TypeError in Vue: Cannot read property 'XXX' of undefined, what are the solutions?

Vue is a popular front-end framework that is often used to develop web applications. However, during the development process using Vue, errors such as "TypeError: Cannot read property 'XXX' of undefined" may sometimes occur. When this error occurs, how should we solve it? This article details how to resolve this error.

First, let’s understand what the “TypeError: Cannot read property ‘XXX’ of undefined” error is. This error is usually caused by accessing an undefined property or method in the code. For example, in a Vue component, we may encounter a situation where an undefined data property is accessed in the component, as shown below:

export default {
  data() {
    return {
      message: 'Hello, World!'
    }
  },
  methods: {
    showMessage() {
      console.log(this.message.toUpperCase()); // TypeError: Cannot read property 'toUpperCase' of undefined
    }
  }
}

In the showMessage method of the above code, an attempt is made to access a non-existent data property. property this.message.toUpperCase(), so the "TypeError: Cannot read property 'toUpperCase' of undefined" error will be thrown.

Next, we will introduce ways to solve this error.

1. Check whether the object has been defined

First, check whether the object accessed in the code has been defined. If it is not defined, the error will be thrown. Therefore, we need to use an if statement or the ternary operator to ensure that the object is defined.

export default {
  data() {
    return {
      message: 'Hello, World!'
    }
  },
  methods: {
    showMessage() {
      if (this.message) {
        console.log(this.message.toUpperCase());
      }
    }
  }
}

In the above code, we use the if statement to check whether this.message has been defined. Only when this.message has been defined, the console.log statement will be executed.

2. Use the optional chain operator (?.)

In Vue3.0 and above, you can use the optional chain operator (?.) to avoid accessing undefined Property or method. This operator means that if the left-hand expression is undefined or null, the right-hand expression will not be executed.

export default {
  data() {
    return {
      message: 'Hello, World!'
    }
  },
  methods: {
    showMessage() {
      console.log(this.message?.toUpperCase()); // 如果this.message未定义，则不会执行toUpperCase()方法
    }
  }
}

In the above code, we use the optional chain operator (?.) to access the this.message property. If this.message is undefined, the toUpperCase() method will not be executed, so it will not be thrown. TypeError.

3. Use default values ​​

You can also use default values ​​when accessing undefined properties or methods to avoid throwing TypeError errors.

export default {
  data() {
    return {
      message: 'Hello, World!'
    }
  },
  methods: {
    showMessage() {
      console.log((this.message || '').toUpperCase()); // 如果this.message未定义，则返回空字符串
    }
  }
}

In the above code, we use the OR operator (||) to set the default value (empty string) for this.message to avoid throwing TypeError errors.

To sum up, when the "TypeError: Cannot read property 'XXX' of undefined" error occurs, we can use the above three methods to solve the error. When writing Vue code, be careful to check whether the object is defined to avoid such errors.

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