How to solve [Vue warn]: Avoid mutating a prop directly since the value will be overwritten on vue.js 2?

Question

My view is like this :

<div class="col-md-8">
    ...
        <star :value="{{ $data['rating'] }}"></star>
    ...
</div>

My star component is like this :

<template>
    <span class="rating" :class='{"disable-all-rating": !!value}'>
        <template v-for="item in items">
            <label class="radio-inline input-star" :class="{'is-selected': ((starValue>= item.value) && starValue!= null)}">
                <input type="radio" class="input-rating" v-model="starValue" @click="rate(item.value)">
            </label>
        </template>
    </span>
</template>
<script>
    export default{
        props: {
            'value': null
        },
        computed: {
            starValue () {
                return this.temp_value
            }
        },
        data(){
            return{
                items: [
                    {value: 5},
                    {value: 4},
                    {value: 3},
                    {value: 2},
                    {value: 1}
                ],
                temp_value: null,
            }
        },
        methods:{
            rate: function (star) {           
               this.$http.post(window.BaseUrl + '/star', {star: star});                         
               this.temp_value = star;                         
            },
        }
    }
</script>

My css is like this :

span.rating {
  direction: rtl;
  display: inline-block;
}

span.rating .input-star {
  background: url("../img/star.png") 0 -16px;
  padding-left: 0;
  margin-left: 0;
  width: 16px;
  height: 16px;
}

span.rating .input-star:hover, span.rating .input-star:hover ~ .input-star {
  background-position: 0 0;
}

span.rating .is-selected{
   background-position: 0 0;
}

span.rating .is-disabled{
   cursor: default;
}

span.rating .input-star .input-rating {
  display: none;
}

When I click the star, there exist error on the console like this :

[Vue warn]: Avoid mutating a prop directly since the value will be overwritten whenever the parent component re-renders. Instead, use a data or computed property based on the prop's value. Prop being mutated: "value" (found in at C:\xampp\htdocs\myshop\resources\assets\js\components\Star.vue)

How can I solve it?

Answer 1

You need to make the computed with a getter and a setter and then use $emit to update the prop e.g:

    computed: {
        starValue:{ 
            get:function () {
                return this.temp_value
            },
            set: function(newValue){
                // $emit is the correct way to update props:
                this.$emit('update:value', newValue);
            }
        }
    }

Answer 2

You're changing the value property here.

return this.value = star;

And possibly here.

v-model="value"

The warning means that whenever your view is re-rendered, the value property is going to be set to $data['rating'], overwriting whatever you did inside the start component.

Instead of mutating the property inside your component, when someone clicks a star, you probably want to $emit that the component has changed and let your view change $data['rating'], which will re-render the star component properly.

See the Vue documentation regarding component composition.

Answer 3

The warning suggests not modifying a prop's value directly as doing so you would lose the change anyway if the data at the parent component changes.

That said, here in your method:

methods: {
  rate: function (star) {
    var self = this;
    if (!this.disabled) {
        this.$http.post(window.BaseUrl + '/star', {star: star}).then(function (response) {
            console.log('submitted');
        });
        this.temp_value = star;
        // return this.value = star; - remove this line
    }
  }
}

and add a computed property like so:

computed: {
  starValue () {
    return this.temp_value
  }
}