Promise源码

const PENDING = "pending";
const FULFILLED = "fulfilled";
const REJECTED = "rejected";

class MyPromise {
  constructor(executor) {
    try {
      executor(this.resolve, this.reject);
    } catch (e) {
      this.reject(e);
    }
  }
  // promsie 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 = [];
  resolve = value => {
    //如果状态不是等待，阻止程序向下指向
    if (this.status !== PENDING) return;
    //将状态改为成功
    this.status = FULFILLED;
    //保存成功之后的值
    this.value = value;
    //判断成功回调是否存在，如果存在就直接调用
    // this.successCallback && this.successCallback(this.value = value)
    while (this.successCallback.length) this.successCallback.shift()();
  };
  reject = reason => {
    //如果状态不是等待，阻止程序向下指向
    if (this.status !== PENDING) return;
    //将状态改为失败
    this.status = REJECTED;
    //保存失败之后的原因
    this.reason = reason;
    //判断失败回调是否存在，如果存在就直接调用

    // this.failCallback && this.failCallback(this.reason = reason)
    while (this.failCallback.length) this.failCallback.shift()();
  };
  then(successCallback, failCallback) {
    // 参数可选
    successCallback = successCallback ? successCallback : value => value;
    // 参数可选
    failCallback = failCallback
      ? failCallback
      : reason => {
          throw reason;
        };
    let promise2 = new MyPromise((resolve, reject) => {
      //判断状态
      if (this.status === FULFILLED) {
        setTimeout(() => {
          try {
            let x = successCallback(this.value);
            // 判断 x 的值是普通值还是promise对象
            // 如果是普通值 直接调用resolve
            // 如果是promise对象 查看promsie对象返回的结果
            // 再根据promise对象返回的结果 决定调用resolve 还是调用reject
            resolvePromise(promise2, x, resolve, reject);
            //把错误信息传递给下一个Promise的回调函数
          } catch (error) {
            reject(error);
          }
        }, 0);
      } else if (this.status === REJECTED) {
        setTimeout(() => {
          try {
            let x = failCallback(this.reason);
            // 判断 x 的值是普通值还是promise对象
            // 如果是普通值 直接调用resolve
            // 如果是promise对象 查看promsie对象返回的结果
            // 再根据promise对象返回的结果 决定调用resolve 还是调用reject
            resolvePromise(promise2, x, resolve, reject);
            //把错误信息传递给下一个Promise的回调函数
          } catch (error) {
            reject(error);
          }
        }, 0);
      } else {
        //等待状态
        this.successCallback.push(() => {
          setTimeout(() => {
            try {
              let x = successCallback(this.value);
              // 判断 x 的值是普通值还是promise对象
              // 如果是普通值 直接调用resolve
              // 如果是promise对象 查看promsie对象返回的结果
              // 再根据promise对象返回的结果 决定调用resolve 还是调用reject
              resolvePromise(promise2, x, resolve, reject);
              //把错误信息传递给下一个Promise的回调函数
            } catch (error) {
              reject(error);
            }
          }, 0);
        });
        this.failCallback.push(() => {
          setTimeout(() => {
            try {
              let x = failCallback(this.reason);
              // 判断 x 的值是普通值还是promise对象
              // 如果是普通值 直接调用resolve
              // 如果是promise对象 查看promsie对象返回的结果
              // 再根据promise对象返回的结果 决定调用resolve 还是调用reject
              resolvePromise(promise2, x, resolve, reject);
              //把错误信息传递给下一个Promise的回调函数
            } catch (error) {
              reject(error);
            }
          }, 0);
        });
      }
    });
    return promise2;
  }
  finally(callback) {
    return this.then(
      value => {
        return MyPromise.resolve(callback()).then(value => {});
      },
      reason => {
        return MyPromise.resolve(callback()).then(value => {
          throw reason;
        });
      }
    );
  }
  catch(failCallback) {
    return this.then(undefined, failCallback);
  }
  static all(array) {
    let result = [];
    let index = 0;
    return new MyPromise((resolve, reject) => {
      function addData(key, value) {
        result[key] = value;
        index++;
        if (index == array.length) {
          resolve(result);
        }
      }
      for (let i = 0; i < array.length; i++) {
        let current = array[i];
        if (current instanceof MyPromise) {
          //promise对象
          current.then(
            value => addData(i, value),
            reason => reject(reason)
          );
        } else {
          //普通值
          addData(i, array[i]);
        }
      }
    });
  }
  static resolve(value) {
    if (value instanceof MyPromise) return value;
    return new MyPromise(resolve => resolve(value));
  }
}

function resolvePromise(promise2, x, resolve, reject) {
  //X 是 自己创建的的Promise
  //resolve,reject是构造函数中then方法返回的Promise的参数
  if (promise2 === x) {
    return reject(new TypeError("Chaining cycle detected for promise #<Promise>"));
  }
  if (x instanceof MyPromise) {
    // promise 对象
    x.then(
      value => {
        resolve(value);
      },
      reason => {
        reject(reason);
      }
    );
    // x.then(resolve, reject);
  } else {
    // 普通值
    resolve(x);
  }
}
module.exports = MyPromise;