Getters

Sometimes we may need to compute derived state based on store state, for example filtering through a list of items and counting them. So in your component you can have computed property that returns the number of done todos -

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. You can think of Getters as computed properties for stores. Like computed properties, a getter's result is cached based on its dependencies, and will only re-evaluate when some of its dependencies have changed.

Getters will receive the state as their 1st argument:

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

Property-Style Access

The getters will be exposed on the store.getters object, and you access values as properties:

store.getters.doneTodos // -> [{ id: 1, text: '...', done: true }]

Getters will also receive other getters as the 2nd argument:

getters: {
  // ...
  doneTodosCount: (state, getters) => {
    return getters.doneTodos.length
  }
}

// Accessing
store.getters.doneTodosCount // -> 1

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

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

Note that getters accessed as properties are cached as part of Vue's reactivity system.

Method-Style Access

You can also pass arguments to getters by returning a function. This is particularly useful when you want to query an array in the store:

getters: {
  // ...
  getTodoById: (state) => (id) => {
    return state.todos.find(todo => todo.id === id)
  }
}

store.getters.getTodoById(2) // -> { id: 2, text: '...', done: false }

Note that getters accessed via methods will run each time you call them, and the result is not cached.

mapGetters

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

import { mapGetters } 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 `this.$store.getters.doneTodosCount`
  doneCount: 'doneTodosCount'
})