In-depth understanding of the use of this in Vue3

During the development of Vue3,thisHow to use andVue2There are significant differences, especially after the introduction of Composition API. This article will explore in depth in Vue3thisThe use of the , parse its underlying source code, explore the reasons behind this design, and share some interview skills.

1. How to use this in Vue3

1. Use in the Optional API

In Vue2's Options API,thisUsually pointing to the current component instance. You can passthisAccess the component's data, methods, computed and other properties.

export default {
  data() {
    return {
      message: 'Hello Vue3!'
    };
  },
  methods: {
    greet() {
      (); // Output: Hello Vue3!    }
  }
};

2. Changes in the Combination API

Vue3's combination API no longer depends onthisto access component instances. Instead, it uses the concept of functional programming, throughref、reactiveetc. to create responsive states and throughsetupFunctions to organize logic.

import { ref } from 'vue';

export default {
  setup() {
    const message = ref('Hello Vue3!');
    const greet = () => {
      (); // Output: Hello Vue3!    };
    return {
      message,
      greet
    };
  }
};

3. This limitation in Vue3

In Vue3, due to the promotion of combined APIs,thisThe usage scenarios are greatly restricted. existsetupYou cannot use the functionthisto access component instances. This is becausesetupThe function is called before the component is instantiated, at this timethisNot bound to the component instance.

4. How to access component instances in a composite API

AlthoughsetupCan't be used directly in the functionthis, but you can passgetCurrentInstanceFunction to get the current component instance. However, this is usually not the recommended approach, as it breaks the functional programming style of combinatorial APIs.

import { getCurrentInstance } from 'vue';

export default {
  setup() {
    const instance = getCurrentInstance();
    (instance); // Output the current component instance  }
};

2. Analysis of this related underlying source code

1. The creation process of component instances

In Vue3, the creation process of component instances goes through multiple stages. First, Vue parses the component's configuration object and then creates the initial state of the component instance. In this process,thisNot bound to the component instance. untilsetupVue will not release the function after thethisBind to the component instance and execute other lifecycle hook functions.

2. The execution time of the setup function

setupFunctions are called before component instantiation. This meanssetupYou cannot use the functionthisto access any properties or methods of component instances. This is also an important concept of Vue3 combined API design: separate logic from instance state to support better code reuse and testing.

3. The implementation principle of getCurrentInstance

getCurrentInstanceThe function gets the current component instance by accessing the internal state of Vue. This function is very useful in Vue's development environment, but should be avoided in production environments as it increases unnecessary complexity and performance overhead.

3. Why is it designed like this?

1. Support better code reuse

Vue3's combined API separates logic from instance state through the concept of functional programming. This makes the code easier to reuse, as you can export logic functions and reuse them in other components without worrying aboutthisPointed to the question.

2. Improve the testability of the code

Since the combination API does not depend onthis, so you can write unit tests more easily. You can call it directlysetupFunction and pass in necessary parameters without mocking the entire component instance.

3. Cater to future trends

With the rise of functional and responsive programming, Vue3's combo API caters to these future trends. By abandoningthis, Vue3 provides developers with a more flexible and powerful programming model.

4. Interview skills

1. Be familiar with the basic concepts of combination APIs

During the interview, the interviewer may ask you about your understanding of the Vue3 combination API. You need to be familiar with itref、reactiveThe use of functions such assetupFunction execution timing and limitations.

2. Explain this changes in Vue3

The interviewer may let you compare Vue2 and Vue3thisUse differences. You need to be able to clearly explain in Vue3thislimitations and how to access component instances in a composite API.

3. Show code cases

In an interview, it is very effective to articulate your point of view by presenting code cases. You can prepare some simple code examples to show how to use it in Vue3thisand combination API.

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