[Front-end interview questions] 2023 front-end interview questions - Vue chapter

There are always ups and downs in a person's life. It will not always rise like the rising sun, nor will it always be miserable. Repeated ups and downs are training for a person. Therefore, those who are floating above do not need to be proud; those who are sinking below do not need to be pessimistic. We must be frank and humble, optimistic and enterprising, and move forward. ——Konosuke Matsushita

Hello everyone, my name is Jiang Chen. In today's Internet environment, everyone must have felt it to some extent. In this impetuous society, only by constantly maintaining one's character can one perceive different gains and encourage each other.

A collection of the latest interview questions in 2023, so be prepared at all times.

This article was first published on WeChat public account: Wild Programmer Jiang Chen

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What are the advantages and differences of Vue.js compared to other front-end frameworks such as React and Angular?

Simplicity and ease of use:

Vue.js is a lightweight framework that is easy to learn and use. It provides an intuitive API and clear documentation, allowing developers to quickly build applications.
React and Angular are in some ways more complex and require more learning costs.

Progressive framework:

Vue.js is known as a progressive framework, allowing you to gradually adopt its features. This means you can integrate Vue.js into your existing project without having to rewrite the entire application at once.
React and Angular may require more work when integrated into existing projects.

Two-way data binding:

Vue.js provides direct two-way data binding to keep data in sync between views and models. This makes it easier for developers to manage the state of their applications.
React and Angular also support data binding, but they implement it slightly differently.

Component development:

Vue.js, React, and Angular all encourage component-based development, but Vue.js excels in this regard. The definition of Vue components is very simple and easy to reuse and maintain.
React uses JSX to create components, Angular uses templates. The component systems of these frameworks are also powerful, but may require more configuration.

Ecosystem and Community:

React and Angular have huge ecosystems and active community support, with rich third-party libraries and plug-ins.
The Vue.js ecosystem is also growing, and although it is relatively small, the community is also very active.

performance:

Vue.js performs well in terms of performance and has a virtual DOM mechanism that can update views efficiently.
React also uses virtual DOM, and its performance is also very good. Angular may require more performance optimization work in some cases.

Tools and Ecosystem:

Vue.js provides some powerful tools, such as Vue CLI, for quickly building projects and integrating with official libraries such as Vue Router and Vuex.
React and Angular have similar tools and libraries, but Vue's tool ecosystem is in some ways more intuitive and easier to use.

Use Cases:

Vue.js is suitable for small to medium-sized applications and single-page applications (SPA), as well as projects that require rapid prototyping.
React and Angular are suitable for applications of all sizes, including large enterprise applications. In summary, choosing which front-end framework to use depends on the needs of the project and the preferences of the team. Vue.js has advantages in simplicity, ease of use, and progressive development, making it suitable for many projects, but React and Angular also have their advantages in large-scale applications and enterprise-level projects.

What is the difference between a Vue instance and a component? How do they communicate?

Vue Instance and Components in Vue.js are two different concepts. There are some important differences between them, and there are also different ways to communicate.

1. Vue Instance:

Vue instances are one of the core concepts of Vue.js. It is an independent Vue object used to manage the state, behavior and life cycle of the application.
Typically, a root instance of a Vue application is created, which manages the data and methods of the entire application. You can use new Vue()to create a Vue instance.

2. Components:

Components are reusable blocks of code in Vue.js that are used to build user interfaces. Each component has its own state, behavior and templates.
Components can be used in templates like tags, allowing you to build complex user interfaces by breaking them down into maintainable parts.
Components are defined via Vue.componentor using single-file components ( files)..vue

way of communication:

In Vue.js, Vue instances and components can communicate in the following ways:

1. Props:

Parent components can pass data to child components through props. Child components receive data through props and use it in their own templates.
This is a one-way data flow method, where parent components pass data to child components.

2. Custom events:

Child components can notify parent components of event occurrences by triggering custom events. Parent components can listen to these events and perform appropriate actions.
This is a way of communicating from child components to parent components.

3. State management (such as Vuex):

For large applications, you can use a state management library such as Vuex to manage the application's state. It provides a centralized state store where all components can access and modify the data.
This is a high-level way of communicating across components.

4. Dependency injection:

Vue.js provides a dependency injection mechanism that allows you to register some data in ancestor components and then access this data in descendant components without passing props layer by layer.
Dependency injection is usually used to pass some global configuration or theme styles.

Summary:
Vue instances are the root objects of the entire application, and components are reusable modules in the application. Communication between them is mainly achieved through props and custom events, but for more complex state management, you can use Vuex or other state management libraries.

What is the lifecycle hook function in Vue? What is the order in which they are executed?

Lifecycle hooks in Vue.js are a specific set of functions that allow you to execute code during different lifecycle stages of your component. These hook functions can be used to perform tasks such as initialization, data loading, DOM operations, etc. The life cycle hook functions of Vue components are executed in the following order:

beforeCreate (before creation):
- Called immediately before the component instance is created.
- At this time, the component's data and events have not yet been initialized.
created (after creation):
- Called immediately after the component instance is created.
- The component's data has been initialized, but it has not yet been mounted to the DOM.
beforeMount (before mounting):
- Called immediately before the component is mounted to the DOM.
- At this point, the template compilation is complete, but the component has not yet been rendered to the page.
mounted (after mounting):
- Called immediately after the component is mounted to the DOM.
- At this point, the component has been rendered to the page and DOM operations can be performed.
beforeUpdate (before update):
- Called immediately before component data is updated.
- Within this hook function, you can access the previous state, but the latest data has not yet been applied.
updated:
- Called immediately after the component data is updated.
- At this point the component has been re-rendered and DOM operations can be performed.
beforeDestroy (before destruction):
- Called immediately before the component is destroyed.
- At this point the component is still available and you can perform some cleanup work.
destroyed (after destruction):
- Called immediately after the component is destroyed.
- At this point the component has been completely destroyed and is no longer available.

These lifecycle hook functions allow you to execute code at different stages to meet the needs of your application. For example, createddata initialization can be performed in hooks, and mountedDOM operations can be performed in hooks. Please note that different life cycle hooks are suitable for different purposes, and you should choose the appropriate hook function to perform the corresponding tasks according to your needs.

How is Vue's two-way data binding implemented? Please give an example.

Vue.js's two-way data binding is implemented through its unique reactive system. This system uses the ES6 Proxy object or the Object.defineProperty() method to notify the view to update when the data changes. This means that when you modify the data model, the views associated with it are automatically updated, and vice versa.

Here is a simple example that demonstrates how to implement two-way data binding in Vue.js:

HTML template:

<div id="app">
  <input v-model="message" type="text">
  <p>{
   
   { message }}</p>
</div>

JavaScript code for Vue instance:

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

In this example, we use v-modeldirectives to bidirectionally bind <input>elements to properties in the Vue instance message. This means that when you enter text in the input box, messagethe value of will automatically update, and messagewhen the value of changes, the text will also update automatically.

When you enter text in the input box, Vue will automatically update the entered value to messagethe attribute, thus achieving an update from the view to the data. In turn, if you modify messagethe value of the attribute in JavaScript code, the text in the view will automatically update, achieving an update from data to view.

This two-way data binding keeps data in sync with the view, greatly simplifying the task of handling user input and data presentation in front-end development.

What is the role of computed properties and observers in Vue? What's the difference between them?

In Vue.js, Computed Properties and Observers are both used to handle changes in data, but they have different roles and uses.

Computed Properties:

A computed property is a property type in Vue.js whose value is calculated based on other data properties, similar to a function. The main function of calculated properties is to encapsulate calculation logic so that it can be referenced directly in the template, and they have a caching mechanism that will only be recalculated when the dependent data changes.

Main features and functions:

Used to derive or calculate a value based on an existing data attribute.
With a caching mechanism, it will only be recalculated when relevant data changes, improving performance.
It can be referenced directly in the template like a normal attribute.
Computed properties are generally used for simple data conversion, filtering, formatting and other operations.

Example:

<template>
  <div>
    <p>{
   
   { fullName }}</p>
  </div>
</template>

<script>
export default {
  data() {
    return {
      firstName: 'John',
      lastName: 'Doe'
    }
  },
  computed: {
    fullName() {
      return this.firstName + ' ' + this.lastName;
    }
  }
}
</script>

Watchers:

Observers are a way in Vue.js to perform custom asynchronous or expensive operations when data changes. You can listen for changes in one or more data properties and execute specific functions when the data changes.

Main features and functions:

Used to perform custom operations when data changes, such as asynchronous requests or complex data processing.
There is no caching mechanism, and execution will be triggered every time data changes.
Observer functions need to be written manually to handle data changes.
Can monitor changes in multiple data attributes.

Example:

<template>
  <div>
    <p>{
   
   { message }}</p>
  </div>
</template>

<script>
export default {
  data() {
    return {
      value: 'Initial Value',
      message: ''
    }
  },
  watch: {
    value(newValue, oldValue) {
      // 在value属性变化时执行的操作
      this.message = 'Value changed: ' + newValue;
    }
  }
}
</script>

the difference:

Computed attributes are mainly used for data conversion and derivation. They have a caching mechanism and will only be recalculated when the relevant data changes. They are suitable for simple data processing. They can be referenced directly in the template like normal properties.
Observers are used to perform customized operations when data changes. There is no caching mechanism, and execution will be triggered every time the data changes. Suitable for handling complex asynchronous operations or situations where multiple data changes need to be monitored.

Depending on your specific needs, you can choose to use computed properties or observers to handle data changes. Generally, computed properties are preferred because they are simpler and more performant, while observers are used only when special handling of data changes is required.

Talk about your understanding of Vue components. How to create a Vue component?

Vue components are reusable modules in Vue.js applications. They split a page into multiple independent parts, each part has its own state, template and behavior. Componentization is one of the core concepts of Vue.js, which makes front-end development more modular, maintainable and reusable.

The basic steps to create a Vue component are as follows:

Define the component: First, you need to define a Vue component. Components can be defined using Vue.componentthe method or using single-file components (.vue files). Here is Vue.componentan example of defining a component using :

Vue.component('my-component', {
  // 组件的选项
  template: '<div>This is a custom component</div>'
})

Using components in templates: Once a component is defined, you can use it in the parent component's template. For example:

<template>
  <div>
    <my-component></my-component>
  </div>
</template>

Pass data to the component: You can pass data to the component through the component's props, so that the component can receive external data and use it in the template. For example:

<template>
  <div>
    <my-component :message="message"></my-component>
  </div>
</template>

<script>
export default {
  data() {
    return {
      message: 'Hello from parent component'
    }
  }
}
</script>

Inside the component, you can use propsto receive this data and use it in the template:

<template>
  <div>
    <p>{
   
   { message }}</p>
  </div>
</template>

<script>
export default {
  props: ['message']
}
</script>

Component life cycle: Components also have life cycle hook functions that allow you to execute code at different life cycle stages. These hook functions include beforeCreate, created, beforeMount, mountedetc., which are used to perform tasks such as initialization, data loading, and DOM operations.
Custom events: Components can communicate with each other through custom events. Child components can trigger custom events, and parent components can listen to these events and perform corresponding actions.
Communication between components: In addition to props and custom events, you can also use state management tools like Vuex to achieve communication and data sharing between components.

In short, Vue components are one of the core concepts in Vue.js applications. It makes front-end development more modular and maintainable, allowing you to split the interface into multiple reusable parts, each part has its own state and behavior. Creating and using components is an important part of Vue.js development, helping you build more efficient and maintainable front-end applications.

What are directives in Vue? List some commonly used instructions and briefly introduce their functions.

In Vue.js, directives are special tokens that can be used in templates to represent actions on DOM elements. A directive v-begins with , followed by the name of the directive, for example v-bind, v-ifetc. Instructions are used to bind data in the template to DOM elements and control the display, hiding, rendering and behavior of elements.

Here are some commonly used Vue directives and what they do:

v-bind：
- Function: Used to bind the attributes of elements and bind the attribute values of elements to the data of Vue instances.
- Example:<img v-bind:src="imageUrl">
v-model：
- Function: Used to implement two-way binding between form elements and Vue instance data, so that user input can automatically update data, and vice versa.
- Example:<input v-model="message">
v-for：
- Function: Used to loop through the data of an array or object and generate multiple elements.
- Example:<li v-for="item in items">{ { item }}</li>
v-if / v-else-if / v-else：
- Function: Used to control the display and hiding of elements based on conditions, similar to conditional statements in JavaScript.
- Example:<div v-if="show">This is shown</div>
v-show：
- Function: Used to control the display and hiding of elements based on conditions. Different from this v-if, it is controlled through CSS displayproperties and does not destroy and recreate elements.
- Example:<div v-show="isVisible">This is shown</div>
v-on：
- Function: Used to listen to DOM events and execute the specified method when the event is triggered.
- Example:<button v-on:click="handleClick">Click me</button>
v-pre：
- Function: Skip the compilation process of this element and its sub-elements, and output them directly as raw HTML.
- Example:<div v-pre>{ { message }}</div>
v-cloak：
- Function: Remain hidden between the element and the Vue instance until Vue compilation is completed.
- Example:<div v-cloak>{ { message }}</div>
v-once：
- Function: Only render elements and components once, no responsive updates will be performed.
- Example:<span v-once>{ { message }}</span>

These instructions allow you to easily manipulate DOM elements in the template and dynamically update the view based on changes in data. Each directive has its own specific role, allowing you to define the interaction and logic of your page in a declarative way. You can use these directives in templates as needed to build powerful Vue.js applications.

What is Vuex? What does it do? Please describe the basic structure of a Vuex application.

Vuex is a state management library developed specifically for Vue.js applications. It is mainly used to manage shared state (such as data, status, configuration information, etc.) in Vue.js applications in order to better organize, maintain and track the data flow in the application. The core idea of Vuex is to centrally store the state of the application in a global store, making state changes predictable and maintainable.

The main functions of Vuex include:

Centralized state management: Vuex allows the state of an application to be stored in a single place, called a store. This store is a responsive state tree. Multiple components can share and access this state without passing data through props.
State changes can be tracked: Vuex uses a strict state change tracking mechanism. Every time the state changes, there will be clear records and logs to facilitate developers to track and debug applications.
Component communication: Vuex provides a unified way to manage communication between components. Components can modify state by submitting mutations, and can also trigger asynchronous operations by dispatching actions, and these operations are predictable and controllable.
Middleware: Vuex supports middleware, which can perform some additional logic when the state changes, such as logging, data persistence, etc.

A basic Vuex application typically includes the following components:

State: Stores the application's state data, usually a JavaScript object.
Mutations: Methods used to modify state. Each mutation has a type and a handler function that performs the actual state modification operation.
Actions: Similar to mutations, but it can contain asynchronous operations, usually used to handle interaction with the server, data acquisition, etc. Actions are responsible for submitting mutations to modify the state.
Getters (Computed Properties): Used to derive some new data from the state, similar to calculated properties, and can be used directly by components.
Store: An object that centrally manages states, mutations, actions, and getters. It is the core of Vuex.

Here is an example of the basic structure of a simple Vuex application:

import Vue from 'vue'
import Vuex from 'vuex'

Vue.use(Vuex)

const store = new Vuex.Store({
    
    
  state: {
    
    
    count: 0
  },
  mutations: {
    
    
    increment(state) {
    
    
      state.count++
    },
    decrement(state) {
    
    
      state.count--
    }
  },
  actions: {
    
    
    incrementAsync(context) {
    
    
      setTimeout(() => {
    
    
        context.commit('increment')
      }, 1000)
    }
  },
  getters: {
    
    
    doubleCount(state) {
    
    
      return state.count * 2
    }
  }
})

export default store

In the above example, we defined a Vuex store containing states, mutations, actions, and computed properties. This store can be referenced in Vue components and is used to manage and manipulate the application's state. The use of Vuex can greatly simplify state management and component communication, especially in large applications.

What is Vue Router? What does it do? Please describe the basic usage of Vue Router.

Vue Router is the official routing management library of Vue.js, used to build single-page applications (SPA). It allows you to implement navigation between pages, routing jumps and URL management in Vue applications. The main function of Vue Router is to associate different view components with different routes (URL addresses) of the application to achieve switching and navigation between pages.

Basic usage of Vue Router includes the following steps:

Install Vue Router: First, install Vue Router in your Vue.js project. You can install it using npm or yarn:
```
npm install vue-router
# 或者
yarn add vue-router
```

Create a routing configuration: Create a routing configuration file in your project, usually named router.js, and import Vue and Vue Router:

import Vue from 'vue'
import VueRouter from 'vue-router'

Vue.use(VueRouter)

const routes = [
  {
      
      
    path: '/',         // 路由路径
    component: Home    // 对应的视图组件
  },
  {
      
      
    path: '/about',
    component: About
  }
  // 其他路由配置
]

const router = new VueRouter({
      
      
  routes // 使用配置文件中的路由规则
})

export default router

Create view components: Create corresponding view components for each routing path. These components can be normal Vue components such as Home.vueand About.vue.

Using the Router in the root component: Use the Vue Router in the root Vue instance, usually in main.js:

import Vue from 'vue'
import App from './App.vue'
import router from './router' // 导入路由配置

new Vue({
      
      
  el: '#app',
  router, // 使用路由配置
  render: h => h(App)
})

Use <router-link>and <router-view>: Use the tag in the template <router-link>to create navigation links, and use <router-view>the tag to render the view component of the current route. For example:
```
<template>
  <div>
    <router-link to="/">Home</router-link>
    <router-link to="/about">About</router-link>
    
    <router-view></router-view>
  </div>
</template>
```
Navigation and route jump: You can use <router-link>to implement route navigation, or you can use the method in the component this.$router.push()to perform programmatic route jump.

These are the basic usage methods of Vue Router. It allows you to easily implement navigation and route switching between pages in your Vue.js application, making the development of single-page applications more convenient and maintainable. By defining route configurations and associating view components, you can build rich single-page applications and associate different view components with different URL routes.

What is the difference between Vue2 and Vue3?

There are some important differences and improvements between Vue.js 2 and Vue.js 3. Here are some of the main differences and features:

Performance optimization:
- Vue 3 has made many performance optimizations under the hood, including upgrades to the virtual DOM, making it faster and more efficient.
- Vue 3 introduces optimization strategies such as lazy loading (Lazy Loading) and static hoisting (Static Hoisting) to further improve performance.
Composition API：
- Vue 3 introduces the Composition API, a function-based API that allows for more flexible organization and reuse of component logic.
- The Composition API allows developers to divide code by function, improving code readability and maintainability.
Smaller package size:
- Vue 3's core library is smaller and therefore loads faster.
- Vue 3 supports on-demand loading, allowing only the required functions to be introduced, further reducing the package size.
Teleport：
- Vue 3 introduced Teleport, which allows the content of components to be rendered to any location in the DOM, which is very useful in handling modal boxes, pop-up menus and other scenarios.
Fragments：
- Vue 3 supports Fragments, allowing components to return multiple root elements without the need for additional container elements.
Global API modifications:
- Vue 3 has made some changes to the global API to make it more consistent with modern JavaScript standards.
- For example, Vue.componentnow change to app.component, Vue.directivechange to app.directive, Vue.mixinchange to app.mixin.
New lifecycle hooks:
- Vue 3 introduces new lifecycle hooks such as onBeforeMountand onBeforeUpdateto provide more precise control and better performance optimization opportunities.
TypeScript support improvements:
- Vue 3 has more complete support for TypeScript and provides better type inference and type checking.
Improvements to responsive systems:
- Vue 3 improves the responsive system, provides better TypeScript support, and is more efficient.

Overall, Vue.js 3 has significant improvements in performance, development experience, and maintainability. However, Vue 2 is still a stable version with a wide ecosystem and support, and developers can choose which version to use based on project needs. If you are starting a new project, Vue 3 may be a better choice because of its many advantages and improvements. If you are maintaining a Vue 2 project, you may also consider gradually migrating to Vue 3 to gain improvements in performance and development experience.

Can you list some new features in Vue3?

Here are some important new features and improvements in Vue.js 3:

Composition API: Composition API is one of the most striking new features of Vue 3. It allows you to divide code by function and organize related code logic together, improving maintainability and code reusability.
Teleport: Teleport is a new feature that allows you to render the content of a component to other locations in the DOM. This is useful for creating components such as modals, popup menus, etc.
Fragments: Vue 3 supports Fragments, allowing a component to return multiple root elements without the need for additional wrapping container elements.
Modifications to the global API: Vue 3 has made some modifications to the global API to make it more consistent with modern JavaScript standards. For example, Vue.componentnow change to app.component.
Performance optimization: Vue 3 has performed many performance optimizations at the bottom level, including virtual DOM upgrade, lazy loading, static promotion and other strategies to make the application faster and more efficient.
Responsive system improvements: Vue 3 has improved the responsive system, providing better TypeScript support and more efficient responsive data tracking.
TypeScript support: Vue 3 has more complete support for TypeScript, providing better type inference and type checking.
Smaller package size: The core library of Vue 3 is smaller, loads faster, and supports on-demand loading, reducing the package size.
Improvements in lifecycle hooks: Vue 3 introduces new lifecycle hooks such as onBeforeMountand onBeforeUpdate, providing more precise control and opportunities for performance optimization.
Suspense: Vue 3 supports the Suspense feature, which allows you to handle the loading status of asynchronous components gracefully and provide a better user experience.
Custom renderers: Vue 3 allows you to create custom renderers, which allows you to use Vue in different target environments, such as server-side rendering (SSR) or native applications.
Improvements to V-model: Vue 3 improves the syntax of v-model, making it more flexible and can be used for two-way binding of custom components.

These new features and improvements make Vue.js 3 a more powerful, efficient and flexible front-end framework, helping developers build better single-page applications and user interfaces.

Please explain what Composition API is and what are its advantages?

Composition API is a new component organization method introduced in Vue.js 3. It allows you to divide and organize the code logic of components by function. This is a function-based API style, as opposed to the traditional Options API. Its main advantages include:

More flexible code organization: The Composition API allows you to divide the code logic of a component into multiple functionally related parts, each part is an independent function. This makes the code clearer and easier to maintain and test. You can reuse code logic more easily and apply it to multiple components.
Better type inference: When the Composition API is used with TypeScript,

What performance optimizations are there in Vue 3?

Vue 3 introduces many new features and improvements in performance optimization to improve the performance of your applications. Here are some performance optimization measures in Vue 3:

Virtual DOM rewrite : Vue 3’s virtual DOM implementation has been rewritten to make it faster and lighter. This means higher rendering and update performance.
Static tree promotion : Vue 3 can detect static subtrees and promote them to static vnodes to avoid unnecessary re-rendering and comparison operations.
Sloth loading : Vue 3 supports sloth loading, which only renders subcomponents when needed, reducing the burden of initial rendering.
Better event processing : Vue 3 adopts a more efficient event listening and processing method to improve event processing performance.
Compiler optimization : Vue 3’s template compiler has been optimized to generate more efficient rendering functions and reduce runtime overhead.
Fragment and Teleport : Vue 3 introduced Fragment and Teleport. These features can help you organize your components more effectively and reduce unnecessary nesting and rendering nodes.
Suspense : The Suspense feature in Vue 3 allows you to display placeholders when an asynchronous component loads, which helps improve user experience while reducing unnecessary rendering.
Reactive system rewritten : Vue 3’s reactive system has been rewritten to make it faster and scalable. It uses a Proxy proxy, which is more efficient than Vue 2's Object.defineProperty.
Composition API : Vue 3 introduces the Composition API, which allows you to organize and reuse code more flexibly, which helps improve the performance and maintainability of your code.
Tree-Shaking : Since Vue 3 uses ES modules to organize code, build tools such as Webpack can more easily perform Tree-Shaking, including only the code actually used by the application, reducing the size of the package.

These performance optimization measures make Vue 3 a faster and more efficient front-end framework, helping to build more responsive and fluid web applications. But please note that performance optimization also depends on your specific application and usage, so in actual projects, you may need further performance analysis and adjustments.

What are Teleports and Fragments, and what are their roles in Vue 3?

In Vue 3, Teleport and Fragments are two new features, which are used to improve the rendering structure and rendering position control of components respectively. Here's what they do and how to use them:

Teleport :
- Function : Teleport allows you to render the content of a component to different locations in the DOM structure without being restricted by the parent component. This is useful for handling modals, dialog boxes, notification messages, etc. that need to be rendered at different locations on the page.
- Usage : You can use the element in a template <teleport>and toset its attribute to a target selector to specify which DOM element the content should be rendered into. For example:
```
<template>
  <div>
    <button @click="showModal">Show Modal</button>
    <teleport to="#modal-container">
      <Modal v-if="isModalVisible" @close="closeModal" />
    </teleport>
  </div>
</template>
```
In the above example, the content of the Modal component will be rendered id="modal-container"inside the DOM element with in the page.
Fragments :
- Function : Fragments allow you to wrap multiple child elements in a parent element without introducing additional DOM elements. This helps reduce the nesting of the DOM structure, making the code clearer and more concise.
- Usage : You can use <template>the element or the special syntax provided by Vue 3 v-fragmentto create a Fragment. For example:
```
<template>
  <div>
    <p>Paragraph 1</p>
    <p>Paragraph 2</p>
    <v-fragment>
      <p>Paragraph 3</p>
      <p>Paragraph 4</p>
    </v-fragment>
  </div>
</template>
```
  In the above example, <v-fragment>two elements are wrapped <p>, but the final rendered DOM structure does not contain additional parent elements.

Teleport and Fragments are two powerful tools in Vue 3. They help manage the rendering structure of components more flexibly and clearly, while improving the readability and maintainability of the code. These two features are especially useful when dealing with complex layouts and reusable components.

What changes have been made to the global API in Vue 3? How to use these modified APIs?

Vue 3 has made some modifications to the global API to provide better performance and functionality. Here are some of the major changes and how to use these modified APIs:

Create a Vue instance :
- Before modification (Vue 2) : In Vue 2, you can new Vue()create a root Vue instance using .
- After modification (Vue 3) : In Vue 3, you can use createApp()to create application instances, for example:
```
import {
        
         createApp } from 'vue';
const app = createApp(App);
app.mount('#app');
```
Registration of global components :
- Before modification (Vue 2) : In Vue 2, you can use Vue.component()global registration components.
- After modification (Vue 3) : In Vue 3, you can use app.component()to register global components, for example:
```
app.component('my-component', MyComponent);
```
Filter :
- Before modification (Vue 2) : Vue 2 supports filters, but the concept of filters has been removed in Vue 3. You can use computed properties or methods to replace the functionality of filters.

Mixins :

Before modification (Vue 2) : In Vue 2, you can use mixinsoptions to mix in component options.

After modification (Vue 3) : In Vue 3, you can use mixfunctions to achieve similar functions, for example:

import {
        
         defineComponent, ref, mix } from 'vue';

const mixin = {
        
        
  data() {
        
        
    return {
        
        
      message: 'Hello from mixin'
    };
  }
};

const MyComponent = defineComponent({
        
        
  mixins: [mixin],
  setup() {
        
        
    const count = ref(0);
    return {
        
        
      count
    };
  },
  template: `
    <div>
      {
         
         { message }}
      {
         
         { count }}
    </div>
  `
});

Custom instructions :
- Before modification (Vue 2) : In Vue 2, you can use Vue.directive()to register global custom directives.
- After modification (Vue 3) : In Vue 3, you can use to app.directive()register global custom directives, for example:
```
app.directive('my-directive', {
        
        
  // 自定义指令的定义
});
```

These are some major global API changes. In Vue 3, the global API is used a little differently than in Vue 2, so you need to pay attention to these changes when migrating your project or writing new Vue 3 code. You need to use the new API on a case-by-case basis to ensure that your application can take full advantage of Vue 3's functionality and performance benefits.

Please explain how the reactive system works in Vue 3?

One of the core features of Vue 3 is its reactive system, which allows you to automatically synchronize data and views in your application. Here's a brief explanation of how the reactive system in Vue 3 works:

Initialization :
- When you create a Vue 3 component or application, Vue initializes an instance of the reactive system.
Data definition :
- You setupcreate reactive data in the component's function. This can be achieved with ref, reactive, or .computed
Data dependency tracking :
- When a component renders, Vue automatically tracks data property dependencies. This means that Vue knows which data properties are used to render the view.
Reactive dependency collection :
- Vue will collect the dependencies of data properties during component rendering and build a dependency graph.
Triggered when data changes :
- When a reactive data property changes, Vue notifies views that depend on that data property to update.
Batch update :
- Vue 3 will batch notifications of multiple data changes to minimize DOM update operations and improve performance.
Asynchronous update queue :
- Vue 3 uses microtask queues (such as Promiseor nextTick) to handle data updates, ensuring that multiple data changes in the same event loop only trigger one view update.
View update :
- Once data changes are notified to the view, Vue 3 will re-render the relevant component parts to keep them in sync with the latest data.
Computed properties and listeners :
- Vue 3 allows you to use computed properties ( computed) and listeners ( watch) to handle derivation of data and listen for changes. These features also rely on reactive systems to work.

In general, Vue 3's responsive system achieves automatic synchronization between data and views through data dependency tracking and automatic view update mechanisms. This allows developers to focus more on data processing without having to manually manipulate the DOM, improving development efficiency and improving code maintainability.

What is the difference between Ref and Reactive?

refand reactiveare two different ways to create reactive data in Vue 3, and they have some important differences:

Reference type :
- ref: refUsed to create a single responsive data. It wraps a normal JavaScript value (such as a number, string, etc.) in an .valueobject with properties, making it responsive data.
- reactive: reactiveUsed to create a responsive object containing multiple properties. It accepts an ordinary JavaScript object and returns a responsive proxy object, which can turn the properties in the object into responsive data.
Access method :
- ref: You can .valueaccess refthe values in through the properties. For example: myRef.value.
- reactive: You can directly access reactiveproperties within the object. For example: myReactiveObj.someProperty.
Purpose :
- ref: Usually used to wrap basic data types, such as numbers, strings, Boolean values, etc., or to wrap .valuedata that needs to be updated through .
- reactive: Usually used to create responsive data objects containing multiple properties, such as complex configuration objects or component states.

Example :

Use to refcreate reactive data:

import {
        
         ref } from 'vue';

const count = ref(0); // 创建一个包装数字的 ref

Use to reactivecreate reactive objects:

import {
        
         reactive } from 'vue';

const person = reactive({
        
        
  name: 'Alice',
  age: 30
}); // 创建一个包含多个属性的响应式对象

In general, refused to create a single reactive data, usually used to wrap basic data types. It reactiveis used to create reactive objects containing multiple properties, usually for the management of complex data structures or component states. Which method you choose to use depends on your specific needs and data structure.

What improvements have been made to Vue 3’s support for TypeScript? How to use TypeScript with Vue 3?

Vue 3 has many improvements in its support for TypeScript, making the development using TypeScript and Vue 3 more fluid and type-safe. Here are some key improvements and guidelines for using TypeScript:

1. Type inference and type declaration :

Vue 3 provides more powerful type inference, allowing you to get more accurate type checking.
Vue 3 comes with TypeScript declaration files, so you don't need to install additional declaration files.

2. Single file component :

Parts in single-file components (.vue files) <script>can be written using TypeScript.
You can add type declarations to the component's props, data, methodsetc. parts for better type checking.

3. Provide more type definitions :

Vue 3 provides a wealth of type definitions, including type definitions for ref, reactive, computed, watch, provide, and other functions.inject

4. Composition API：

Vue 3's Composition API has powerful TypeScript support, making it easier to write reusable logic.
defineComponentType-safe components can be easily defined using functions.

5. Type-safe Props :

In components, you can use PropTypeto define the type of props.
Use TypeScript's optional properties and default values to ensure type safety of props.

6. Automated type inference :

Vue 3 can automatically infer the types of many properties, reducing the need to manually add type declarations.

7. Type-safe hook function :

Vue 3 supports type-safe lifecycle hook functions, such as onMounted, onUpdatedetc.

8. TypeScript decorator support :

Vue 3 supports TypeScript decorators, which can be used to create mixins, custom instructions, etc.

9. Rich TypeScript documentation :

The Vue 3 documentation provides a wealth of TypeScript examples and instructions to help developers better understand how to use TypeScript in Vue 3.

Guidelines for using TypeScript :

Install Vue 3: Make sure Vue 3 and TypeScript are installed in your project.
Create components: Use .vuefiles or Composition API to create components. You can add type declarations to define the props and data of the component.
Take advantage of editor support: Use an editor that supports TypeScript (such as VS Code) for better type checking and auto-completion.
Follow the Vue 3 documentation: Check out the official Vue 3 documentation, which has detailed instructions and examples on how to use TypeScript.

Overall, Vue 3 provides powerful TypeScript support, making it easier and more reliable to use TypeScript in Vue 3 projects. You can take advantage of these features to improve code quality, maintainability, and development efficiency.

Please explain how to create custom directives and custom components in Vue 3.

Some new lifecycle hook functions have been added to Vue 3 to extend the lifecycle management and logic of components. Here are the new lifecycle hooks and their uses:

beforeMount (new):
- Purpose: Called before the component is mounted. At this stage, the virtual DOM is prepared but has not yet been rendered into the real DOM. Can be used to perform some preparation work.
beforeUpdate (new):
- Purpose: Called before the component is updated. At this stage, the virtual DOM has been updated but has not yet been rendered into the real DOM. Can be used to perform preparatory work before updating.
updated (new):
- Purpose: Called after the component is updated. At this stage, the component's data has been synchronized into the view. Can be used to perform some operations related to the updated DOM.
beforeUnmount (new):
- Purpose: Called before the component is uninstalled. At this stage, the component is still fully available. Can be used to perform some cleanup work.
unmounted (new):
- Purpose: Called after the component is uninstalled. At this stage, all resources of the component have been released and are no longer available. Can be used to perform some final cleanup work.

These new lifecycle hook functions are mainly used for more fine-grained lifecycle management, allowing you to perform specific operations at different lifecycle stages of components. For example, you can beforeMountperform some operations related to pre-rendering preparation in hooks, or updatedperform some tasks related to updated DOM operations in hooks.

In addition to the new life cycle hooks, Vue 3 still supports other life cycle hooks in Vue 2, such as created, mounted, beforeDestroyand destroyedetc. These lifecycle hooks allow you to manage component lifecycles more flexibly to meet different needs.