数据绑定在刚开始只是和内存建立联系，并没有真正的和后台的数据挂上钩，所以一开始的res.data只是一个空值，必须在之前加个if判断，确保有值以后再开始计算length。
if(res.data){ this.num = res.data.length; }
这样就不会在控制台显示报错了。
- quot undefined property Cannot lengthquot undefined property cannot properties undefined reading cannot typeerror undefined property cannot clearvalidate undefined property cannot access_token undefined property cannot undefined property项目cannot quot properties typeerror undefined properties typeerror undefined reading properties typeerror undefined uncaught undefined generate firedac cannot