How to deal with '[Vue warn]: Discarded one or more' error

How to deal with the "[Vue warn]: Discarded one or more" error

During the development process of using Vue.js, we may encounter some warning prompts, One of the common warnings is "[Vue warn]: Discarded one or more". This warning usually appears when a component uses the v-if or v-show directive, which means that Vue.js discarded certain elements during the rendering process. This article explains the cause of this warning and how to deal with it.

There are usually two reasons for warnings:

1. Conditions are not met: When the conditions of the v-if or v-show directive are not met, Vue will remove the element from Removed from DOM. When the condition is met again, Vue will re-render the element. During this process, if Vue finds that there is state or event binding on the element, it will issue this warning.
2. Subcomponent is destroyed: The warning may also appear when a subcomponent is destroyed. If there are some asynchronous operations or delayed execution code inside the subcomponent, these codes may still be executed when the subcomponent is destroyed. If these codes use the state or event binding of the Vue instance, a warning will appear.

In order to solve this warning, we can take the following methods:

1. Use v-if instead of v-show: when the conditions are not met, use v- if instead of v-show can avoid warnings. v-if will render the element if the condition is met, and completely remove it from the DOM if the condition is not met. The disadvantage of this is that there may be performance issues when switching conditions frequently.
2. Use the key attribute: In the v-for directive, using the key attribute can help Vue accurately track the status of each element. This way, when conditions change, Vue will re-render the corresponding elements instead of re-creating them.

<template>
  <div>
    <div v-for="item in items" :key="item.id">{{ item.name }}</div>
  </div>
</template>

3. Cancel asynchronous operations when the component is destroyed: When the child component is destroyed, we can use Vue's beforeDestroy hook function to cancel possible asynchronous operations or clean up event bindings.

export default {
  beforeDestroy() {
    // 取消定时器
    clearTimeout(this.timer);
    // 取消事件监听
    window.removeEventListener('resize', this.handleResize);
  },
  created() {
    // 异步操作
    this.timer = setTimeout(() => {
      // do something
    }, 1000);
    // 事件监听
    window.addEventListener('resize', this.handleResize);
  }
}

4. Use Vue's $destroy method to destroy the child component: If the child component really needs to perform some cleanup operations when destroyed, we can manually call the $destroy method in the parent component to destroy the child. components. This will trigger the child component's beforeDestroy hook function and remove the child component from the DOM.

export default {
  methods: {
    destroyChildComponent() {
      this.$refs.childComponent.$destroy();
    }
  }
}

To summarize, the key to dealing with the "[Vue warn]: Discarded one or more" error is to understand the cause of the warning and take appropriate steps to resolve it. We can use v-if instead of v-show to avoid warnings when conditions are not met, use key attributes to track the state of elements, cancel asynchronous operations and clean up event bindings, and manually call the $destroy method to destroy child components. Through these methods, we can improve the performance of the application and avoid this warning.

Hope this article can help you deal with the "[Vue warn]: Discarded one or more" error and better use Vue.js to develop applications.

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