手写 promise

// 定义为常量好处: 复用并且写代码时有提示
const PENDING = "pending"; // 等待
const FULFILLED = "fulfilled"; // 成功
const REJECTED = "rejected"; // 失败

class MyPromise {
  constructor(exectur) {
    // 执行器
    try {
      exectur(this.resolve, this.reject);
    } catch (error) {
      this.reject(error);
    }
  }
  /* 
      Promise 状态是每一个 Promise 独有的,所以我们定义为实例属性
      因为我们需要频繁的去使用它 这里我们可以将三种状态定义为常量
      默认为 等待状态
    */
  status = PENDING;
  // 成功之后的值
  value = undefined;
  // 失败后的原因
  reason = undefined;
  // 成功回调
  successCallback = [];
  // 失败回调
  failCallback = [];
  /*
      定义成箭头函数原因:
        调用的时候是直接调用的 如: resolve() 
        为了改变this指向,箭头函数的this指向的是执行上下文环境,在这就是指向 MyPromise的实例对象 
    */
  resolve = (value) => {
    // 如果状态不是等待,阻止程序向下执行
    if (this.status !== PENDING) return;
    // 将状态更改为成功
    this.status = FULFILLED;
    // 保存成功之后的值
    this.value = value;
    // 判断成功回调是否存在 如果存在 就调用
    // this.successCallback && this.successCallback(this.value);
    while (this.successCallback.length) this.successCallback.shift()();
  };
  reject = (reason) => {
    // 如果状态不是等待,阻止程序向下执行
    if (this.status !== PENDING) return;
    // 将状态更改为失败
    this.status = REJECTED;
    // 保存失败后的原因
    this.reason = reason;
    // 判断失败回调是否存在 如果存在 就调用
    // this.failCallback && this.failCallback(this.reason);
    while (this.failCallback.length) this.failCallback.shift()();
  };
  then(successCallback, failCallback) {
    successCallback = successCallback ? successCallback : (value) => value;
    failCallback = failCallback
      ? failCallback
      : (reason) => {
          throw reason;
        };
    let promise2 = new MyPromise((resolve, reject) => {
      // 判断状态
      if (this.status === FULFILLED) {
        // 为了获得返回的 promise2对象 将代码变成异步代码
        setTimeout(() => {
          try {
            // 拿到成功回调的返回值
            let x = successCallback(this.value);
            /* 
                          判断x的值是普通值还是 promise 对象
                            普通值：直接调用 resolve
                            promise对象：查看promise对象返回的结果，再根据结果 决定调用 resolve 还是 reject
                        */
            resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, 0);
      } else if (this.status === REJECTED) {
        setTimeout(() => {
          try {
            let x = failCallback(this.reason);
            resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, 0);
      } else {
        // 等待状态
        // 将成功回调和失败回调存储起来
        this.successCallback.push(() => {
          setTimeout(() => {
            try {
              let x = successCallback(this.value);
              resolvePromise(promise2, x, resolve, reject);
            } catch (error) {
              reject(error);
            }
          }, 0);
        });
        this.failCallback.push(() => {
          setTimeout(() => {
            try {
              let x = failCallback(this.reason);
              resolvePromise(promise2, x, resolve, reject);
            } catch (error) {
              reject(error);
            }
          }, 0);
        });
      }
    });
    return promise2;
  }
  finally(callback) {
    // 得到当前 Promise对象的状态
    return this.then(
      (value) => {
        // 等待 finally中的 Promise对象执行完成后，再返回 value
        return MyPromise.resolve(callback()).then(() => value);
      },
      (reason) => {
        return MyPromise.reject(callback()).then(() => {
          throw reason;
        });
      }
    );
  }
  catch(failCallback) {
    return this.then(undefined, failCallback);
  }
  static all(array) {
    // 用来存放结果的数组
    let result = [];
    let index = 0;
    return new MyPromise((resolve, reject) => {
      function addData(key, value) {
        result[key] = value;
        index++;
        /* 
                  因为参数有可能有异步状态
                  等待所有异步操作完成后才能调用resolve方法 
                */
        if (index === array.length) {
          resolve(result);
        }
      }

      for (let i = 0, len = array.length; i < len; i++) {
        let current = array[i];
        // 判断 current是否是MyPromise 的实例
        if (current instanceof MyPromise) {
          // promise对象
          current.then((value) => addData(i, value), reject);
        } else {
          // 普通值
          addData(i, current);
        }
      }
    });
  }
  // 只要有一个成功或者失败就返回
  static race(array) {
    let promise = new MyPromise((resolve, reject) => {
      for (let i = 0; i < array.length; i++) {
        let curr = array[i];
        // MyPromise实例 结果处理
        if (curr instanceof MyPromise) {
          curr.then(resolve, reject);
        } else {
          // 非MyPromise实例处理
          resolve(curr);
        }
      }
    });
    return promise;
  }
  static resolve(value) {
    if (value instanceof MyPromise) return value;
    return new MyPromise((resolve) => resolve(value));
  }
  static reject(reason) {
    // 如果是MyPromise 实例 则直接返回
    if (reason instanceof MyPromise) return reason;
    // 如果不是MyPromise 实例 否则返回一个 MyPromise实例
    return new MyPromise((resolve, reject) => reject(reason));
  }
}

function resolvePromise(promise2, x, resolve, reject) {
  if (promise2 === x) {
    return reject(
      new TypeError("Chaining cycle detected for promise #<Promise>")
    );
  }
  // 判断 x是否是MyPromise 的实例
  if (x instanceof MyPromise) {
    // promise对象
    // x.then(value => resolve(value),reason => reject(reason));
    // => 简化
    x.then(resolve, reject);
  } else {
    // 普通值
    resolve(x);
  }
}

module.exports = MyPromise;

function p2() {
  return new MyPromise((resolve, reject) => {
    // resolve('p2 resolve');
    reject("p2 reject");
  });
}

p2()
  .then((value) => {
    console.log(value);
  })
  .catch((reason) => {
    console.log(reason);
  });