Add class to VNode in slot

addClassToSlot.vue

<script>
export default {
  name: "ButtonGroup",
  
  // eslint-disable-next-line no-unused-vars
  render(h) {
    return (
      <div class="button-group">
        {this.$slots.default &&
          this.$slots.default.map(vnode => {
            vnode.data.class = {};
            vnode.data.class["button-group__item"] = true;
            return vnode;
          })}
      </div>
    );
  }
};
</script>