The difference between vue root instance and component instance

Vue.js is a very popular front-end framework in front-end development. Its core is to build reusable web components to achieve rapid development and maintenance. In Vue.js, there are two types of instances, namely root instances and component instances. This article will introduce the differences in detail.

1. Root instance

The root instance is also called the root Vue instance, which is the first instance created and mounted to the real DOM in Vue.js. The root instance is usually created by new Vue(), passing an options object to initialize the application.

The following is a simple example of a root instance:

new Vue({
  el: '#app',
  data: {
    message: 'Hello world!'
  }
})

In the above code, the el attribute specifies the DOM element to which the Vue instance is to be mounted (in this case it is an id app div element), the data attribute defines the data of the instance.

As the entry point of the Vue.js application, the root instance will automatically create the DOM elements required by the root Vue instance and mount them on the specified DOM node. Therefore, the root instance can access the application's global state and configuration through the instance properties of the Vue instance.

2. Component instance

Component instance refers to the instance created by the Vue component constructor, also called a local Vue instance. Each component instance has its own scope, data, and lifecycle hook functions. In a Vue.js application, component instances can be nested within another component instance or the root instance.

The following is a simple example of a component instance:

Vue.component('my-component', {
  template: '<div>{{ message }}</div>',
  data: function() {
    return {
      message: 'Hello from My Component!'
    }
  }
})

new Vue({
  el: '#app'
})

In the above code, we define a component named my-component. This component has a template, which specifies the component's HTML markup. Inside the component, we define a data attribute and assign it the value 'Hello from My Component!'. Finally, we create the root instance.

When we use the my-component tag in the template, Vue.js will automatically create the corresponding component instance and add it to the DOM.

3. The difference between root instance and component instance

1. Difference 1: Different creation methods

The root instance is created by new Vue(), The component instance is created by Vue.component().

2. Difference 2: Different scopes

The root instance is the root instance of the entire Vue.js application. It has a global scope and can customize global instructions and filters. and other global elements. Component instances have their own independent scope, variables between different components are independent of each other, and component instances can only access their own data.

3. Difference three: different life cycles

Vue.js provides some life cycle hook functions to help us execute code at specific points in time. Different types of Vue instances have different life cycle hook functions. The component instance hook function is more flexible than the root instance hook function.

Life cycle hook functions of the root instance: beforeCreate, created, beforeMount, mounted, beforeUpdate, updated, beforeDestroy and destroyed.

Component life cycle hook functions: beforeCreate, created, beforeMount, mounted, beforeUpdate, updated, beforeDestroy and destroyed, as well as component-specific activated and deactivated.

4. Summary

This article briefly introduces the difference between root instances and component instances in Vue.js. The root instance is the entry point of the entire Vue.js application. The component instance is a local Vue instance with an independent and isolated scope. The life cycle hook function is more flexible than the root instance. Understanding these differences can help us better understand the architecture and design of the entire application when developing Vue.js applications.

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