Promise实现原理，给一份实现代码

Promise 是一种异步编程的解决方案，其本质上是一个状态机，包含三种状态：pending、fulfilled 和 rejected。

以下是 Promise 的基本实现代码：

class Promise {
  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((callback) => callback());
      }
    };

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

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

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

    const promise2 = new Promise((resolve, reject) => {
      if (this.state === 'fulfilled') {
        setTimeout(() => {
          try {
            const x = onFulfilled(this.value);
            resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, 0);
      } else if (this.state === 'rejected') {
        setTimeout(() => {
          try {
            const x = onRejected(this.reason);
            resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, 0);
      } else {
        this.onFulfilledCallbacks.push(() => {
          setTimeout(() => {
            try {
              const x = onFulfilled(this.value);
              resolvePromise(promise2, x, resolve, reject);
            } catch (error) {
              reject(error);
            }
          }, 0);
        });

        this.onRejectedCallbacks.push(() => {
          setTimeout(() => {
            try {
              const x = onRejected(this.reason);
              resolvePromise(promise2, x, resolve, reject);
            } catch (error) {
              reject(error);
            }
          }, 0);
        });
      }
    });

    return promise2;
  }

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

const resolvePromise = (promise2, x, resolve, reject) => {
  if (promise2 === x) {
    reject(new TypeError('Chaining cycle detected for promise'));
  }

  let called = false;

  if (x !== null && (typeof x === 'object' || typeof x === 'function')) {
    try {
      const then = x.then;

      if (typeof then === 'function') {
        then.call(
          x,
          (y) => {
            if (called) return;
            called = true;
            resolvePromise(promise2, y, resolve, reject);
          },
          (r) => {
            if (called) return;
            called = true;
            reject(r);
          }
        );
      } else {
        resolve(x);
      }
    } catch (error) {
      if (called) return;
      called = true;
      reject(error);
    }
  } else {
    resolve(x);
  }
};

这个实现代码中，我们首先定义了一个 Promise 类，其构造函数接受一个 executor 函数，如果在 executor 函数中调用了 resolve 方法，则 Promise 的状态会变为 fulfilled，如果调用了 reject 方法，则 Promise 的状态会变为 rejected。

在 then 方法中，我们首先对 onFulfilled 和 onRejected 进行了类型判断，然后根据当前 Promise 的状态，分别执行对应的回调函数。如果当前 Promise 的状态是 pending，我们则将回调函数存储到 onFulfilledCallbacks 或 onRejectedCallbacks 中，等待 Promise 的状态发生变化后再执行。

在 resolvePromise 方法中，我们首先判断 x 是否等于 promise2，如果是则说明出现了循环引用的情况，直接抛出错误。然后我们判断 x 是否为对象或函数，如果是，则尝试获取其 then 方法。如果获取到了，则调用 then 方法，并将 resolve 和 reject 方法作为参数传递给 then 方法。如果 then 方法返回值仍然是一个 Promise，则递归调用 resolvePromise 方法，直到返回值不为 Promise 为止。否则，直接将返回值作为 Promise 的结果传递给下一个 then 方法。如果 x 不是对象或函数，则直接将其传递给下一个 then 方法。