实现 Promise

问题

实现一个支持 then、catch 和 resolve/reject 的基本 Promise。

解答

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

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

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

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

  then(onFulfilled, onRejected) {
    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v;
    onRejected = typeof onRejected === 'function' ? onRejected : e => { throw e };

    return new MyPromise((resolve, reject) => {
      if (this.state === 'fulfilled') {
        setTimeout(() => {
          try {
            const result = onFulfilled(this.value);
            resolve(result);
          } catch (error) {
            reject(error);
          }
        });
      }

      if (this.state === 'rejected') {
        setTimeout(() => {
          try {
            const result = onRejected(this.reason);
            resolve(result);
          } catch (error) {
            reject(error);
          }
        });
      }

      if (this.state === 'pending') {
        this.onFulfilledCallbacks.push(() => {
          setTimeout(() => {
            try {
              const result = onFulfilled(this.value);
              resolve(result);
            } catch (error) {
              reject(error);
            }
          });
        });
        this.onRejectedCallbacks.push(() => {
          setTimeout(() => {
            try {
              const result = onRejected(this.reason);
              resolve(result);
            } catch (error) {
              reject(error);
            }
          });
        });
      }
    });
  }

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

使用示例

const p = new MyPromise((resolve, reject) => {
  setTimeout(() => resolve('成功!'), 1000);
});

p.then(value => console.log(value)); // "成功!"