//JS部分
data:{
    countDownNum: 3,
    timer: '', //定时器
}
//定时器
  countDown: function () {
    let that = this;
    let countDownNum = 3; //获取倒计时初始值 
    that.setData({
      countDownNum: countDownNum
    })
    that.setData({
      timer: setInterval(function () {
        countDownNum--;
        that.setData({
          countDownNum: countDownNum
        })
        if (countDownNum == 0) {
          clearInterval(that.data.timer);
          //关闭定时器之后，可作其他处理     
          wx.switchTab({
            url: '../home/home',
          })
        }
      }, 1000)

    })
  },
 /**
   * 生命周期函数--监听页面初次渲染完成
   */
  onReady: function () {
     this.countDown()
  },

HTML部分
    <view class="skp">
                {{countDownNum}}秒后自动进入首页
    </view>