Understanding the 'mounted' Lifecycle Hook in Vue.js

In Vue.js, a popular JavaScript framework for building user interfaces, there's a concept known as lifecycle hooks. These are predefined methods that get called at different stages of a Vue object’s existence. One of these lifecycle hooks is 'mounted'.

The 'mounted' lifecycle hook in Vue.js gets triggered when the component has been inserted into the DOM (Document Object Model). DOM is a cross-platform and language-independent interface that treats an XML or HTML document as a tree structure. Each node in this tree is an object representing a part of the document.

When a Vue instance goes through the process of creating and inserting its components into the DOM, the 'mounted' hook comes into play. This happens after the instance has been created, and after the el and data properties have been initialized. Now that the Vue instance is bound to our root element and data has been combined with our methods, the Vue instance will be mounted to our DOM element defined in the el property.

Although the Vue instance and data are available before the 'mounted' phase, they are not yet inserted into the DOM. By the 'mounted' phase, our template has fully replaced the 'el' element defined in our Vue instance and all bindings have been compiled.

Here's a practical example:

new Vue({
  el: '#app',
  data: {
    message: 'Hello Vue!'
  },
  mounted: function () {
    console.log(this.$el.textContent) // 'Hello Vue!'
  }
})

In this code snippet, when the 'mounted' hook is triggered, Vue logs the content 'Hello Vue!' into the console. This implies that, at this point, the Vue component has been fully inserted into the DOM.

Remember, the 'mounted' hook is widely used for interacting with JavaScript or jQuery libraries, because Vue can only guarantee that all the components' templates have been rendered into the DOM at this lifecycle hook. However, it's always recommended to create a Vue component whenever possible rather than manipulating the DOM directly.

By understanding the 'mounted' lifecycle hook, you can gain greater control over what happens during the initialization and rendering of your Vue components. This ultimately helps in creating more efficient and effective Vue.js applications.