Vue.js Computed Properties: Cache Calculations for Improved Performance

In this blog post, we will explore how to use Vue Computed Properties to cache calculations. Computed properties allow us to perform complex logic computations within our Vue components, giving us more declarative and efficient templates.

What is a Computed Property?

In Vue.js, we can output data values using double curly braces like {{ value }} in our templates. While we can perform simple computations using expressions within these curly braces, we are limited to a single JavaScript expression. Furthermore, it is recommended to keep templates focused on displaying data rather than performing logic computations.

This is where computed properties come in. Computed properties allow us to perform more complex computations and store the result in a cache. This makes them suitable for scenarios where we need to calculate a value based on other data properties.

To define a computed property in a Vue component, we need to add a computed property to the component’s options object:

<script>
export default {
  computed: {
    // Define your computed properties here
  }
}
</script>

An example of a computed property

Let’s take a look at an example to understand how computed properties work. In this example, we have an array called items and we want to calculate the count of items multiplied by 10 using a computed property called count.

<template>
  <p>{{ count }}</p>
</template>

<script>
export default {
  data() {
    return {
      items: [1, 2, 3]
    };
  },
  computed: {
    count: function() {
      return 'The count is ' + this.items.length * 10;
    }
  }
}
</script>

In this code, notice that we didn’t have to call count() anywhere in the template. Vue.js automatically invokes the computed property for us. We also used a regular function (not an arrow function) to define the count computed property because we need to be able to access the component instance through this.

Computed Properties vs Methods

You may be wondering, what’s the difference between computed properties and methods in Vue.js?

First, methods must be called explicitly, whereas computed properties are automatically evaluated whenever their dependencies change. So if you have a method called count, you would need to call count() to get the result instead of just referencing it like a computed property.

<script>
export default {
  data() {
    return {
      items: [1, 2, 3]
    };
  },
  methods: {
    count: function() {
      return 'The count is ' + this.items.length * 10;
    }
  }
}
</script>

The main advantage of computed properties over methods is that computed properties are cached. The result of a computed property is internally cached until the dependent data property changes. This means that the computed property is only re-evaluated when necessary, improving performance.

On the other hand, methods are not cached. They are re-evaluated every time they are called. So if you have a method that performs a heavy computation, it will be recalculated every time you call it.

<template>
  <p>{{ now }}</p>
</template>

<script>
export default {
  computed: {
    now: function () {
      return Date.now();
    }
  }
}
</script>

In this example, the computed property now will be rendered once and then not be updated even when the component re-renders. To achieve the desired behavior in this case, a method would be more suitable.

In summary, computed properties offer a convenient way to cache calculations and improve performance in Vue.js. They are especially useful when you need to calculate a value based on other data properties. Methods, on the other hand, are better suited for situations where you need to perform a computation every time it is called.

Tags: Vue.js, Computed Properties, Caching, Performance

What is a Computed Property?#

An example of a computed property#

Computed Properties vs Methods#

What is a Computed Property?

An example of a computed property

Computed Properties vs Methods