Understanding the 'destroyed' Lifecycle Hook in Vue.js

In Vue.js, the lifecycle of a component follows specific stages - from creation to mounting, updating, and finally destruction. Vue.js provides several lifecycle hooks that you can utilize for executing custom functionalities at various stages of the component's lifecycle. Among these lifecycle hooks is the destroyed hook, which holds unique importance being one of the last hooks to run in the component lifecycle.

The 'destroyed' lifecycle hook in Vue.js is called after a Vue instance has been destroyed. It's during this stage that all directives of the Vue instance have been unbound, all event listeners have been removed, and all child components have also been destroyed.

This cleanup process is essential to freeing up resources and preventing memory leaks in your application. For instance, if your component was set up to listen for events from a global event bus, the 'destroyed' hook is the perfect place to remove these event listeners:

export default {
  created() {
    eventBus.$on('event', this.handleEvent)
  },
  destroyed() {
    eventBus.$off('event', this.handleEvent)
  },
  methods: {
    handleEvent() {
      // handle event
    }
  }
}

In this example, the 'destroyed' hook is used to call $off to remove the listener from the event bus, thus preventing the case where the listener might still be active and responding to events even when the component itself no longer exists.

It's important to note, however, that the 'destroyed' hook does not guarantee that all child components have also been destroyed. If you need to remove event listeners or perform other cleanup in child components, the 'beforeDestroy' hook should be used instead.

Contrary to some misconceptions, the 'destroyed' hook cannot be used to prevent a component from being destroyed. Once this point in the lifecycle process has been reached, the Vue instance is in the process of being destroyed and can't be stopped from doing so.

Understanding and appropriately using Vue’s lifecycle hooks enables developers to interact with their components at crucial points during their lifecycle. They create broad possibilities for extra functionalities and validations which can enhance application performance, maintainability, and user experience.