How to update reactive Objects (and properties in general)?

Question

Vue.js documentation on Reactivity in Depth mentions that

a property must be present in the data object in order for Vue to convert it and make it r

Answer 1

It's not that the tags object is declared, it's that the properties on the tags object do not exist when the Vue is instantiated.

In the second case you are adding two new properties to the tags object by using an indexer. Vue cannot detect that those properties were added.

This is why the $set method exists. When you add a new property to an object, you need to add it via $set or Vue.set

 Vue.set(vm.tags, 'hello', true)

or, if you are inside a Vue method,

this.$set(this.tags, 'hello', true)

In the first case, you are adding a completely different object that has the properties. That being the case, Vue is aware of the properties and converts them into reactive properties when the new value is added to data.

If instead you added a new empty object and then added the properties, you would be back in the same case as your second example.

Typically, you just want to initialize the object with the empty properties.

data: {
    tags:{
        hello: false,
        world: false
    }
}

In which case, the properties will be converted to reactive properties and changes will be detected.

Edit

@WoJ posted a comment with a pen with code that looks like this:

var vm = new Vue({
  el: "#root",
  data: {
    posts: {},
    tags: {}
  }
});

vm.tags = {
  hello: true,
  world: true
};

vm.posts["bonjour"] = true
vm.posts["monde"] = true

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.1.10/vue.min.js"></script>
<div id="root">
  <!-- show all tags which are on -->
  {{tags}}
  {{posts}}
</div>

In this code it appears that the posts property of the Vue is updated with new reactive properties because they are displayed in the output when the Vue is rendered. What is happening here is that the properties are added to the posts object, that's just how javascript works, you can add properties to objects, but Vue doesn't know they are there. More specifically, these properties are not added as reactive properties (getters/setters) which is how Vue knows when changes occur to data that has been added to the Vue. In fact, adding these properties to the posts object does not trigger a render.

So, why do the new properties show up in the output? The reason the new posts properties show up in the output is because setting the tags property to a new object triggers a render to be scheduled. It's important to know that Vue renders are not synchronous, they are asynchronous (see here).

... 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.

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.

In the code example above, the update to tags triggers a render to be scheduled. Then, the posts object is updated with two new properties that are not converted to reactive properties because Vue doesn't know they exist. Then some time later, the scheduled render occurs and Vue updates the HTML with the current state of the objects in data. Since posts does have those two new properties, those properties are rendered to the screen. Updates to those properties, however, will never trigger an update to render.

To see this is the case, simply comment out the update to the tags property.

var vm = new Vue({
  el: "#root",
  data: {
    posts: {},
    tags: {}
  }
});

//vm.tags = {
//  hello: true,
//  world: true
//};

vm.posts["bonjour"] = true
vm.posts["monde"] = true

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.1.10/vue.min.js"></script>
<div id="root">
  <!-- show all tags which are on -->
  {{tags}}
  {{posts}}
</div>

Notice, in this case, the rendered Vue never changes.

Answer 2

This is because in the first case, you are running a setter on tags (because you are reassigning it) - which Vue has wrapped and can detect. In the second case, you're running setters on nested properties that were not in your data: { tags: { definition, so they are not reactive.

The Change Detection Caveats section in the documentation covers this, though not exactly the same as your case (the nested properties situation). You would have to declare your data like:

data: {
  tags: {
    hello: null,
    world: null,
  }
}