手写Promise

手写Promise

class MyPromise {
  constructor(excutorCallBack) {
    this.status = "pending";
    this.value = undefined;
    this.resolveCallbackArr = [];
    this.rejectCallbackArr = []; 
    const resolveFn = (result) => {
      // 这里加setTimeout延时，主要是为了让then里面的回调先被添加到对应的数组中，之后有回调可以循环执行
      setTimeout(() => {
        if (this.status === "pending") {
          this.status = "resolved";
          this.value = result;
          if (this.resolveCallbackArr.length) this.resolveCallbackArr.forEach(item => item(this.value));
        };
      }, 0)
    }
    const rejectFn = (reason) => {
      // 这里加setTimeout延时，主要是为了让then里面的回调先被添加到对应的数组中，之后有回调可以循环执行
      setTimeout(() => {
        if (this.status === "pending") {
          this.status = "rejected";
          this.value = reason;
          if (this.rejectCallbackArr.length) this.rejectCallbackArr.forEach(item => item(this.value));
        }
      }, 0)
    }
    // 执行执行器函数(包含异常捕获)
    try {
      excutorCallBack && excutorCallBack(resolveFn, rejectFn);
    } catch (error) {
      rejectFn(error)
    }
  }
  // 实例方法
  then(resolveCallback, rejectCallback) {
    typeof resolveCallback !== "function" ? resolveCallback = result => result : null;
    typeof rejectCallback !== "function" ? rejectCallback = reason => {throw new Error(reason.message)} : null;
    return new MyPromise((resolve, reject) => {
      this.resolveCallbackArr.push(() => {
        try {
          let x = resolveCallback(this.value);
          x instanceof Promise ? x.then(resolve, reject) : resolve(x);
        }catch(err) {
          reject(err);
        }
      })
      this.rejectCallbackArr.push(() => {
        try{
          let x = rejectCallback(this.value);
          x instanceof Promise ? x.then(resolve, reject) : resolve(x);
        }catch(err) {
          reject(err);
        }
      })
    })    
  }
  catch(rejectCallback) {
    return this.then(null, rejectCallback)
  }
}
export default MyPromise;