Instance

Constructor

Every Vue vm is bootstrapped by creating a root Vue instance with the Vue constructor function:

var vm = new Vue({
  // options
})

Although not strictly associated with the MVVM pattern, Vue’s design was no doubt inspired by it. As a convention, we often use the variable vm (short for ViewModel) to refer to our Vue instances.

The Vue constructor can be extended to create reusable component constructors with pre-defined options:

var MyComponent = Vue.extend({
  // extension options
})
// all instances of `MyComponent` are created with
// the pre-defined extension options
var myComponentInstance = new MyComponent()

Although it is possible to create extended instances imperatively, most of the time it is recommended to compose them declaratively in templates as custom elements. We will talk about the component system in detail later. For now, you just need to know that all Vue components are essentially extended Vue instances

Properties and Methods

It should be noted that only these proxied properties are reactive. If you attach a new property to the instance after it has been created, it will not trigger any view updates. We will discuss the reactivity system in detail later.

In addition to data properties, Vue instances expose a number of useful instance properties and methods. These properties and methods are prefixed with $ to differentiate them from proxied data properties. For example:

var data = { a: 1 }
var vm = new Vue({
  el: '#example',
  data: data
})
vm.$data === data // -> true
vm.$el === document.getElementById('example') // -> true
// $watch is an instance method
vm.$watch('a', function (newVal, oldVal) {
  // this callback will be called when `vm.a` changes
})

Instance Lifecycle Hooks

Each Vue instance goes through a series of initialization steps when it is created - for example, it needs to set up data observation, compile the template, mount the instance to the DOM, and update the DOM when data changes. Along the way, it will also invoke some lifecycle hooks, which give us the opportunity to execute custom logic. For example, the created hook is called after the instance is created:

var vm = new Vue({
  data: {
    a: 1
  },
  created: function () {
    // `this` points to the vm instance
    console.log('a is: ' + this.a)
  }
})
// -> "a is: 1"

There are also other hooks which will be called at different stages of the instance’s lifecycle, for example mounted, updated, and destroyed. All lifecycle hooks are called with their this context pointing to the Vue instance invoking it. Some users may have been wondering where the concept of “controllers” lives in the Vue world and the answer is: there are no controllers. Your custom logic for a component would be split among these lifecycle hooks.