vuex/docs/en/getters.md

Getters

Sometimes we may need to compute derived state based on store state, for example filtering through a list of items and counting them:

computed: {
  doneTodosCount () {
    return this.$store.state.todos.filter(todo => todo.done).length
  }
}

If more than one component needs to make use of this, we have to either duplicate the function, or extract it into a shared helper and import it in multiple places - both are less than ideal.

Vuex allows us to define "getters" in the store (think of them as computed properties for stores):

const store = new Vuex.Store({
  state: {
    todos: [
      { id: 1, text: '...', done: true },
      { id: 2, text: '...', done: false }
    ]
  },
  getters: {
    doneTodosCount: state => {
      return state.todos.filter(todo => todo.done).length
    }
  }
})

The getters will be exposed on the store.getters object:

store.getters.doneTodosCount // -> 1

We can now easily make use of it inside any component:

computed: {
  doneTodosCount () {
    return this.$store.getters.doneTodosCount
  }
}

The mapGetters Helper

The mapGetters helper simply maps store getters to local computed properties:

import { mapState } from 'vuex'

export default {
  // ...
  computed: {
    // mix the getters into computed with object spread operator
    ...mapGetters([
      'doneTodosCount',
      'anotherGetter',
      // ...
    ])
  }
}

If you want to map a getter to a different name, use an object:

mapGetters({
  // map this.doneCount to store.getters.doneTodosCount
  doneCount: 'doneTodosCount'
})