Understanding Vue.js Watchers and Their Applications

Vue.js is a progressive Javascript framework that aids developers in building user interfaces. One of the key features of Vue.js, that simplifies the handling of state updates and changes, is the concept of Watchers.

So, to answer the question, "Which Vue.js feature allows you to perform side effects in response to reactive data changes?" The right answer is Watchers.

What are Vue.js Watchers?

In Vue.js, Watchers provide a way to react to changes in data, allowing developers to perform side effects, like making API calls or validating input, in response to these changes. Instead of running a function every time something changes, Vue.js 'watches' the data and waits until something actually changes before taking action.

This can be quite beneficial for scenarios where you wish to perform certain actions when your data changes.

Here's an example of using a watcher:

var vm = new Vue({
  data: {
    a: 1
  },
  watch: {
    a: function(newValue, oldValue) {
      // this will be called when `vm.a` changes
    }
  }
})

In the above code, whenever the value of the data property a changes, our watcher gets triggered, thus executing our piece of code described in the function attached to the watcher.

Key Applications and Best Practices

Vue.js watchers are exceptionally useful when it comes to handling complex state changes. Here are a few real-world use cases:

• Making API calls in response to user input.
• Performing complex calculations or manipulations based on data changes.
• Debouncing API calls.

Although Vue.js watchers are extremely useful, it's essential to understand when to use watchers and when to use computed properties. Computed properties are cached based on their reactive dependencies and only re-evaluate when some of its dependencies have changed. Hence, for simpler dependencies, computed properties could be more appropriate.

Besides this, avoid initiating heavy computations or side effects in watchers if they are not necessary. Unnecessary use of watchers could lead to slower performance and harder-to-read code.

In conclusion, Watchers provide a powerful way for developers to control actions in response to reactive data changes in Vue.js. However, how and when they are used should be carefully considered to maintain efficient and readable code.