How to solve '[Vue warn]: Avoid mutating a prop directly' error

How to solve the "[Vue warn]: Avoid mutating a prop directly" error

In Vue development, we often encounter a common warning message: " [Vue warn]: Avoid mutating a prop directly". The meaning of this warning message is that we should not directly modify the value of a props, but should modify it through other methods.

In order to better understand and solve this problem, let us look at a specific code example:

<template>
  <div>
    <h1 id="message">{{ message }}</h1>
    <button @click="changeMessage">Change Message</button>
  </div>
</template>

<script>
export default {
  props: {
    message: String
  },
  methods: {
    changeMessage() {
      this.message = "New Message"; // 直接修改props的值
    }
  }
};
</script>

In this code, we define a props named "message" , and then directly modified the value of props in the "changeMessage" method. However, this approach is not recommended.

Why is it not recommended to modify the value of props directly? This is because props, as a one-way data flow, can only be passed from parent components to child components. Child components should not directly modify the values ​​​​of props, because this may lead to data inconsistency and difficult-to-debug bugs.

So, how to solve this warning message? The solution is to use the "emit" method provided by Vue to send an event, and then listen to this event in the parent component to modify the value of props. Let us modify the above code:

<template>
  <div>
    <h1 id="message">{{ message }}</h1>
    <button @click="changeMessage">Change Message</button>
  </div>
</template>

<script>
export default {
  props: {
    message: String
  },
  methods: {
    changeMessage() {
      this.$emit("update:message", "New Message"); // 发送一个事件
    }
  }
};
</script>

In the above code, we use this.$emit("update:message", "New Message") to send a name For the "update:message" event, the new message is passed as a parameter. Then listen to this event in the parent component and modify the value of props in the event callback function.

<template>
  <div>
    <child-component :message="parentMessage" @update:message="updateParentMessage"></child-component>
  </div>
</template>

<script>
import ChildComponent from "./ChildComponent.vue";
export default {
  components: {
    ChildComponent
  },
  data() {
    return {
      parentMessage: "Hello"
    };
  },
  methods: {
    updateParentMessage(newMessage) {
      this.parentMessage = newMessage; // 在事件回调函数中修改props的值
    }
  }
};
</script>

In the parent component, we pass "parentMessage" as props to the child component, and add the "@update:message" event listener on the child component. When the child component sends this event, the parent component will call the "updateParentMessage" method to modify the value of "parentMessage", thus realizing the modification of props.

By using the "emit" method and event listening, we avoid directly modifying the values ​​​​of props, thereby solving the "[Vue warn]: Avoid mutating a prop directly" warning message.

To summarize, we should develop the habit of not directly modifying the props value. Use the "emit" method in the child component to send an event, and listen to this event in the parent component to modify the props value. This ensures data consistency and makes maintenance and debugging easier.

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