Vue.js Watchers

Watchers are a key feature of Vue.js, which provide a way to watch for changes on a property and react accordingly. When a watched property changes, a function defined in the watcher will be called, allowing the component to update itself in response. This feature allows you to be notified of changes to specific variables in your application and perform some logic whenever the value changes.

Syntax

The watch option can be used in a Vue.js component to watch a particular property and run a callback function whenever the property changes. The basic syntax is as follows:

watch: {
  propertyName: function (newVal, oldVal) {
    // callback function
  }
}

In this syntax, propertyName is the name of the property to watch, and the callback function is what to do when this property changes. newVal and oldVal are the new and previous values of the watched property, respectively.

Example

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

new Vue({
  el: "#app",
  data: {
    message: "Initial message"
  },
  methods: {
    changeMessage() {
      this.message = "Changed message";
    }
  },
  watch: {
    message(newVal, oldVal) {
      console.log(`The message has changed from ${oldVal} to ${newVal}`);
    }
  }
})

In this example, a value of message is watched, and the console logs the changes whenever it changes.

Output

When the changeMessage() method is called by clicking the button, it updates the message data property and triggers the watcher function. The output in the console will be as follows:

The message has changed from Initial message to Changed message

Explanation

The watcher runs the callback function when a specific data property changes. This allows you to run additional logic in response to the change, such as updating the view, updating other data properties, or performing an API call.

Use

The watch option in Vue.js is useful in situations where you want to watch a particular property for changes and perform some logic when the value changes. Some common use cases include:

• Updating other data properties when one value changes
• Performing an API call when a particular property changes
• Updating the view when a conditional value changes

Important Points

• The watch option can be used to watch specific properties for changes in a Vue.js component.
• A callback function is called whenever a watched property changes, with the new and old values passed as arguments.
• Watchers can be used for updating other properties, performing API calls, or updating the view.

Summary

Vue.js watchers are a powerful way to watch specific properties for changes and perform logic in response. Understanding the syntax and use cases for watchers can greatly enhance your Vue.js applications and allow for increased functionality and interactivity.