You can use computed properties to calculate values based on data properties:

<template>
  <Window title="Example" width="400" height="100" margined>
    <Box>
      <Text>Hello, {{ fullName }}</Text>
    </Box>
  </Window>
</template>
​
<script>
export default {
  data() {
    return {
      firstName: 'John',
      lastName: 'Smith'
    };
  },
  computed: {
    fullName() {
      return this.firstName + ' ' + this.lastName;
    }
  }
}
</script>

This creates a fullName property which will be automatically calculated based on the firstName and lastName properties. The example will display "Hello, John Smith" in the Text widget.

The above code can be also written like this:

<Text>Hello, {{ firstName + ' ' + lastName }}</Text>

However, when the expression becomes complex, or is repeated within the template, putting it into a computed property results in simpler and more readable code. Also, the computed property is defined using a regular function, so it can contain more complex code, not just a simple expression.

You can use a computed property just like a regular data property, for example:

methods: {
  printName() {
    console.log( this.fullName );
  }
}

Thanks to Vue.js reactivity, the value of the computed property will only be recalculated when necessary, in this case when the firstName or lastName is changed. Because of that, computed properties are more efficient than regular methods.

A computed property can also have a setter:

computed: {
  fullName: {
    get() {
      return this.firstName + ' ' + this.lastName;
    },
    set( newValue ) {
      const names = newValue.split( ' ' );
      this.firstName = names[ 0 ];
      this.lastName = names[ names.length - 1 ];
    }
  }
}

This way, the base properties are automatically modified when the you assign a value directly to the computed property.

In some cases it's necessary to plug directly into the Vue.js reactivity system, for example to initiate an asynchronous operation when a data property changes. You can declare a watcher to do that:

watch: {
  page( oldValue, newValue ) {
    // this function is called when the value of the page property is changed
  }
}

See also Computed Properties and Watchers in the Vue.js documentation for more details.