VueJS data() being called twice on load

gistfile1.txt

var Component = Vue.extend({

  template: "<div>Hello world</div>",
  
  data: function () {
    console.log('Component data()');
    return {};
  },
  
  ready: function () {
    console.log('Component ready()');
  }
  
});



var vm = new Component({
  el: "#app"
});