Detailed explanation of the watch function in Vue3: application for monitoring data changes

Vue3 is one of the most popular JavaScript frameworks currently. Many web developers and front-end developers will encounter the need to monitor data changes when using Vue3. Vue3 provides the watch function to achieve this purpose. In this article, we will explain the watch function in Vue3 in detail and introduce how to use the watch function to monitor data changes in Vue components.

The watch function is one of the very important functions in Vue3. The watch function can be used to monitor data changes in Vue components and perform corresponding operations when the data changes. In Vue3, the basic syntax of the watch function is as follows:

  watch: {
    propertyName: function (newValue, oldValue) {
      //在数据变化时执行的操作
    }
  }

In this syntax, propertyName is the name of the data to be monitored. newValue and oldValue represent the new value and the old value respectively. When the data changes, the watch function will automatically call this function, and then pass the new value and the old value as parameters. We can determine whether the data has changed by comparing the size of the new value with the old value.

In addition to basic syntax, Vue3's watch function also supports some advanced usage, such as in-depth monitoring, immediate execution, calculated properties, etc. Let’s introduce these advanced usages respectively.

Depth Monitoring

By default, Vue3's watch function only monitors whether the data references are equal. In other words, if we change the attribute value of the data, but the reference of the data does not change, then the watch function will not be executed. To solve this problem, Vue3 provides in-depth monitoring functionality. We can add the deep option to the watch function to enable in-depth monitoring. For example:

  watch: {
    propertyName: {
      handler: function (newValue, oldValue) {
        //在数据变化时执行的操作
      },
      deep: true
    }
  }

In this example, we enable in-depth monitoring by setting deep to true. In this way, when the attribute value of the data changes, the watch function will be called.

Execute immediately

By default, Vue3's watch function will only be executed when the data changes. If we need to execute the watch function once when the component is initialized, we can add the immediate option to the watch function. For example:

  watch: {
    propertyName: {
      handler: function (newValue, oldValue) {
        //在数据变化时执行的操作
      },
      immediate: true
    }
  }

In this example, we set immediate to true so that the watch function will be executed once when the component is initialized.

Computed properties

In Vue3, we can use calculated properties to generate monitored data. The basic syntax of calculated properties is as follows:

  computed: {
    propertyName: function () {
      //计算属性的逻辑
      return someValue
    }
  }

In this syntax, propertyName is the name of the monitored data, and someValue is the calculated result of the calculated property. We can use calculated properties as monitored data, and then use the watch function to monitor changes in calculated properties. For example:

  computed: {
    propertyName: function () {
      //计算属性的逻辑
      return someValue
    }
  },
  watch: {
    propertyName: function (newValue, oldValue) {
      //在数据变化时执行的操作
    }
  }

In this example, we use the calculated property propertyName to generate monitored data, and then use the watch function to monitor changes in propertyName. When propertyName changes, the watch function will be called.

Summary

In this article, we have a detailed explanation of the watch function in Vue3. We introduced the basic syntax of the watch function, as well as some advanced usage, such as in-depth monitoring, immediate execution, calculated properties, etc. Vue3's watch function is a very convenient and practical function. It allows us to easily monitor data changes in Vue components and helps us write more elegant and efficient code. If you are learning Vue3 or already using Vue3, you must understand and master the watch function.

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