element-ui中表格 type="expand"设置全部展开

 <el-table
      :data="tableData"
      :expand-row-keys="expends"  //expends是数组，设置你要展开行的id
      :row-key="getRowKeys" //通过getRowKeys方法获取到row的行id值
      >

2、data中设置

 data() {
    return {
      expends:[],
      }
   }

3、methods中设置

 methods: {
    //设置table中的扩展项，展开的id，此处我需要全部展开
    getExpends(){
      var Id = this.tableData.map(item => item.id)
            this.expends = Id
    },
    getRowKeys(row){
      return row.id
    },
    }

4、created()中调用 getExpends()，使一打开网页就全部展开

created() {
    this.getExpends();
  },