How to solve the '[Vue warn]: Cannot assign to read only property' error

How to solve the "[Vue warn]: Cannot assign to read only property" error

In the process of developing using Vue.js, we often encounter Some error messages. One of the common errors is "[Vue warn]: Cannot assign to read only property". This error is usually caused by trying to modify read-only properties in the Vue instance. This article details the cause of this error and provides a solution and associated code examples.

Error reason

There are two types of data in Vue: responsive data and non-responsive data. Reactive data is data in a Vue instance. When the data changes, Vue will automatically update the view. Non-responsive data refers to data outside the Vue instance, and Vue will not track its changes.

When we use Vue for data binding, Vue will convert the data into reactive data. However, if we try to modify the read-only property, a "[Vue warn]: Cannot assign to read only property" error will appear.

Solution

The solution to this error is to avoid modifying read-only properties. According to the error message, we can determine which attribute was incorrectly modified. Next, we will introduce two common situations and corresponding solutions.

Scenario 1: Modify the props attribute

When using the props attribute to pass data from the parent component to the child component, the props attribute is read-only. Therefore, if we try to modify the props attribute in the child component, this error will appear. To solve this problem, we should use a special method provided by Vue, which is to use events to update the properties of the parent component.

The following is a sample code that demonstrates how to use the props attribute correctly:

// 父组件
<template>
  <div>
    <child-component :message="message" @updateMessage="updateMessage"></child-component>
  </div>
</template>

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

export default {
  components: {
    ChildComponent
  },
  data() {
    return {
      message: 'Hello Vue!'
    }
  },
  methods: {
    updateMessage(newMessage) {
      this.message = newMessage;
    }
  }
}
</script>

// 子组件
<template>
  <div>
    <button @click="changeMessage">Change Message</button>
  </div>
</template>

<script>
export default {
  props: {
    message: {
      type: String,
      required: true
    }
  },
  methods: {
    changeMessage() {
      this.$emit('updateMessage', 'New Message');
    }
  }
}
</script>

In this example, the parent component passes the message to the child component through the props attribute. The button click event in the child component calls the changeMessage method and uses this.$emit to trigger a custom event to pass the new message back to the updateMessage method in the parent component. This way we avoid modifying read-only props properties.

Scenario 2: Modify calculated properties

Computed properties are properties calculated based on other properties and have a caching function in Vue. By default, computed properties are read-only. If we try to modify the value of a computed property, the "[Vue warn]: Cannot assign to read only property" error will also appear. To solve this problem, we should modify the dependency properties of the computed properties.

The following is a sample code that demonstrates how to use calculated properties and dependent properties correctly:

<template>
  <div>
    <input v-model="firstName">
    <input v-model="lastName">
    <p>{{ fullName }}</p>
  </div>
</template>

<script>
export default {
  data() {
    return {
      firstName: 'John',
      lastName: 'Doe'
    }
  },
  computed: {
    fullName: {
      get() {
        return this.firstName + ' ' + this.lastName;
      },
      set(value) {
        const names = value.split(' ');
        this.firstName = names[0];
        this.lastName = names[1];
      }
    }
  }
}
</script>

In this example, we use v-model to bind the values ​​​​of the input boxes to on the firstName and lastName properties. The computed property fullName calculates the full name based on firstName and lastName. Note that in the set function of the computed property, we modify the dependent property, not the computed property itself. This way, we avoid the error of modifying read-only properties.

Summary

In Vue development, we often encounter "[Vue warn]: Cannot assign to read only property" error. This error is usually caused by trying to modify a read-only property. To resolve this error, we should avoid modifying read-only properties. For different situations, we can use events to update props attributes or modify dependency attributes of calculated attributes to solve this problem. I hope the solutions and code examples in this article are helpful!

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