Can I pass parameters in computed properties in Vue.Js

is this possible to pass parameter in computed properties in Vue.Js. I can see when having getters/setter using computed, they can take a parameter and assign it to a variable. like here from documentation:

// ...
computed: {
  fullName: {
    // getter
    get: function () {
      return this.firstName + ' ' + this.lastName
    },
    // setter
    set: function (newValue) {
      var names = newValue.split(' ')
      this.firstName = names[0]
      this.lastName = names[names.length - 1]
    }
  }
}
// ...

Is this also possible:

// ...
computed: {
  fullName: function (salut) {
      return salut + ' ' + this.firstName + ' ' + this.lastName    
  }
}
// ...

Where computed property takes a argument and returns desired output. However when I try this, I am getting this error:

vue.common.js:2250 Uncaught TypeError: fullName is not a function(…)

Should I be using methods for such cases?

Solution 1:

Most probably you want to use a method

<span>{{ fullName('Hi') }}</span>

methods: {
  fullName(salut) {
      return `${salut} ${this.firstName} ${this.lastName}`
  }
}

Longer explanation

Technically you can use a computed property with a parameter like this:

computed: {
   fullName() {
      return salut => `${salut} ${this.firstName} ${this.lastName}`
   }
}

(Thanks Unirgy for the base code for this.)

The difference between a computed property and a method is that computed properties are cached and change only when their dependencies change. A method will evaluate every time it's called.

If you need parameters, there are usually no benefits of using a computed property function over a method in such a case. Though it allows you to have a parametrized getter function bound to the Vue instance, you lose caching so not really any gain there, in fact, you may break reactivity (AFAIU). You can read more about this in Vue documentation https://vuejs.org/v2/guide/computed.html#Computed-Caching-vs-Methods

The only useful situation is when you have to use a getter and need to have it parametrized. For instance, this situation happens in Vuex. In Vuex it's the only way to synchronously get parametrized result from the store (actions are async). Thus this approach is listed by official Vuex documentation for its getters https://vuex.vuejs.org/guide/getters.html#method-style-access

Solution 2:

You can use methods, but I prefer still to use computed properties instead of methods, if they're not mutating data or do not have external effects.

You can pass arguments to computed properties this way (not documented, but suggested by maintainers, don't remember where):

computed: {
   fullName: function () {
      var vm = this;
      return function (salut) {
          return salut + ' ' + vm.firstName + ' ' + vm.lastName;  
      };
   }
}

EDIT: Please do not use this solution, it only complicates code without any benefits.

Solution 3:

Well, technically speaking we can pass a parameter to a computed function, the same way we can pass a parameter to a getter function in vuex. Such a function is a function that returns a function.

For instance, in the getters of a store:

{
  itemById: function(state) {
    return (id) => state.itemPool[id];
  }
}

This getter can be mapped to the computed functions of a component:

computed: {
  ...mapGetters([
    'ids',
    'itemById'
  ])
}

And we can use this computed function in our template as follows:

<div v-for="id in ids" :key="id">{{itemById(id).description}}</div>

We can apply the same approach to create a computed method that takes a parameter.

computed: {
  ...mapGetters([
    'ids',
    'itemById'
  ]),
  descriptionById: function() {
    return (id) => this.itemById(id).description;
  }
}

And use it in our template:

<div v-for="id in ids" :key="id">{{descriptionById(id)}}</div>

This being said, I'm not saying here that it's the right way of doing things with Vue.

However, I could observe that when the item with the specified ID is mutated in the store, the view does refresh its contents automatically with the new properties of this item (the binding seems to be working just fine).

Solution 4:

[Vue2] Filters are a functionality provided by Vue components that let you apply formatting and transformations to any part of your template dynamic data.

They don’t change a component’s data or anything, but they only affect the output.

Say you are printing a name:

new Vue({
  el: '#container',
  data() {
    return {
      name: 'Maria',
      lastname: 'Silva'
    }
  },
  filters: {
    prepend: (name, lastname, prefix) => {
      return `${prefix} ${name} ${lastname}`
    }
  }
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
<div id="container">
  <p>{{ name, lastname | prepend('Hello') }}!</p>
</div>

Notice the syntax to apply a filter, which is | filterName. If you're familiar with Unix, that's the Unix pipe operator, which is used to pass the output of an operation as an input to the next one.

The filters property of the component is an object. A single filter is a function that accepts a value and returns another value.

The returned value is the one that’s actually printed in the Vue.js template.