To manage the state of an application you should use [Vuex][vuex-docs].
_Note:_ All of the below is explained in more detail in the official [Vuex documentation][vuex-docs].
## Separation of concerns
Vuex is composed of State, Getters, Mutations, Actions and Modules.
When a user clicks on an action, we need to `dispatch` it. This action will `commit` a mutation that will change the state.
_Note:_ The action itself will not update the state, only a mutation should update the state.
## File structure
When using Vuex at GitLab, separate this concerns into different files to improve readability:
```
└── store
├── index.js # where we assemble modules and export the store
├── actions.js # actions
├── mutations.js # mutations
├── getters.js # getters
├── state.js # state
└── mutation_types.js # mutation types
```
The following example shows an application that lists and adds users to the state.
(For a more complex example implementation take a look at the security applications store in [here](https://gitlab.com/gitlab-org/gitlab-ee/tree/master/ee/app/assets/javascripts/vue_shared/security_reports/store))
### `index.js`
This is the entry point for our store. You can use the following as a guide:
Sometimes we may need to get derived state based on store state, like filtering for a specific prop.
Using a getter will also cache the result based on dependencies due to [how computed props work](https://vuejs.org/v2/guide/computed.html#Computed-Caching-vs-Methods)
> It is a commonly seen pattern to use constants for mutation types in various Flux implementations. This allows the code to take advantage of tooling like linters, and putting all constants in a single file allows your collaborators to get an at-a-glance view of what mutations are possible in the entire application.
```javascript
export const ADD_USER = 'ADD_USER';
```
### How to include the store in your application
The store should be included in the main component of your application:
```javascript
// app.vue
import store from 'store'; // it will include the index.js file
export default {
name: 'application',
store,
...
};
```
### Communicating with the Store
```javascript
<script>
import { mapActions, mapState, mapGetters } from 'vuex';
1. Do not call a mutation directly. Always use an action to commit a mutation. Doing so will keep consistency throughout the application. From Vuex docs:
> why don't we just call store.commit('action') directly? Well, remember that mutations must be synchronous? Actions aren't. We can perform asynchronous operations inside an action.
```javascript
// component.vue
// bad
created() {
this.$store.commit('mutation');
}
// good
created() {
this.$store.dispatch('action');
}
```
1. Use mutation types instead of hardcoding strings. It will be less error prone.
1. The State will be accessible in all components descending from the use where the store is instantiated.
### Testing Vuex
#### Testing Vuex concerns
Refer to [vuex docs][vuex-testing] regarding testing Actions, Getters and Mutations.
#### Testing components that need a store
Smaller components might use `store` properties to access the data.
In order to write unit tests for those components, we need to include the store and provide the correct state:
Because we're currently using [`babel-plugin-rewire`](https://github.com/speedskater/babel-plugin-rewire), you may encounter the following error when testing your Vuex actions and getters:
`[vuex] actions should be function or object with "handler" function`
To prevent this error from happening, you need to export an empty function as `default`:
```
// getters.js or actions.js
// prevent babel-plugin-rewire from generating an invalid default during karma tests
