手写Promise

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

function resolvePromise(promise2, x, resolve, reject) {
  if (promise2 === x) {
    return reject(
      new TypeError("Chaining cycle detected for promise #<MyPromise>")
    );
  }

  let called = false;
  if ((typeof x === "object" && typeof x !== null) || typeof x === "function") {
    try {
      const then = x.then;
      if (typeof then === "function") {
        then.call(
          x,
          (y) => {
            if (called) return;
            called = true;
            resolvePromise(x, 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);
  }
}

class MyPromise {
  constructor(executor) {
    this.status = PENDING;
    this.value = undefined;
    this.reason = undefined;
    this.onFulfiledCallbacks = [];
    this.onRejectedCallbacks = [];

    const resolve = (value) => {
      if (this.status === PENDING) {
        this.status = FULFILLED;
        this.value = value;
        this.onFulfiledCallbacks.map((fn) => fn());
      }
    };

    const reject = (reason) => {
      if (this.status === PENDING) {
        this.status = REJECTED;
        this.reason = reason;
        this.onRejectedCallbacks.map((fn) => fn());
      }
    };

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

  then(onFulfiled, onRejected) {
    onFulfiled =
      typeof onFulfiled === "function" ? onFulfiled : (value) => value;
    onRejected =
      typeof onRejected === "function"
        ? onRejected
        : (reason) => {
            throw reason;
          };
    const promise2 = new MyPromise((resolve, reject) => {
      if (this.status === FULFILLED) {
        setTimeout(() => {
          try {
            let x = onFulfiled(this.value);
            resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, 0);
      }
      if (this.status === REJECTED) {
        setTimeout(() => {
          try {
            let x = onRejected(this.reason);
            resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, 0);
      }

      if (this.status === PENDING) {
        this.onFulfiledCallbacks.push(() => {
          try {
            let x = onFulfiled(this.value);
            resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        });
        this.onRejectedCallbacks.push(() => {
          try {
            let x = onRejected(this.reason);
            resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        });
      }
    });
    return promise2;
  }
  catch(errorCallback) {
    this.then(null, errorCallback);
  }
}

module.exports = MyPromise;