When you pass a plain JavaScript object to a Vue instance as its data option, Vue will walk through all of its properties and convert them to getter/setters using Object.defineProperty. This is an ES5-only and un-shimmable feature, which is why Vue doesn’t support IE8 and below.
The getter/setters are invisible to the user, but under the hood they enable Vue to perform dependency-tracking and change-notification when properties are accessed or modified. One caveat is that browser consoles format getter/setters differently when converted data objects are logged, so you may want to install vue-devtools for a more inspection-friendly interface.
Every component instance has a corresponding watcher instance, which records any properties “touched” during the component’s render as dependencies. Later on when a dependency’s setter is triggered, it notifies the watcher, which in turn causes the component to re-render.
Adding new properties to an object
Setting items on an array
Setting the length of an array
Due to the limitations of modern JavaScript (and the abandonment of Object.observe), Vue cannot detect property addition or deletion.
Vue does not allow dynamically adding new root-level reactive properties to an already created instance. However, it’s possible to add reactive properties to a nested object using the Vue.set(object, key, value) method:
Sometimes you may want to assign a number of properties to an existing object, for example using Object.assign() or _.extend(). However, new properties added to the object will not trigger changes. In such cases, create a fresh object with properties from both the original object and the mixin object
