startCountdown() {

    let that = this; // 保存this引用

    this.setData({

      timer: setInterval(() => {

        let currentCountdown = that.data.countdown - 1;

        that.setData({

          countdown: currentCountdown

        });

        if (currentCountdown <= 0) {

          console.log("倒计时结束");

          wx.exitMiniProgram({

            success: (res) => {

              console.log("退出小程序");

            },

            fail: (err) => {

              console.log("退出小程序失败", err);

            }

          });

          clearInterval(that.data.timer);

          // 倒计时结束后的操作

        }

      }, 1000)

    });

  },