How to solve the problem 'Error: 'xxx' has already been declared as a data property.' when using vuex in a Vue application?

In the development process of Vue applications, it is very common to use vuex to manage application status. However, in the process of using vuex, sometimes we may encounter such an error message: "Error: 'xxx' has already been declared as a data property." This error message seems baffling, but it is actually due to the error in Vue. In the component, it is caused by using repeated variable names to define data attributes and vuex state attributes.

So, how to solve this problem? Below I will discuss the following aspects.

1. Understand the problem of variable name conflicts in vuex and Vue components

First of all, it needs to be clear that the data attribute in the Vue component, the state attribute of vuex, and the computed attribute are all considered Vue's state properties. Therefore, when using vuex, if the same name as in the data attribute is defined, it will cause a variable name conflict and the above error message will appear.

For example, in the following code, a variable named count is defined in the data attribute, and a variable with the same name is also defined in the state of vuex:

<template>
  <div>
    <h1>{{ count }}</h1>
  </div>
</template>

<script>
  import { mapState } from 'vuex';
  export default {
    data() {
      return {
        count: 0
      }
    },
    computed: {
      ...mapState(['count'])
    }
  }
</script>

At this time, in When running the Vue application, the above error message will appear.

2. Solution 1: Modify the variable name

There are many ways to solve this problem. One of the simple and effective methods is to modify the repeated variable name.

In the above code, we can change the count variable name in the data attribute to another name, such as "dataCount", and then the problem of variable name conflict will no longer occur. The code is as follows:

<template>
  <div>
    <h1>{{ count }}</h1>
  </div>
</template>

<script>
  import { mapState } from 'vuex';
  export default {
    data() {
      return {
        dataCount: 0 // 把变量名改成dataCount
      }
    },
    computed: {
      ...mapState(['count'])
    }
  }
</script>

3. Solution 2: Use the namespaced attribute

Another way to solve this problem is to use the namespaced attribute in the vuex store. The purpose of this attribute is to specify a namespace for the state property of vuex to avoid conflict with variable names in the Vue component.

For example, we can change the vuex state attribute in the above code to the following form:

const store = new Vuex.Store({
  namespaced: true, // 新增一个 namespaced 属性
  state: {
    count: 0
  }
})

After defining the vuex state attribute, we also need to specify it when we use the mapState function in the Vue component Namespaces. For example, modify the above code as follows:

<template>
  <div>
    <h1>{{ count }}</h1>
  </div>
</template>

<script>
  import { mapState } from 'vuex';
  export default {
    computed: {
      ...mapState({
        count: state => state.exampleModule.count // exampleModule 是命名空间
      })
    }
  }
</script>

By using the namespaced attribute, we can avoid the problem of variable name conflicts and make the code more standardized and clear.

Summary

When using vuex in a Vue application, it is very important to avoid variable name conflicts. When the error message "Error: 'xxx' has already been declared as a data property." appears, we can solve this problem by modifying the variable name or using the namespaced attribute. For more information on how to use vuex, you can refer to the official documentation: https://vuex.vuejs.org/zh/

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