Understanding Vue.js Lifecycle Hooks: The 'updated' Hook

In Vue.js, a JavaScript framework for building user interfaces, lifecycle hooks allow developers to add their own functionality at specific stages of a component's lifecycle. Among the lifecycle hooks Vue.js offers, the 'updated' hook is a critical one.

The updated hook in Vue.js is called right after the DOM has been updated. This means it's invoked once the data changes on your component and the re-render cycle has been completed.

var vm = new Vue({
  data: {
    message: 'Hello, world!'
  },
  updated: function () {
    console.log('The DOM has been updated.')
  }
});

In the example above, whenever message changes, Vue.js will re-render the component to reflect the new state and then call the updated hook function, logging 'The DOM has been updated.' to the console.

This hook is particularly useful when you need to do something in response to a change in your data. For example, you might use it to trigger a network request to fetch more data when your component's state changes, or to manipulate the DOM directly in response to data changes.

While the updated hook is useful, it's critical to understand its place in the Vue lifecycle for best practice and efficient application design. Here are some useful insights on how to use it effectively:

1. Avoid changing state during this hook: When the updated hook is called, the component’s data has already been changed, and Vue’s reactivity system has updated the DOM to reflect these changes. If you attempt to change state during this phase, you’ll potentially create an infinite loop of updates, which will slow down your application.

2. Use for non-reactive data changes: One best use-case scenario would be getting updated data that is not reactive or manipulating the DOM in a way that is not reactive — Vue doesn’t need to track such changes, and they won’t trigger further updates.

Understanding Vue.js lifecycle hooks like 'updated' allows you to create interactive applications that efficiently react to changes in state, leading to efficient application flow and goal-oriented functionality.