处理不同的情况

通常情况下，我们在.then时返回的时一个Promise对象，而不是向上节案例中的 return 2;

p.then((res) => {
  console.log(res);
  return new Promise((resolve, reject) => {
    resolve(2);
  });
}).then((res) => {
  console.log(res); // Promise
});

此时在下一个 .then 的时候，打印出来的结果是一个Promise对象

原因

let x = onFulfilled(this.value); // 得到一个Promise
resolve(x); // 没做任何处理直接返回

分析情况

• 如果是一个普通值，直接调用 resolve
• 如果是Promise对象，看Promise是成功还是失败 如果成功resolve；如果失败reject
• 在很多地方都需要进行该返回值的判断处理，因此将该方法封装起来

class Promise {
  constructor(executor) {
    this.state = "pending";
    this.value = undefined;
    this.reason = undefined;
    this.onResolvedCallbacks = [];
    this.onRejectedCallbacks = [];
    let resolve = (value) => {
      if (this.state === "pending") {
        this.state = "fulfilled";
        this.value = value;
        this.onResolvedCallbacks.forEach((fn) => fn());
      }
    };
    let reject = (reason) => {
      if (this.state === "pending") {
        this.state = "rejected";
        this.reason = reason;
        this.onRejectedCallbacks.forEach((fn) => fn());
      }
    };
    try {
      executor(resolve, reject);
    } catch (err) {
      reject(err);
    }
  }
  then(onFulfilled, onRejected) {
    let p2 = new Promise((resolve, reject) => {
      if (this.state === "fulfilled") {
        let x = onFulfilled(this.value);
        resolvePromise(x, resolve, reject);
      }
      if (this.state === "rejected") {
        onRejected(this.reason);
      }
      if (this.state === "pending") {
        this.onResolvedCallbacks.push(() => {
          onFulfilled(this.value);
        });
        this.onRejectedCallbacks.push(() => {
          onRejected(this.reason);
        });
      }
    });
    return p2;
  }
}

function resolvePromise(x, resolve, reject) {
  // 如果是Promise对象
  if (x instanceof Promise) {
    x.then(
      (value) => {
        // 成功时进行resolve的回调
        resolve(value);
      },
      (err) => {
        // 失败时进行reject的回调
        reject(err);
      }
    );
  } else {
    resolve(x);
  }
}

const p = new Promise((resolve, reject) => {
  resolve(1);
});

p.then((res) => {
  console.log(res);
  return new Promise((resolve, reject) => {
    resolve(2);
  });
}).then((res) => {
  console.log(res);
});