实现基础版Promise

class Prom {
  static resolve(value) {
    if (value && value.then) {
      return value;
    }
    return new Prom((resolve) => resolve(value));
  }

  constructor(fn) {
    this.value = undefined;
    this.reason = undefined;
    this.status = "PENDING";

    // 维护一个 resolve/pending 的函数队列
    this.resolveFns = [];
    this.rejectFns = [];

    const resolve = (value) => {
      // 注意此处的 setTimeout
      setTimeout(() => {
        this.status = "RESOLVED";
        this.value = value;
        this.resolveFns.forEach(({ fn, resolve: res, reject: rej }) =>
          res(fn(value))
        );
      });
    };

    const reject = (e) => {
      setTimeout(() => {
        this.status = "REJECTED";
        this.reason = e;
        this.rejectFns.forEach(({ fn, resolve: res, reject: rej }) =>
          rej(fn(e))
        );
      });
    };

    fn(resolve, reject);
  }

  then(fn) {
    if (this.status === "RESOLVED") {
      const result = fn(this.value);
      // 需要返回一个 Promise
      // 如果状态为 resolved，直接执行
      return Prom.resolve(result);
    }
    if (this.status === "PENDING") {
      // 也是返回一个 Promise
      return new Prom((resolve, reject) => {
        // 推进队列中，resolved 后统一执行
        this.resolveFns.push({ fn, resolve, reject });
      });
    }
  }

  catch(fn) {
    if (this.status === "REJECTED") {
      const result = fn(this.value);
      return Prom.resolve(result);
    }
    if (this.status === "PENDING") {
      return new Prom((resolve, reject) => {
        this.rejectFns.push({ fn, resolve, reject });
      });
    }
  }
}

Prom.resolve(10)
  .then((o) => o * 10)
  .then((o) => o + 10)
  .then((o) => {
    console.log(o);
  });