Is there no way to reliably handle slot contents in Vue 3?

Question

I'm currently migrating a Vue 2 codebase to Vue 3.

<card>
    <template #default>
        <card-header>
            <title level="3">Header</title>
        </card-header>
        <card-body
            v-for="b in bodyCount"
            :key="'b' b">
            Body {{ b }}
        </card-body>
        <card-footer
            v-for="f in footerCount"
            :key="'f' f">
            <text>Footer {{ f }}</text>
        </card-footer>
    </template>
</card>

The Card component has a render function that calls this.$slots.default(). However, in Vue 3 this returns different content than in Vue 2. So if I do console.log(this.$slots.default()) - I get an array with 3 elements, [v node of type "header", type It is a v node of Symbol(Fragment), a v node of type Symbol(Fragment)]

More specifically, it doesn't recognize the card footer/card body as components, instead I see the symbol (fragment) in the type. Card-header is actually fine since it's not inside the v-for loop.

There is a logic inside the rendering function of the card component that needs to know how many text/footer components it has. So I need to be able to tell the component type. Is this not possible in Vue 3? I can guess this has something to do with the v-for here, but I'm not sure how to actually get the final v-nodes. In this example, I want the array to contain card headers all card bodies all card footers instead of 3 elements.

Answer 1

The design of your card component doesn't look meaningful to me. I suggest you reconsider.

This is a simple playground rendering slot

const { createApp, h } = Vue;

const Card = {
  setup(props, { attrs, slots, emit, expose }) {
      return () => [           
          h('label', 'Card'),
          h('div', 'Slots:'),
          h('div', {}, slots.default()),
          h('div', {}, slots.header()),
          h('div', {}, slots.body()),
          h('div', {}, slots.footer())
      ]
  }
}

const App = { 
  components: { Card },
  data() {
    return {
    }  
  }
}
const app = createApp(App)
app.mount('#app')

#app { line-height: 2; }
[v-cloak] { display: none; }
label { font-weight: bold; }

<div id="app">
<card>
    <template #default>
       Default
        <card-header>
            <title level="3">Header</title>
        </card-header>
    </template>
    <template #header>
       Header
    </template>
    <template #body>
       Body
    </template>
    <template #footer>
       Footer
    </template>
</card>
</div>
<script src="https://unpkg.com/vue@3/dist/vue.global.js"></script>

Please heed the warning. Vue won't render card-header here because it can't parse it.

[Vue warn]: Failed to resolve component: card-header
If this is a native custom elemen

If you have access to the data, why are you passing the slots action card-body/card-footer items directly through bodyCount?

If it doesn't look reasonable to me.

const { createApp, h } = Vue;

const CardHeader = {
    template: '<div class="card-header">card-header: <slot></slot></div>'
}
const CardBody = {
    template: '<div class="card-body">card-body: <slot></slot></div>'
}
const CardFooter = {
    template: '<div class="card-footer">card-footer: <slot></slot></div>'
}

const Card = {
  components: {
    CardHeader, CardBody, CardFooter
  },
  setup(props, { attrs, slots, emit, expose }) {
      //console.log(slots.default())
      return () => [           
          h('label', 'Card'),
          h('div', 'Slots:'),
          h('div', {}, slots.default()),
      ]
  }
}

const App = { 
  components: { Card, CardHeader, CardBody, CardFooter },
  data() {
    return {
      bodyCount: ['a','b'],
      footerCount: ['a','b']
    }  
  }
}
const app = createApp(App)
app.mount('#app')

#app { line-height: 2; }
[v-cloak] { display: none; }
label { font-weight: bold; }
.card-header, .card-body, .card-footer {
  border: 1px solid #F0F0F0;
}

<div id="app">
<card>
    <template #default>
        <card-header>
            <div level="3">Header</div>
        </card-header>
        bodyCount: {{bodyCount.length}}
        <card-body
            v-for="b in bodyCount"
            :key="'b' + b">
            Body {{ b }}
        </card-body>
        footerCount: {{footerCount.length}}
        <card-footer
            v-for="f in footerCount"
            :key="'f' + f">
            <text>Footer {{ f }}</text>
        </card-footer>
    </template>
</card>
</div>
<script src="https://unpkg.com/vue@3/dist/vue.global.prod.js"></script>

Answer 2

It turns out - fragments can be expanded via the Children property on each element of the array. So in this case, v-for produces a v node, which is a fragment, but we can infer more by looking at the Children property.