Vue JS CheckBoxGroup with a prop array as v-model

Question

I am stuck at making a CheckBoxGroup with a prop array as v-model. I have read the vuejs guide: https://v2.vuejs.org/v2/guide/forms.html#Checkbox which has the v-model array in the data of the same component, but it is obviously pretty useless if I want to make this component reusable and insert the v-model via props and for example check some of the boxes from "outside". So I tried following:

CheckBoxgroup.vue

<template>
  <div>
    <label v-for="day in allDays" :key="day">
      <input v-model="checkedDays" type="checkbox" :value="day" />
      <span>{{ day }}</span>
    </label>
    <div>Checked days: {{ checkedDays }}</div>
 </div>
</template>
<script lang="ts">
import Vue from 'vue'
import { Component, Prop } from 'vue-property-decorator'

@Component
export default class CheckBoxGroup extends Vue {
  @Prop() checkedDays!: string[]

  @Prop() allDays!: string[]
}
</script>

Index.vue

<template>
  <div>
    <checkbox-group :checked-days="checkedDays" :all-days="allDays" />
  </div>
</template>

<script lang="ts">
import { Vue, Component } from 'vue-property-decorator'
import CheckboxGroup from './checkBoxGroup.vue'

@Component({
  components: { CheckboxGroup },
})
export default class Index extends Vue {

  // This list would usually come from an API
  allDays = ['Monday', 'Tuesday', 'Wednesday']

  checkedDays = ['Monday']
}
</script>

So the code is working almost fine, but I am getting

[Vue warn]: Avoid mutating a prop directly since the value will be overwritten whenever the parent component re-renders...

Is there any way around it? Any help would be appriciated.

Answer 1

you can't mutate the parent state from the children directly, however you can emit the event from child to parent to mutate from there as below:

Vue.component('check-box-group', {
  template: `
    <div>
      <label v-for="day in allDays" :key="day">
        <input 
          v-model="checkedDays" 
          :value="day" 
          @click="$emit('update-checked-days', { newCheckedDay: day })"
          type="checkbox" 
        />
        <span>{{ day }}</span>
      </label>
      <div>Checked days: {{ checkedDays }}</div>
    </div>
  `,
  props: {
    checkedDays: {
      type: Array, default: () => ([])
    },
    allDays: {
      type: Array, default: () => ([])
    },
  }
})

new Vue({
  el: "#app",
  data() {
    return {
      allDays: ['Monday', 'Tuesday', 'Wednesday'],

      checkedDays: ['Monday']
    }
  },
  methods: {
    HandleUpdateCheckedDays({newCheckedDay}) {
      const indexOfCheckedDay = this.checkedDays.findIndex(checkedDay => checkedDay === newCheckedDay)

      if (indexOfCheckedDay === -1) { // if not exists then add to checkedDays
        this.checkedDays.push(newCheckedDay)
      } else {
        this.checkedDays = this.checkedDays.filter((_, i) => i !== indexOfCheckedDay)
      }
    }
  }
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.6.11/vue.min.js"></script>

<div id="app">
  <check-box-group 
    :checked-days="checkedDays" 
    :all-days="allDays" 
    @update-checked-days="HandleUpdateCheckedDays"
  />
</div>

note: remember that TS class composition is deprecated.