readme.md

What is a Computed Property

In Vue.js you can output any data value using parentheses:

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

<script>
export default {
  data() {
    return {
      count: 1
    }
  }
}
</script>

This property can host some small computations, for example

<template>
  {{ count * 10 }}
</template>

but you're limited to a single JavaScript expression.

In addition to this technical limitation, you also need to consider that templates should only be concerned with displaying data to the user, not perform logic computations.

To do something more a single expression, and to have more declarative templates, that you use a computed property.

Computed properties are defined in the computed property of the Vue component:

<script>
export default {
  computed: {

  }
}
</script>

An example of a computed property

Here's an example code that uses a computed property count to calculate the output. Notice:

1. I didn't have to call {{ count() }}. Vue.js automatically invokes the function
2. I used a regular function (not an arrow function) to define the count computed property because I need to be able to access the component instance through this.

<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>

Computed properties vs methods

If you already know about Vue methods, you may wonder what's the difference.

First, methods must be called, not just referenced, so you'd need to do:

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

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

But the main difference is that computed properties are cached.

The result of the count computed property is internally cached until the items data property changes.

Important: computed properties are only updated when a reactive source updates. Regular JavaScript methods are not reactive, so a common example is to use Date.now():

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

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

It will render once, and then it will not be updated even when the component re-renders. Just on a page refresh, when the Vue component is quit and reinitialized.

In this case, a method is better suited for your needs.