The role of the watch option in vue

The watch option in Vue.js can monitor changes in computed properties or data and run user-defined callback functions when changes occur, which can be used to update the UI, perform asynchronous tasks, and control component behavior.

The role of the watch option in Vue

Answer the question:
Vue. The watch option in js is used to monitor changes in the computed property or data, and run a user-defined callback function when changes occur.

Detailed explanation:

The watch option is a method of a Vue instance, accepting two parameters:

• To be monitored Attribute or expression: can be a computed attribute or an attribute in the data object.

• Callback function: Function called when the monitored property changes. This function accepts two parameters:

  • newValue: The new value of the attribute.
  • oldValue: The old value of the attribute.

#The purpose of the watch option is to allow you to perform specific tasks when data changes. This is useful in the following situations:

• Updating UI: When a data property is modified, you may need to update the UI elements associated with that data.
• Perform asynchronous tasks: You may need to trigger asynchronous operations when data changes, such as sending an API request.
• Control the behavior of a component: You can change the internal state or behavior of a component in the watch callback function.

Usage example:

<code class="javascript">// 在 Vue 实例中使用 watch
watch: {
  // 监视 computed 属性
  computedProp: {
    handler(newValue, oldValue) {
      // 在 computedProp 改变时执行
    },
    immediate: true // 立即执行一次
  },

  // 监视 data 对象中的属性
  dataProp: {
    handler(newValue, oldValue) {
      // 在 dataProp 改变时执行
    }
  }
}</code>

Note: The

• watch option only monitors changed properties, rather than the entire object or array.
• If you want to deeply monitor an object or array, you can use a third-party library, such as Vue.js's vue-deep-watch.
• The watch option can be used in any Vue instance, including components and root instances.

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