Vue.js modify computed property?

Question

I am trying to modify a moment.js instance that resides in vue.js computed property like so.

computed: {
    currentDate() {
        return moment();
    }
}

Whenever I try to call it with a method like this one, nothing happens:

methods: {
    prevMonth() {
        this.currentDate = moment(this.currentDate).subtract(1, 'months');
    }
}

I am guessing this is because computed properties only allow to act as getters(and optionally setters). How can I change this behavior though?

I oversimplified the example since I am using the cmoputed property to fetch data from my vuex store. I am no longer able to manipulate it though.

Is there some way I can populate a local currentDate property with the vuex store's value so I can still manipulate it and add months, etc?

I have though about using the mounted property for this but I only mount my component once. Any help is welcome.

Answer 1

If your currentDate property belongs to your Vuex store, you shouldn't be manipulating it inside your components. You should instead: 1) map the getter locally as a computed property and 2) map the mutation locally as a method.

Here's an example of what your date Vuex module might look like:

export default {
    state: {
        currentDate: moment()
    },
    mutations: {
        subtractMonth (state, date) {
            state.currentDate = moment(state.currentDate).subtract(1, 'months');
        }
    },
    getters: {
        getCurrentDate: (state) => {
            return state.currentDate
        }
    }
}

And this is how the component would make use of it, without actually doing anything "locally":

import { mapGetters, mapMutations } from 'vuex'
export default {
    computed: {
        ...mapGetters({
            currentDate: 'getCurrentDate'
        })
    },
    methods: {
        ...mapMutations({
            prevMonth: 'subtractMonth'
        })
    }
}

You'd still be able to bind currentDate inside your component and call prevMonth as before, but now everything is being done via the Vuex single state.

Answer 2

Computed properties are not methods, that you can call. If you want to have such method, move currentDate to methods. The you also can invoke it from mounted.