Correct way to handle v-if with v-for in Vue

Question

How should we handle a v-for combined with a v-if in this exemple:

Here my data.json:

[
  {
    image: 'foo.jpg'
  },
  {
    image: 'foobar.jpg',
    video: 'bar.mp4'
  },
  {
    image: 'barfoo.jpg'
  }
]

In my vue template, I'd like to render something like this:

<div v-for="(item, key, index) in data" : key="index">
   <img :src="item.image">
   <video v-if="!!item.video" :src="item.video"></video>
</div >

But as the documentation say, we should avoid v-if with v-for and instead use a computed value. But there I need to render the image even if the json entry doesn't have a video.

That's not a big deal, I've find a solution but I'd like to know the best way to handle this.

Answer 1

I think that you are misunderstanding the documentation. In this instance, it makes sense to use both v-for and v-if because you are always going to render at least the <img> and you might render the <video>. The documentation is trying to show that you should pre-filter a list of items if the v-if and v-for are on the exact same element.

The example the docs are showing are trying to get you to cut a list that would show only only 2 of the 3 total elements down to just 2 elements and letting the computed prop tell you when there are more. Otherwise you have to iterate through the entire list every time you need to re-render.

Consider this difference:

// An array of 1000 items but only 1 has a video element
const data = [0...999];

data.push({
  image: "some.jpg",
  video: "some.mp4"
});

<!--
We always render 1000 items but only some will have video 
-->
<div 
v-for="(item, key, index) in data" 
:key="index">
  <img :src="item.image">
  <video v-if="!!item.video" :src="item.video"></video>
</div>


<!-- 
We have to visit 1000 elements but 
only 1 will actually end up rendering
-->
<div 
v-if="!!item.video" 
v-for="(item, key, index) in data" 
:key="index">
  <img :src="item.image">
  <video :src="item.video"></video>
</div>