I am trying to give a dynamic class to my vue3 app's main div.

When I create the app with the mands down below, it creates a main div tag which has an id app. I can change the styling with css but since I don't have it in the page I can not change class dynamically.

Code to create the app

const app = createApp(App)
app.use(store)
app.use(router)
app.mount('#app')

What I want to do

<div id="app" :class="myClass"></div>

I am trying to give a dynamic class to my vue3 app's main div.

When I create the app with the mands down below, it creates a main div tag which has an id app. I can change the styling with css but since I don't have it in the page I can not change class dynamically.

Code to create the app

const app = createApp(App)
app.use(store)
app.use(router)
app.mount('#app')

What I want to do

<div id="app" :class="myClass"></div>

• javascript
• html
• vue.js
• vuejs3

• 1 What's stopping you from doing that? – Dan Knights Commented Sep 30, 2020 at 21:57
• 1 In Vue3 when you create the app it creates the main <div> tag with app id. But you can not reach it in the code because it is being created in the building process. @Daniel_Knights – Aiden Pearce Commented Oct 1, 2020 at 10:22
• Oh, I see. Good to know – Dan Knights Commented Oct 1, 2020 at 10:35

2 Answers 2

You could add property that represent your class then use watch to update the your root element using document.querySelector('#app').classList = [val], i made the following example that illustrate a real work example which is the mode change from dark to light and vice-versa, i added property called mode and i change it using a button click event then i watch it to update the root element class :

const {
  createApp
} = Vue;
const App = {
  data() {
    return {
      mode: 'is-light'
    }
  },
  watch: {
    mode(val) {
      document.querySelector('#app').classList = [val]
    }
  }
}
const app = createApp(App)
app.mount('#app')

#app {
  padding: 12px;
  border-radius: 4px
}

.is-dark {
  background: #454545;
  color: white
}

.is-light {
  background: #f9f9f9;
  color: #222
}

button {
  -webkit-transition: all .2s ease;
  transition: all .2s ease;
  padding: 10px;
  border: 0;
  border-radius: 6px;
  cursor: pointer;
  position: relative;
  overflow: hidden;
  color: #fff;
  -webkit-box-sizing: border-box;
  box-sizing: border-box;
  background: transparent;
  background: #4545ee
}

<script src="https://unpkg./[email protected]/dist/vue.global.prod.js"></script>

<div id="app" class="is-dark">

  <button @click="mode= 'is-dark'" class="is-dark">
Dark
</button>

  <button @click="mode= 'is-light'" class="is-light">
Light
</button>
  <p>
    Lorem, ipsum dolor sit amet consectetur adipisicing elit. Corrupti quidem aspernatur provident fugit impedit esse itaque iusto iure voluptatem eaque?
  </p>

</div>

Unfortunately, you're not going to be able to do that with Vue directly.

Defining :class="myClass" on root element will not be reactive.

...but you can handle that manually.

you can use this.$el.parentElement in a method (like mounted) and that will allow you to manage the class attribute. (there are several methods like addClass, toggleClass on the classList API)

If you want that to be reactive, you can use a puted or watch to modify that based on some other parameter, but you'd need to provide more context to have more detail on that.

If possible though (depends on the setup/need), you could nest the application in another level of div, so you can control the class.