How to solve the problem 'Cannot read property 'xyz' of undefined' in Vue application?

Vue is a very popular JavaScript framework that helps developers build interactive web applications. When we develop Vue applications, error messages such as "Cannot read property 'xyz' of undefined" sometimes appear. This error message means that we are trying to access an undefined variable, or accessing an object property, but in fact the property does not exist. This article will introduce some methods to solve this error.

1. Check whether the variable is correctly declared and initialized

The "Cannot read property 'xyz' of undefined" error message is likely caused by incorrect declaration or initialization of the variable. caused. In Vue, we usually use the data attribute to declare and initialize variables in components. Therefore, make sure you have correctly declared and initialized the variables you want to use in the data attribute. For example, in the code below, we are referencing an uninitialized variable foo, which will result in the above error:

<template>
  <div>{{ foo.bar }}</div>
</template>
<script>
export default {
  data() {
    // 未初始化变量foo
  }
}
</script>

To solve this problem, we just need to initialize the foo variable in the data attribute:

<template>
  <div>{{ foo.bar }}</div>
</template>
<script>
export default {
  data() {
    return {
      foo: { bar: 'Hello World' }
    }
  }
}
</script>

2. Check whether the object property name is correct

If the "Cannot read property 'xyz' of undefined" error message appears when we access an object property in the Vue component, Then it's probably because we used a wrong attribute name. Check that the property exists in the object you are trying to access, and make sure you use the correct property name. For example, in the code below, we are using a non-existent property baz, which will result in the above error:

<template>
  <div>{{ foo.baz }}</div>
</template>
<script>
export default {
  data() {
    return {
      foo: { bar: 'Hello World' }
    }
  }
}
</script>

To solve this problem, we just need to add a correct property baz to the object i.e. Available:

<template>
  <div>{{ foo.baz }}</div>
</template>
<script>
export default {
  data() {
    return {
      foo: { bar: 'Hello World', baz: 'Hello Vue' }
    }
  }
}
</script>

3. Check whether asynchronous requests are used

If we use asynchronous requests to obtain data in the Vue component and use the data in the component, then Before the data is retrieved, the data will be undefined, which will result in a "Cannot read property 'xyz' of undefined" error message. For example, in the code below, we use an asynchronous request to store data into the foo variable, but before the data is loaded, we try to access the foo.bar property, which will result in the above error:

<template>
  <div>{{ foo.bar }}</div>
</template>
<script>
export default {
  data() {
    return {
      foo: {}
    }
  },
  created() {
    this.getData();
  },
  methods: {
    getData() {
      axios.get('/api/data')
        .then(response => {
          this.foo = response.data;
        })
    }
  }
}
</script>

To To solve this problem, we can use the v-if directive to hide the relevant component before the data is loaded, and then display the component after the data is loaded. For example:

<template>
  <div v-if="foo.bar">{{ foo.bar }}</div>
</template>
<script>
export default {
  data() {
    return {
      foo: {}
    }
  },
  created() {
    this.getData();
  },
  methods: {
    getData() {
      axios.get('/api/data')
        .then(response => {
          this.foo = response.data;
        })
    }
  }
}
</script>

In the above code, we use the v-if directive to determine whether the foo.bar attribute has been loaded. The component will not be displayed until the data is loaded. The component will be displayed after the data is loaded.

When developing a Vue application, when encountering the "Cannot read property 'xyz' of undefined" error message, we should first check whether the variables are correctly declared and initialized, and check whether the correct object properties are used. name. If we use an asynchronous request to get data, make sure to hide the data during component loading and not show the component until the data is loaded. With these methods, we can easily resolve this error and continue developing our Vue application.

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