TypeError: Cannot read property 'XXX' of undefined in Vue project, how to deal with it?

When developing in the Vue project, I occasionally encounter errors such as TypeError: Cannot read property 'XXX' of undefined. The page shows that the page cannot be rendered and the data cannot be obtained. And other issues. This kind of error may occur in various scenarios such as data transmission, component communication, asynchronous requests, etc. when using the Vue framework. This article will detail the causes and solutions to this error.

1. Causes

TypeError: Cannot read property 'XXX' of undefined has various reasons. The following are some common situations:

1. Access Undefined object property

In this case, it can be encountered when the page is rendered, or it can be detected when the page is loaded.

For example:

const obj = {};
console.log(obj.xxx); //TypeError: Cannot read property 'xxx' of undefined

This error indicates an attempt to access an undefined property xxx.

2. Error in obtaining asynchronous request data

When requesting data, due to various reasons, the data returned by the request may be empty or undefined.

For example:

axios.get('/api/data').then(res => {
  const data = res.data;
  console.log(data.xxx); //TypeError: Cannot read property 'xxx' of undefined
}).catch(err => {
  console.log(err);
})

In this case, if the data returned by the server is empty or undefined, an error will be reported when trying to access its properties.

3. Accessing a destroyed component

In the Vue component, if the component's properties or methods are still being accessed when the component is destroyed, this error will occur.

For example:

export default {
  data () {
    return {
      list: []
    }
  },
  created () {
    this.timer = setTimeout(() => {
      this.list = [1, 2, 3];
    }, 1000);
  },
  beforeDestroy () {
    clearTimeout(this.timer);
  }
}

In this component, a timer is created, and the list attribute in the data in the component is modified after 1 second. This error will occur if the component is destroyed before the timer is executed.

4. The parent component does not pass the props attribute

In the Vue component, if you need to pass the props attribute through the parent component, if the attribute is not passed in the parent component, it will This error occurs.

For example:

<template>
  <my-component :list="list"></my-component>
</template>

<script>
import MyComponent from './MyComponent.vue';

export default {
  components: {
    MyComponent
  }
}
</script>

In this example, the list attribute is not defined in the parent component, but the list attribute is bound to the child component through props, which will cause this error.

2. Solution

1. Check whether there are undefined properties in the code

TypeError: Cannot read property 'XXX' of undefined appears. When this error occurs, the first step should be to check whether the code is accessing undefined properties. You can find the specific error location by adding breakpoints, printing error objects, etc.

2. Processing data of asynchronous requests

When making an asynchronous request, the returned data needs to be verified to ensure that it is not undefined.

For example:

axios.get('/api/data').then(res => {
  const data = res.data;
  if (data) {
    console.log(data.xxx);
  } else {
    console.log('返回的数据为空或者为 undefined');
  }
}).catch(err => {
  console.log(err);
});

Avoid errors caused by accessing undefined by verifying the returned data before obtaining the data.

3. Add component life cycle hook

In the component cycle hook, you can control the creation and destruction of components to avoid accessing destroyed components.

For example:

export default {
  data () {
    return {
      list: []
    }
  },
  created () {
    this.timer = setTimeout(() => {
      if (!this._isDestroyed) {
        this.list = [1, 2, 3];
      }
    }, 1000);
  },
  beforeDestroy () {
    clearTimeout(this.timer);
    this._isDestroyed = true;
  }
}

By adding the _isDestroyed attribute, you can prevent the component's properties from being accessed when the component has been destroyed.

4. Set the default value

When using the props attribute, you should set the default value to avoid causing errors if it is not defined in the parent component.

For example:

export default {
  props: {
    list: {
      type: Array,
      default: () => []
    }
  }
}

When the parent component does not pass the list attribute, the default empty array is used.

In short, when developing a Vue project and an error like TypeError: Cannot read property 'XXX' of undefined occurs, you need to first find the cause of the error and then adopt an appropriate solution strategy. These solutions not only solve the problem, but also help us better understand Vue's mechanisms such as data flow and component communication.

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