vuejs repeat a legend row to table every 25 or 50 records

I already have VueJS v-for working fine:

<tr v-for="request in requests">
    <td>{{request.name}}</td>
    <td> .. etc .. </td>
</tr>

Now I need to add a legend/guiding row say every 25 or 50 records, something like this:

<span v-for="(request, index) in requests">
    <tr>
        <td>{{request.name}}</td>
        <td> .. etc .. </td>
    </tr>
    <tr v-if="index % 25 == 0">
        <th>Name</th>
        <th> .. etc .. </th>
    </tr>
</span>

To my surprise, not only does the v-if part not work but I get back an error: "ReferenceError: request is not defined" (even if I left the v-if directive out, or even removed the extra <tr> entirely), so VueJS is making considerations on DOM structure perhaps that I don't understand yet.

Either way, how would I do this?

And by the way, is there a purely HTML/CSS way to do this?

Solution

Your code has invalid HTML. You can't have spans wrapping trs.

Usually invalid HTML is not much of a big deal, but the browsers are extremelly buggy when handling invalid tr/tds placement (the spec is not clear about what they should do in case of error, so they behave each in a specific way for specific cases/errors).

The correct approach is to use <template>s, aka "Conditional Groups":

<table>
    <template v-for="(request, index) in requests">
        <tr>
            <td>{{request.name}}</td>
            <td> .. etc .. </td>
        </tr>
        <tr v-if="index % 25 == 0">
            <th>Name</th>
            <th> .. etc .. </th>
        </tr>
    </template>

Demo reproducing your error:

new Vue({
  el: '#app',
  data: {
    requests: [{name: 'a1'},{name: 'a2'},{name: 'a3'},{name: 'a4'},{name: 'a5'},{name: 'a6'},{name: 'a7'},{name: 'a8'}]
  }
})

<script src="https://unpkg.com/vue"></script>

<div id="app">

  <table border="1">
    <span v-for="(request, index) in requests">
      <tr>
        <td>{{request.name}}</td>
        <td> .. etc .. </td>
      </tr>
      <tr v-if="index % 3 == 0">
        <th>Name</th>
        <th> .. etc .. </th>
      </tr>
    </span>
  </table>
  
</div>

Demo with the fix:

new Vue({
  el: '#app',
  data: {
    requests: [{name: 'a1'},{name: 'a2'},{name: 'a3'},{name: 'a4'},{name: 'a5'},{name: 'a6'},{name: 'a7'},{name: 'a8'}]
  }
})

<script src="https://unpkg.com/vue"></script>

<div id="app">

  <table border="1">
    <template v-for="(request, index) in requests">
      <tr>
        <td>{{request.name}}</td>
        <td> .. etc .. </td>
      </tr>
      <tr v-if="index % 3 == 0">
        <th>Name</th>
        <th> .. etc .. </th>
      </tr>
    </template>
  </table>
  
</div>