How to solve 'TypeError: Cannot set property 'abc' of undefined' in Vue application?

Vue is a popular JavaScript framework for building modern web applications. It enables developers to build interactive UI interfaces and interact with backend APIs. But as the size of the application increases, maintaining the code becomes more difficult, and sometimes you encounter some bugs. In this article, we will explore a common error in Vue, namely "TypeError: Cannot set property 'abc' of undefined", and introduce how to solve it.

Error analysis

The undefined property cannot be set error usually occurs when trying to access an undefined variable or object property in a Vue component. For example, accessing undefined component properties, or incorrectly initializing data objects, etc. Let's look at some common code snippets that may trigger this kind of error: Initializing the userList array will trigger the "TypeError: Cannot set property 'push' of undefined" error.

1. Accessing undefined component properties

export default {
  data() {
    return {
      userList: [],
    };
  },
  async mounted() {
    // 错误示例 - userList数组未被初始化
    await fetchUsers().then((users) => {
      this.userList.push(...users);
    });
  },
};

In the above example, because the user property is not defined, "TypeError: Cannot read property 'firstName' of undefined will be triggered "mistake.

Solution

Because this error is usually caused by accessing undefined component properties, we You need to check that all properties in the component are defined in the correct place. For example, in the example above, we should make sure to define a property called "user" so that it can be accessed in the computed property and avoid throwing an error.

<template>
  <div>{{ user.name }}</div>
</template>

<script>
export default {
  computed: {
    // 错误示例 - user属性未被定义
    userFullName() {
      return this.user.firstName + ' ' + this.user.lastName;
    },
  },
};
</script>

1. Check whether the data object is correctly initialized

Correctly initializing the data object of the Vue component is another way to avoid errors caused by accessing undefined objects. We can solve such problems by setting default values ​​for the data objects or initializing the data when the component instance is mounted. For example, in the code below, we set a default value for the userList array and initialize it when the component instance is mounted.

<template>
  <div>{{ user.name }}</div>
</template>

<script>
export default {
  props: {
    user: {
      type: Object,
      required: true,
    },
  },
  computed: {
    userFullName() {
      return this.user.firstName + ' ' + this.user.lastName;
    },
  },
};
</script>

Summary
2. In Vue applications, encountering "TypeError: Cannot set property 'abc' of undefined" error is usually caused by accessing undefined component properties or incorrectly initializing data objects. caused. We can solve such problems by checking that the component properties are correctly defined and the data objects are initialized correctly. These methods will help us avoid common mistakes in Vue applications and allow us to build modern web applications faster.

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