基本promise

    function myPromise(fn) {
      var value = null,
        callbacks = [];

      this.then = function (onFulfilled) {
        callbacks.push(onFulfilled);
      };

      function resolve(value) {
        callbacks.forEach(function (callback) {
          callback(value);
        });
      }

      fn(resolve);
    }
    let mss = new myPromise(function (resolve) {
      setTimeout(() => {
        resolve(233)
      }, 100);
    })