Understanding Vue.js Deep Watching

In Vue.js, it is often necessary to observe or "watch" a data object and then act in response to changes to that object. This process is often referred to as 'Reactivity' in Vue.js.

One common use case for this is to 'deep watch' an object, that is to watch not just the object, but all of its nested properties. This is used when we want to trigger a side effect when something inside an object has changed.

The correct way to deep watch an object in Vue.js, as noted in the quiz answer, is as follows:

watch: { object: { handler: 'method', deep: true } }

This syntax sets up a watcher on object, and specifies method as the handler that gets triggered whenever object or any of its nested properties change.

The deep option here is what makes this a deep watch. It tells Vue.js to also observe nested properties within the object and subsequently call the handler if any of these properties change.

Practical Example

The following simple example illustrates the usage of Vue.js's deep watch.

new Vue({
  el: '#app',
  data: {
    user: {
      name: '',
      age: ''
    }
  },
  watch: {
    user: {
      handler: 'updateUser',
      deep: true
    }
  },
  methods: {
    updateUser() {
      console.log("User data has been updated.");
    }
  }
});

In the above example, any changes to either user.name or user.age will trigger the updateUser method thanks to setting deep: true.

Best Practices

While the deep watcher is a powerful tool and can be incredibly useful, it's essential to be careful when using it. Observing changes on large objects with many nested properties can have performance implications.

Consider the case where you only need to watch for changes to a specific nested property. In such cases, directly watch that nested object like so: watch: { 'object.property': 'method' }.

Remember, Vue.js's watch feature and its powerful options, like 'deep', are excellent tools when used appropriately. However, like all powerful tools, they require good understanding and judicious use.