The role of watch in vue

Watch in Vue.js is used to monitor changes in responsive data properties and execute callback functions. The specific method of use is to use the watch option in the Vue instance to specify a map or array of expressions. Each change in the expression or change in the array elements will trigger the corresponding callback function. Benefits of watch include reactive change detection, callback functions, initial load triggering, and multiple usage scenarios such as loading data, updating DOM, and handling array changes.

The role of watch in Vue.js

In Vue.js, watch is a built-in responsive Feature that allows you to monitor and respond to changes in reactive data properties. In short, watch will execute the specified function or callback when the relevant data changes.

How to use watch

To use watch, you can use the watch option in your Vue instance. This option accepts an object containing an expression or array to callback function mapping, as shown below:

<code class="javascript">export default {
  watch: {
    // 表达式：当表达式值发生变化时执行回调函数
    '$route.params.id': (newValue, oldValue) => {
      // ...
    },

    // 函数：当指定函数返回的新值与旧值不相等时执行回调函数
    computedProp: (newValue, oldValue) => {
      // ...
    },

    // 数组：监视数组中的每个项目的变动并执行回调函数
    items: {
      handler: (newValue, oldValue) => {
        // ...
      },

      // 可选的：允许在初始加载时触发回调函数
      immediate: true
    }
  }
}</code>

Benefits of watch

Using watch has the following benefits:

• Reactive change detection: watch automatically tracks changes to reactive data, so you don't have to manually check for differences.
• Callback function: watch allows you to perform specific actions or update other data when the data changes.
• Initial load: You can use the immediate: true option to trigger a callback function on initial load to perform an action immediately after the page loads.

Usage scenarios

Some typical watch usage scenarios include:

• Loading data based on routing parameters
• Update DOM based on computed properties
• Update the list when the array changes
• Perform asynchronous operations on initial load

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