Simple handwritten promise function

const PENDING = 'pending';
    const FULFILLED = 'fulfilled';
    const REJECT = 'reject';
    function myPromise(fn) {
      let _this = this;
      this.state = PENDING;
      this.value = undefined;
      this.reason = undefined;
      this.onFulfillList = [];
      this.onRejectList = [];
      function reslove(value) {
        _this.state = FULFILLED;
        _this.value = value;
        _this.onFulfillList.forEach((item) = > item(value));
      }
      function reject(reason) {
        _this.state = REJECT;
        _this.reason = reason;
        _this.onRejectList.forEach((item) = > item(reason));
      }
      try {
        fn(reslove, reject);
      } catch (error) {
        reject(error);
      }
      myPromise.prototype.then = function(res, rej) {
        if (this.state === 'fulfilled') {
          typeof res === 'function' && res(this.value);
        }
        if (this.state === 'reject') {
          typeof rej === 'function' && rej(this.reason);
        }
        if (this.state === 'pending') {
          typeof res === 'function' && this.onFulfillList.push(res);
          typeof rej === 'function' && this.onRejectList.push(rej); }};Copy the code

Reference zhuanlan.zhihu.com/p/144058361 if any infringement, please contact me to delete