Can variables still be modified after vue is mounted?

The Vue framework is a popular JavaScript framework that uses declarative rendering to establish the connection between views and models through data binding. Vue applications typically involve creating and setting up Vue instances, defining components and data properties, setting computed properties, and updating the application's state when needed.

This article will explore a common question, that is, whether variables can be modified after the Vue instance is mounted. The answer is yes! In fact, Vue instances provide a variety of ways to modify data properties and state, including using computed properties, event handlers, observable properties, methods, and more.

First, let's take a look at how to define and modify data properties in a Vue instance. In a Vue instance, you can use the data option to define data attributes, for example:

const app = new Vue({
  el: '#app',
  data: {
    message: 'Hello, Vue!'
  }
});

In the above code, we define a Vue instance and set a data attribute, which is message. The value of this property is "Hello, Vue!". How to modify the value of this data attribute after mounting? We can use the Vue.set method to change it:

Vue.set(app, 'message', 'Hello, World!');

Using the Vue.set method tells the Vue instance that we want to update a data property, and Vue will automatically update the view in the component to reflect this change.

Secondly, computed properties are also very useful. They allow us to calculate new values ​​based on existing data properties. The advantage of computed properties is that they cache the results and are not recalculated as long as the dependent data has not changed. This can greatly improve performance.

const app = new Vue({
  el: '#app',
  data: {
    firstName: 'Tom',
    lastName: 'Smith'
  },
  computed: {
    fullName() {
      return `${this.lastName}, ${this.firstName}`;
    }
  }
});

In the above code, we define a calculated property fullName, which is calculated based on the data attributes firstName and lastName. If firstName or lastName is modified, the Vue instance will automatically recalculate the value of fullName and update the view.

In addition to computed properties, Vue also supports observable properties. Observable properties allow you to listen for changes in a variable and perform some action when the change occurs. Suppose we define a data attribute count and an observed attribute squareCount:

const app = new Vue({
  el: '#app',
  data: {
    count: 2,
    squareCount: null
  },
  watch: {
    count() {
      this.squareCount = this.count * this.count;
    }
  }
});

In the above code, we define an observed attribute squareCount, which listens for changes in the count attribute and recalculates the square of the count value. If we want to modify the value of the count property, just execute the following code:

app.count = 3;

The Vue instance will automatically call the observation property to calculate the new value of squareCount. Finally, we can also use methods in the Vue instance to modify data properties. Suppose we define a method that modifies firstname:

const app = new Vue({
  el: '#app',
  data: {
    firstName: 'Tom',
    lastName: 'Smith'
  },
  methods: {
    changeFirstName() {
      this.firstName = 'Mike';
    }
  }
});

In the above code, we define a changeFirstName method that changes firstName to "Mike". If we want to change the firstName property, just call the changeFirstName method at the location that needs to be changed.

In short, the variables in the Vue instance can be modified after mounting. In addition to the above methods, Vue also provides many other methods to deal with data properties and state. We recommend that developers try these methods when building applications to gain more control over their application's state and behavior.

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