vue.js - Why does my Vue component require :key?

Question

I have a small Vue.js component which displays a favorite star icon. Clicking on the icon favorites/unfavorites the element. So far I have only implemented the UI part, which looks like this:

<template>
  <div :key="favorite">
    <a v-on:click="toggleFavorite" style="cursor: pointer">
      <i v-show="favorite" class="text-warning fas fa-star"></i>
      <i v-show="!favorite" class="text-warning far fa-star"></i>
    </a>
  </div>
</template>

<script>
export default {
  data() {
    return {
      favorite: true,
    }
  },
  mounted() {
  },
  methods: {
    toggleFavorite() {
      this.favorite = !this.favorite
    }
  },
  props: ['team-id'],
}
</script>

<style scoped>
</style>

As you can see, the logic is pretty simple.

This works well, but one thing that bothers me is that, if I remove the :key property from my template, the icon is not updated when I click on it (even though I have checked that the underlying property is indeed updated correctly). Adding :key makes it work, I imagine because it forces Vue.js to completely re-render the component when favorite is updated.

Why is this happening? I'm fairly new to the world of JS frameworks, so forgive any obvious stuff I might be missing. I did some research online but couldn't find an explanation. I just want to make sure I'm doing things the right way and not merely hacking around the issue here.

Answer 1

Vue patches with the virtual DOM whenever it is necessary. That is, whenever vue detects the changes on the DOM, it patches them for faster performance. And patching in the DOM will not change the icon or image. You need to replace the DOM instead.

Thus, vue provides the way for us whenever we need to change the DOM by replacing method, we can use :key binding.

So, :key binding can be used to force replacement of an element/component instead of reusing it.

The following whole html div will be replaced whenever there is change in favorite data as we're :key binding on it:

<div :key="favorite">
    <a v-on:click="toggleFavorite" style="cursor: pointer">
      <i v-show="favorite" class="text-warning fas fa-star"></i>
      <i v-show="!favorite" class="text-warning far fa-star"></i>
    </a>
  </div>

---

This is why vue forcefully allows us to use :key binding inside a loop as there's need of replacing the elements inside the loop whenever it detects the changes in the data. This is made compulsory from 2.2.0+ and ESLint also have implemented this feature so that if you miss :key binding inside the loop, then you'll see the error on that line when you use editor that supports eslint, so that you can fix the error.

Just an opinion, the strict requirement of the :key binding should be removed from the vue as we might want a loop of predefined data and don't want to change the DOM but we still use the v-for loop for listing bigger data. But it might be rare case though.

---

Read carefully on the documentation for :key binding and then you'll have an idea.

The :key binding can be useful when you want to:

Properly trigger lifecycle hooks of a component

Trigger transitions

---

1. Use :key binding to replace the DOM. Remember it slower the performance as it replace the whole DOM that is bound to the element.
2. Don't use :key binding when you don't want to replace the DOM or you think there's no data changes detection required. This will allow vue to perform better without :key binding.