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global.Promise = (function () {
  class Promise {
    constructor(callback) {
      this.val = undefined;
      this.status = "pending";
      this.init(callback);
    }
    onReject = (err) => {
      this.val = err;
      this.status = "reject";
      console.log("onReject is call");
    };
    onResolve = (val) => {
      this.val = val;
      this.status = "fulfilled";
      console.log("onResolve is call");
    };
    init(callback) {
      callback(this.onResolve, this.onReject);
    }
    then(resolve, reject) {
      if (this.status === "fulfilled") {
        resolve(this.val);
      } else if (this.status === "reject") {
        if (!reject === undefined) reject(this.val);
      }
      return this;
    }
    finally(cb) {
      if (this.status === "fulfilled" || this.status === "reject") {
        cb();
      }
    }
    catch(cb) {
      cb(this.val);
      return this;
    }
  }
  Promise.reject = function () {};
  Promise.resolve = function () {};

  return Promise;
})();

function getApi(flag = true) {
  return new Promise((resolve, reject) => {
    if (flag) {
      resolve(123);
    } else {
      reject(new Error("你错了！"));
    }
  });
}
getApi(true)
  .then((res) => {
    console.log(res);
    return 123;
  })
  .then((val) => {
    console.log(val);
  });