一级状态变更为resolved

最开始想直接返回this，发觉这样问题大了去了，所以得另外new一个MyPromise实例，代码如下

then(onfulfilled, onrejected) {
    return new MyPromise((resolve, reject) => {
        this._callbacks_resolved.push(data => {
            // 没有处理，直接往后传
            if (!onfulfilled) {
                return resolve(data)
            }
            try {
                let r = onfulfilled(data)
                // 有then函数，就认为是Promise
                if (r && typeof r.then === "function") {
                    return r.then(resolve, reject)
                }
                resolve(r)
            } catch(e) {
                // 有错误直接向后传
                reject(e)
            }
        })
        this._callbacks_rejected.push(err => {
            if (!onrejected) {
                return reject(err)
            }
            try {
                let r = onrejected(err)
                if (r && typeof r.then === "function") {
                    return r.then(resolve, reject)
                }
                resolve(r)
            } catch(e) {
                reject(e)
            }
        })
        this._handle()
    })
}

以上基本功能已经实现完成，最后还有一个catch函数，很简单，包装一下就好了

catch(onrejected) {
    return this.then(undefined, onrejected)
}

随便写了个测试用例

var p = new MyPromise((resolve, reject) => {
  resolve(1);
})
  .then((n) => {
    console.log(n);
    return new MyPromise((resolve, reject) => {
      resolve(2);
    });
  })
  .then(console.log)
  .then(() => {
    throw new Error("error");
  });

p.catch((e) => {
  console.error(e);
});

p.catch((e) => {
  console.error(e);
});

输出

1
2
error
error

完全符合预期

好了，完结

附上全部代码

class MyPromise {
  constructor(excutor) {
    // 状态
    this._state = "pending";
    // 成功回调
    this._callbacks_resolved = [];
    // 失败回调
    this._callbacks_rejected = [];
    // 执行
    excutor(
      (data) => {
        // 避免重复执行
        if (this._state !== "pending") {
          return;
        }
        // 保存结果
        this._data = data;
        // 状态变更
        this._state = "resolved";
        // 处理
        this._handle();
      },
      (err) => {
        if (this._state !== "pending") {
          return;
        }
        // 错误信息
        this._err = err;
        // 状态变更
        this._state = "rejected";
        this._handle();
      }
    );
  }
  then(onfulfilled, onrejected) {
    return new MyPromise((resolve, reject) => {
      this._callbacks_resolved.push((data) => {
        // 没有处理，直接往后传
        if (!onfulfilled) {
          return resolve(data);
        }
        try {
          let r = onfulfilled(data);
          // 有then函数
          if (r && typeof r.then === "function") {
            return r.then(resolve, reject);
          }
          resolve(r);
        } catch (e) {
          // 有错误直接向后传
          reject(e);
        }
      });
      this._callbacks_rejected.push((err) => {
        if (!onrejected) {
          return reject(err);
        }
        try {
          let r = onrejected(err);
          if (r && typeof r.then === "function") {
            return r.then(resolve, reject);
          }
          resolve(r);
        } catch (e) {
          reject(e);
        }
      });
      this._handle()
    });
  }

  catch(onrejected) {
    return this.then(undefined, onrejected);
  }

  _handle() {
    if (this._state === "pending") {
      return;
    }
    // 为什么要延时，then的东西是不能马上执行的，可以看看micro-task和macro-task，这里只能模拟
    setTimeout(() => {
      if (this._state === "resolved") {
        this._callbacks_resolved.forEach((cb) => cb(this._data));
      } else {
        this._callbacks_rejected.forEach((cb) => cb(this._err));
      }
      this._callbacks_resolved = [];
      this._callbacks_rejected = [];
    });
  }
}

链接: https://www.fly63.com/article/detial/8711