el-tree 默认选中第一个节点

export default {
  data() {
    return {
      defaultProps: {
        children: 'children',
        label: 'label'
      },
      treeData: [{
          label: '一级 1',
          children: [{
            label: '二级 1-1',
            children: [{
              label: '三级 1-1-1'
            }]
          }]
        }, {
          label: '一级 2',
          children: [{
            label: '二级 2-1',
            children: [{
              label: '三级 2-1-1'
            }]
          }, {
            label: '二级 2-2',
            children: [{
              label: '三级 2-2-1'
            }]
          }]
        }, {
          label: '一级 3',
          children: [{
            label: '二级 3-1',
            children: [{
              label: '三级 3-1-1'
            }]
          }, {
            label: '二级 3-2',
            children: [{
              label: '三级 3-2-1'
            }]
          }]
        }],
    }
  },
  watch: {
    treeData: {
      handler(val) {
        if(val) {
          this.$nextTick(() => {
            document.querySelector('.el-tree-node__content').click()
          })
        }
      },
      immediate: true
    }
  },
}