- The Vue Instance
- Template Syntax
- Computed Properties and Watchers
- Class and Style Bindings
- Conditional Rendering
- List Rendering
- Event Handling
- Form Input Bindings
- Components Basics
- Component Registration
- Custom Events
- Dynamic & Async Components
- Handling Edge Cases
Transitions & Animation
- Enter/Leave & List Transitions
- State Transitions
Reusability & Composition
- Custom Directives
- Render Functions & JSX
- Single File Components
- TypeScript Support
- Production Deployment
- State Management
- Server-Side Rendering
- Reactivity in Depth
- Migration from Vue 1.x
- Migration from Vue Router 0.7.x
- Migration from Vuex 0.6.x to 1.0
- Migration to Vue 2.7
- Comparison with Other Frameworks
- Join the Vue.js Community!
- Meet the Team
You’re browsing the documentation for v2.x and earlier. For v3.x, click here.
Reactivity in Depth
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.
Vue cannot detect property addition or deletion. Since Vue performs the getter/setter conversion process during instance initialization, a property must be present in the
data object in order for Vue to convert it and make it reactive. For example:
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, propertyName, value) method:
You can also use the
vm.$set instance method, which is an alias to the global
Sometimes you may want to assign a number of properties to an existing object, for example using
_.extend(). However, new properties added to the object will not trigger changes. In such cases, create a fresh object with properties from both the original object and the mixin object:
Vue cannot detect the following changes to an array:
- When you directly set an item with the index, e.g.
vm.items[indexOfItem] = newValue
- When you modify the length of the array, e.g.
vm.items.length = newLength
To overcome caveat 1, both of the following will accomplish the same as
vm.items[indexOfItem] = newValue, but will also trigger state updates in the reactivity system:
You can also use the
vm.$set instance method, which is an alias for the global
To deal with caveat 2, you can use
Since Vue doesn’t allow dynamically adding root-level reactive properties, you have to initialize Vue instances by declaring all root-level reactive data properties upfront, even with an empty value:
If you don’t declare
message in the data option, Vue will warn you that the render function is trying to access a property that doesn’t exist.
There are technical reasons behind this restriction - it eliminates a class of edge cases in the dependency tracking system, and also makes Vue instances play nicer with type checking systems. But there is also an important consideration in terms of code maintainability: the
data object is like the schema for your component’s state. Declaring all reactive properties upfront makes the component code easier to understand when revisited later or read by another developer.
In case you haven’t noticed yet, Vue performs DOM updates asynchronously. Whenever a data change is observed, it will open a queue and buffer all the data changes that happen in the same event loop. If the same watcher is triggered multiple times, it will be pushed into the queue only once. This buffered de-duplication is important in avoiding unnecessary calculations and DOM manipulations. Then, in the next event loop “tick”, Vue flushes the queue and performs the actual (already de-duped) work. Internally Vue tries native
setImmediate for the asynchronous queuing and falls back to
For example, when you set
vm.someData = 'new value', the component will not re-render immediately. It will update in the next “tick”, when the queue is flushed. Most of the time we don’t need to care about this, but it can be tricky when you want to do something that depends on the post-update DOM state. Although Vue.js generally encourages developers to think in a “data-driven” fashion and avoid touching the DOM directly, sometimes it might be necessary to get your hands dirty. In order to wait until Vue.js has finished updating the DOM after a data change, you can use
Vue.nextTick(callback) immediately after the data is changed. The callback will be called after the DOM has been updated. For example:
There is also the
vm.$nextTick() instance method, which is especially handy inside components, because it doesn’t need global
Vue and its callback’s
this context will be automatically bound to the current Vue instance:
$nextTick() returns a promise, you can achieve the same as the above using the new ES2017 async/await syntax: