SoFunction
Updated on 2025-04-14

Detailed explanation of the entire process of Vue instance mounting

Preface

When we create an application using , the first step is usually to create a Vue instance and then mount it to the DOM (Document Object Model). Understanding the mount process of Vue instances will not only help us better grasp the core principles of Vue, but also improve our debugging and optimization capabilities in development. This article will analyze in detail the complete process of Vue instances from creation to mounting, aiming to help you understand this critical mechanism in depth.

What is a Vue instance

In Vue, everything revolves around Vue instances. Vue instances are the core of Vue applications, which are responsible for data binding, DOM update, event processing and other tasks.

Creating a Vue instance is simple, you just need to use new Vue() and pass in a configuration object. For example:

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

The above code creates a Vue instance and mounts it to the DOM element on the page with id #app.

Vue instance mount process

1. Instantiate Vue

When we call new Vue() , Vue starts creating a new instance. This process includes the following steps:

  • Initialize configuration: Vue will merge the incoming configuration objects with the default configuration.
  • Initialize life cycle: Vue sets some internal states and prepares the life cycle hook (such as created, mounted, etc.).
  • Initialize event: Vue sets up the event system for communication between components.
  • Initialize rendering: Vue prepares virtual DOM related content.

2. Before Mount

Before mount, the Vue instance will call the beforeMount hook function, and if you define this function in the configuration object, it will be executed at this step. At this point, the template has not been compiled into the virtual DOM.

var app = new Vue({
  el: '#app',
  data: {
    message: 'Hello Vue!'
  },
  beforeMount() {
    ('beforeMount: The instance is about to be mounted');
  }
});

3. Compile templates

If the Vue instance has the el option, Vue will find the corresponding DOM element and compile the template into a render function. Without the el option, the Vue instance is in an unmounted state until the vm.$mount(el) method is called.

4. Mount

Next, Vue will render the virtual DOM tree generated by the rendering function into a real DOM and replace the original DOM element. After this step is completed, the mounted hook function will be called.

var app = new Vue({
  el: '#app',
  data: {
    message: 'Hello Vue!'
  },
  mounted() {
    ('mounted: The instance is mounted');
  }
});

5. Update (update)

Vue automatically updates DOM when the data changes. This process performs efficient differential updates by comparing old and new virtual DOM trees. This is what Vue calls a “responsive system.”

6. Destroy

When a Vue instance is no longer needed, the vm.$destroy() method can be called to destroy it. During the destruction process, the beforeDestroy and destroyed hook functions are called.

var app = new Vue({
  el: '#app',
  data: {
    message: 'Hello Vue!'
  },
  beforeDestroy() {
    ('beforeDestroy: The instance is about to be destroyed');
  },
  destroyed() {
    ('destroyed: The instance has been destroyed');
  }
});

// Some time laterapp.$destroy();

In-depth analysis of the mount process of Vue instances

1. Vue constructor

When we call new Vue(), we are actually calling the constructor of Vue. This constructor is defined in the core part of the Vue source code. It is mainly responsible for initializing various internal states and providing the necessary tools for instances.

function Vue(options) {
  if (!(this instanceof Vue)) {
    ('Vue is a constructor and should be called with the `new` keyword');
  }
  this._init(options);
}

2. _init method

The initialization of the Vue instance is mainly carried out in the _init method. This method will call multiple internal methods in turn to complete the initialization work, including life cycle, events, rendering, data binding, etc.

Here is the core process of the _init method:

  • Merge configuration: Merge the configuration passed in by the user with the default configuration.
  • Initialize lifecycle: Sets the lifecycle state of the instance.
  • Initialize the event system: Add event listening and event triggering functions to the instance.
  • Initialize rendering: Prepare the virtual DOM environment.
  • Data responsiveness: Converts the passed data object into a responsive object.
  • Call hook function: Call lifecycle hook functions at the appropriate time, such as beforeCreate and created.

3. Booting of the mount process

When we pass in the el option, Vue will automatically call the $mount method after initialization to mount the instance. If there is no el option, we need to call the $mount method manually.

.$mount = function (el, hydrating) {
  el = el && query(el);
  return mountComponent(this, el, hydrating);
};

4. mountComponent method

The mountComponent method is the core of the Vue mount process, which is mainly responsible for the following steps:

  • Set the mount point: Determine the mounted DOM element.
  • Compile template: convert the template into a render function.
  • Calling beforeMount** hook**: Called before template compilation.
  • Create render observer: Instantiate an observer object that listens for data changes and re-renders.
  • Call mounted** hook**: Called after the template is rendered and the DOM is inserted.

5. Rendering functions and virtual DOM

Vue uses virtual DOM to describe the real DOM structure. The process of compiling a template is actually converting the template into a rendering function, which returns a virtual DOM tree.

During initial rendering, Vue converts the virtual DOM tree to a real DOM and inserts it into the page. After that, whenever the data changes, the rendering function regenerates a new virtual DOM tree, Vue compares the old and new virtual DOM trees (diff algorithm), and updates only those that actually change. This approach greatly improves performance.

6. Responsive data and update mechanism

Vue's responsive system is one of its core highlights. When we define responsive data in a data object, Vue will use it to intercept the data read and write operations, thereby realizing the monitoring of data changes.

When the responsive data changes, Vue notifies all components that depend on it for re-rendering. This process is automatic and developers do not need to operate manually.

7. Calling timing of life cycle hook

Throughout the lifetime of a Vue instance, Vue provides multiple hook functions that allow developers to insert custom logic at specific times:

  • beforeCreate: After instance is initialized, data observation (data observer) and event configuration are called before.
  • created: The instance has been created, the data observation and event configuration have been completed, but the mount process has not started yet.
  • beforeMount: Called before the mount starts.
  • mounted: Called after the mount is completed.
  • beforeUpdate: Called before update.
  • updated: Called after the update is completed.
  • beforeDestroy: Called before instance is destroyed.
  • destroyed: Called after the instance is destroyed.

8. Things to note in practical application

In actual development, we need to pay attention to the following points to ensure the efficient operation of Vue instances:

  • Avoid using complex expressions in templates: complex expressions will increase the amount of computation and affect rendering performance.
  • Use lifecycle hooks reasonably: Writing logic in a suitable hook function can avoid unnecessary calculations and rendering.
  • The data should be flattened as much as possible: Data structures with too deep nesting levels will increase the overhead of responsive systems.

Summarize

A comprehensive understanding of the mount process of Vue instances is the basis for in-depth understanding. Through the detailed analysis of this article, we explore the stages of Vue instances from creation, mount to update and destruction, and have a deep understanding of their internal operating mechanisms. This not only helps us to improve our efficiency in real development, but also helps us to write more robust and efficient Vue applications.

This is the end of this article about the full process of Vue instance mounting. For more related Vue instance mounting content, please search for my previous articles or continue browsing the related articles below. I hope everyone will support me in the future!