cannot read property 'length' of underfined

vue代码写完，控制台报cannot read property 'length' of underfined错误。以下是代码

 fetchData () {
        var cur_path = this.$route.path; //获取当前路由
        var routers = this.$router.options.routes; // 获取路由对象
        var nav_type = "";
        for (var i = 0; i < routers.length; i++) {
          var children = routers[i].children;
          if(routers[i].children){
            for (var j = 0; j < children.length; j++) {
              var grand_children = children[j].children;
              for (var k = 0; k < grand_children.length; k++) {
                if (grand_children[k].path == cur_path) {
                  nav_type = routers[i].type;
                  break;
                }
              }
            }
          }
        }
        if (nav_type == "home") {
          this.defaultActiveIndex = "/";
          this.homeShow = true;
          this.enterpriseShow = false;
        } else if (nav_type == "enterprise") {
          this.defaultActiveIndex = "/enterpriseManager";
          this.homeShow = false;
          this.enterpriseShow = true;
        }
      },

以下是报错信息