对象遍历

1、从一个数组对象中获取最里层id

        "grades":[   
            {     
                "grade_id":1,
                "grade_name":"一年级",
                "class":[     //item[key2]  
                    {
                        "id":1,
                        "name":"一班"
                    },
                    {
                        "id":2,
                        "name":"二班"
                    },
                    {
                        "id":4,
                        "name":"三班"
                    }
                ]
            },
            {
                "grade_id":2,
                "grade_name":"二年级",
                "class":[
                    {
                        "id":3,
                        "name":"一班"
                    }
                ]
            },
            {
                "grade_id":3,
                "grade_name":"三年级",
                "class":[
                    {
                        "id":8,
                        "name":"一班"
                    },
                    {
                        "id":9,
                        "name":"二班"
                    }
                ]
            }
        ]  

     this.gradesData.filter((item, index) => {
            for (let key in item) { 
              if (key=="class") {
                for (let key2 in item[key]) { 
                  let id = item[key][key2]['id'];
                 this.selectedItem.push(id) 
                }
              }
            }
 console.log(this.selectedItem)//124389