For now, we don't have any Vue app running on our web page. The whole library is based on Vue instances, which are the mediators between your View and your data. So, we need to create a new Vue instance to start our app:

// New Vue instance
var app = new Vue({
  // CSS selector of the root DOM element
  el: '#root',
  // Some data
  data () {
    return {
      message: 'Hello Vue.js!',
    }
  },
})

The Vue constructor is called with the new keyword to create a new instance. It has one argument--the option object. It can have multiple attributes (called options), which we will discover progressively in the following chapters. For now, we are using only two of them.

With the el option, we tell Vue where to add (or "mount") the instance on our web page using a CSS selector. In the example, our instance will use the <div id="root"> DOM element as its root element. We could also use the $mount method of the Vue instance instead of the el option:

var app = new Vue({
  data () {
    return {
      message: 'Hello Vue.js!',
    }
  },
})
// We add the instance to the page
app.$mount('#root')

We will also initialize some data in the data option with a message property that contains a string. Now the Vue app is running, but it doesn't do much, yet.