Vue.js – the Vuex “context” object

vue.jsvuejs2vuex

I am trying to get better understanding of what the "context" object is in Vuex.

The context object is referred to numerous times in the Vuex documentation. For example, in https://vuex.vuejs.org/en/actions.html, we have:

Action handlers receive a context object which exposes the same set of
methods/properties on the store instance, so you can call
context.commit to commit a mutation…

I understand how to use it, and also that we can use destructuring if we only want to use the "commit" from the context object, but was hoping for a little more depth, just so I can better understand what is going on.

As a start, I found a couple ~8.5 year old posts on the "context object" as a pattern:
what is context object design pattern? and
Can you explain the Context design pattern?

However, specifically to Vuex, I'd love a better understanding of:

What is the context object / what is its purpose?
What are all the properties/methods that it is making available to use in Vuex?

Thank you!

Best Answer

From the documentation you pointed out you can read:

We will see why this context object is not the store instance itself when we introduce Modules later.

The main idea of the context object is to abstract the scope of the current Module. If you simply access store.state, it will always be the root state.

The context object of actions and its properties/methods are described here in the source code and also referenced in the API documentation

Here is the list:

{
  state,      // same as store.state, or local state if in modules
  rootState,  // same as store.state, only in modules
  commit,     // same as store.commit
  dispatch,   // same as store.dispatch
  getters,    // same as store.getters, or local getters if in modules
  rootGetters // same as store.getters, only in modules
}

Related Solutions

Vue.js – Accessing getters within Vuex mutations

Vuex store mutation methods do not provide direct access to getters.

This is probably bad practice*, but you could pass a reference to getters when committing a mutation like so:

actions: {
  fooAction({commit, getters}, data) {
    commit('FOO_MUTATION', {data, getters})
  }
},
mutations: {
  FOO_MUTATION(state, {data, getters}) {
    console.log(getters);
  }
}

* It is best practice to keep mutations a simple as possible, only directly affecting the Vuex state. This makes it easier to reason about state changes without having to think about any side effects. Now, if you follow best practices for vuex getters, they should not have any side effects in the first place. But, any logic that needs to reference getters can run in an action, which has read access to getters and state. Then the output of that logic can be passed to the mutation. So, you can pass the reference to the getters object to the mutation, but there's no need to and it muddies the waters.

Javascript – How to watch store values from vuex

Let's say, for example, that you have a basket of fruits, and each time you add or remove a fruit from the basket, you want to (1) display info about fruit count, but you also (2) want to be notified of the count of the fruits in some fancy fashion...

fruit-count-component.vue

<template>
  <!-- We meet our first objective (1) by simply -->
  <!-- binding to the count property. -->
  <p>Fruits: {{ count }}</p>
</template>

<script>
import basket from '../resources/fruit-basket'

export default () {
  computed: {
    count () {
      return basket.state.fruits.length
      // Or return basket.getters.fruitsCount
      // (depends on your design decisions).
    }
  },
  watch: {
    count (newCount, oldCount) {
      // Our fancy notification (2).
      console.log(`We have ${newCount} fruits now, yay!`)
    }
  }
}
</script>

Please note, that the name of the function in the watch object, must match the name of the function in the computed object. In the example above the name is count.

New and old values of a watched property will be passed into watch callback (the count function) as parameters.

The basket store could look like this:

fruit-basket.js

import Vue from 'vue'
import Vuex from 'vuex'

Vue.use(Vuex)

const basket = new Vuex.Store({
  state: {
    fruits: []
  },
  getters: {
    fruitsCount (state) {
      return state.fruits.length
    }
  }
  // Obviously you would need some mutations and actions,
  // but to make example cleaner I'll skip this part.
})

export default basket

You can read more in the following resources:

Best Answer

Related Solutions

Vue.js – Accessing getters within Vuex mutations

Javascript – How to watch store values from vuex

Related Topic