BFE.devAI解答 - JavaScript 代码题
67. 请自行实现Promise

以下是一个可能的解答：

class MyPromise {
  constructor(executor) {
    this.state = 'pending';
    this.value = undefined;
    this.reason = undefined;
    this.onResolvedCallbacks = [];
    this.onRejectedCallbacks = [];

    const resolve = value => {
      if (this.state === 'pending') {
        this.state = 'fulfilled';
        this.value = value;
        this.onResolvedCallbacks.forEach(cb => cb(this.value));
      }
    };

    const reject = reason => {
      if (this.state === 'pending') {
        this.state = 'rejected';
        this.reason = reason;
        this.onRejectedCallbacks.forEach(cb => cb(this.reason));
      }
    };

    try {
      executor(resolve, reject);
    } catch (error) {
      reject(error);
    }
  }

  then(onFulfilled, onRejected) {
    const promise2 = new MyPromise((resolve, reject) => {
      if (this.state === 'fulfilled') {
        setTimeout(() => {
          try {
            const x = onFulfilled(this.value);
            this.resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        })
      } else if (this.state === 'rejected') {
        setTimeout(() => {
          try {
            const x = onRejected(this.reason);
            this.resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        })
      } else if (this.state === 'pending') {
        this.onResolvedCallbacks.push(() => {
          setTimeout(() => {
            try {
              const x = onFulfilled(this.value);
              this.resolvePromise(promise2, x, resolve, reject);
            } catch (error) {
              reject(error);
            }
          })
        })
        this.onRejectedCallbacks.push(() => {
          setTimeout(() => {
            try {
              const x = onRejected(this.reason);
              this.resolvePromise(promise2, x, resolve, reject);
            } catch (error) {
              reject(error);
            }
          })
        })
      }
    })

    return promise2;
  }

  catch(onRejected) {
    return this.then(null, onRejected);
  }

  resolvePromise(promise2, x, resolve, reject) {
    if (x === promise2) {
      return reject(new TypeError('Chaining cycle detected'));
    }

    let called = false;

    if (x instanceof MyPromise) {
      x.then(
        value => {
          if (called) return;
          called = true;
          this.resolvePromise(promise2, value, resolve, reject);
        },
        reason => {
          if (called) return;
          called = true;
          reject(reason)
        }
      )
    } else if (x !== null && (typeof x === 'object' || typeof x === 'function')) {
      try {
        const then = x.then;
        if (typeof then === 'function') {
          then.call(x,
            value => {
              if (called) return;
              called = true;
              this.resolvePromise(promise2, value, resolve, reject);
            },
            reason => {
              if (called) return;
              called = true;
              reject(reason);
            }
          )
        } else {
          resolve(x);
        }
      } catch (error) {
        if (called) return;
        called = true;
        reject(error);
      }
    } else {
      resolve(x);
    }
  }

  static resolve(value) {
    if (value instanceof MyPromise) {
      return value;
    }
    return new MyPromise(resolve => resolve(value));
  }

  static reject(reason) {
    return new MyPromise((_, reject) => reject(reason));
  }
}

这个实现使用类构造函数创建promise，并存储状态、值和回调函数列表。它实现了两个核心器方法 then() 和 catch() 以支持链式调用。

函数 resolvePromise() 处理可能的快速解决（fulfillment）结果，并将其解析为与当前promises级联的结果。

最后，静态方法 resolve() 、reject() 从构造函数中创建新的promises对外提供了帮助函数。