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Mounted application does not replace the element
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Overview

In Vue 2.x, when mounting an application that has a template, the rendered content replaces the element we mount to. In Vue 3.x, the rendered application is appended as a child of such an element, replacing element's innerHTML.

2.x Syntax

In Vue 2.x, we pass an HTML element selector to new Vue() or $mount:

new Vue({
  el: '#app',
  data() {
    return {
      message: 'Hello Vue!'
    }
  },
  template: `
    <div id="rendered">{{ message }}</div>
  `
})

// or
const app = new Vue({
  data() {
    return {
      message: 'Hello Vue!'
    }
  },
  template: `
    <div id="rendered">{{ message }}</div>
  `
})

app.$mount('#app')

When we mount this application to the page that has a div with the passed selector (in our case, it's id="app"):

<body>
  <div id="app">
    Some app content
  </div>
</body>

in the rendered result, the mentioned div will be replaced with the rendered application content:

<body>
  <div id="rendered">Hello Vue!</div>
</body>

3.x Syntax

In Vue 3.x, when we mount an application, its rendered content will replace the innerHTML of the element we pass to mount:

const app = Vue.createApp({
  data() {
    return {
      message: 'Hello Vue!'
    }
  },
  template: `
    <div id="rendered">{{ message }}</div>
  `
})

app.mount('#app')

When this app is mounted to the page that has a div with id="app", this will result in:

<body>
  <div id="app" data-v-app="">
    <div id="rendered">Hello Vue!</div>
  </div>
</body>

Migration Strategy

Migration build flag: GLOBAL_MOUNT_CONTAINER

See Also