Vue 2 - Mutating props vue-warn

I started https://laracasts.com/series/learning-vue-step-by-step series. I stopped on the lesson Vue, Laravel, and AJAX with this error:

vue.js:2574 [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: "list" (found in component )

I have this code in main.js

Vue.component('task', {
    template: '#task-template',
    props: ['list'],
    created() {
        this.list = JSON.parse(this.list);
    }
});
new Vue({
    el: '.container'
})

I know that the problem is in created() when I overwrite the list prop, but I am a newbie in Vue, so I totally don't know how to fix it. Anyone have an idea how (and please explain why) to fix it?

Answers:

Answer

Vue just warns you: you change the prop in the component, but when parent component re-renders, "list" will be overwritten and you lose all your changes. So it is dangerous to do so.

Use computed property instead like this:

Vue.component('task', {
    template: '#task-template',
    props: ['list'],
    computed: {
        listJson: function(){
            return JSON.parse(this.list);
        }
    }
});
Answer

The Vue pattern is props down and events up. It sounds simple, but is easy to forget when writing a custom component.

As of Vue 2.2.0 you can use v-model (with computed properties). I have found this combination creates a simple, clean, and consistent interface between components:

  • Any props passed to your component remains reactive (i.e., it's not cloned nor does it require a watch function to update a local copy when changes are detected).
  • Changes are automatically emitted to the parent.
  • Can be used with multiple levels of components.

A computed property permits the setter and getter to be separately defined. This allows the Task component to be rewritten as follows:

Vue.component('Task', {
    template: '#task-template',
    props: ['list'],
    model: {
        prop: 'list',
        event: 'listchange'
    },
    computed: {
        listLocal: {
            get: function() {
                return this.list
            },
            set: function(value) {
                this.$emit('listchange', value)
            }
        }
    }
})  

The model property defines which prop is associated with v-model, and which event will be emitted on changes. You can then call this component from the parent as follows:

<Task v-model="parentList"></Task>

The listLocal computed property provides a simple getter and setter interface within the component (think of it like being a private variable). Within #task-template you can render listLocal and it will remain reactive (i.e., if parentList changes it will update the Task component). You can also mutate listLocal by calling the setter (e.g., this.listLocal = newList) and it will emit the change to the parent.

What's great about this pattern is that you can pass listLocal to a child component of Task (using v-model), and changes from the child component will propagate to the top level component.

For example, say we have a separate EditTask component for doing some type of modification to the task data. By using the same v-model and computed properties pattern we can pass listLocal to the component (using v-model):

<script type="text/x-template" id="task-template">
    <div>
        <EditTask v-model="listLocal"></EditTask>
    </div>
</script>

If EditTask emits a change it will appropriately call set() on listLocal and thereby propagate the event to the top level. Similarly, the EditTask component could also call other child components (such as form elements) using v-model.

Answer

Props down, events up. That's Vue's Pattern. The point is that if you try to mutate props passing from a parent. It won't work and it just gets overwritten repeatedly by the parent component. Child component can only emit an event to notify parent component to do sth. If you don't like these restrict, you can use VUEX(actually this pattern will suck in complex components structure, you should use VUEX!)

Answer

You should not change the props's value in child component. If you really need to change it you can use .sync. Just like this

<your-component :list.sync="list"></your-component>

Vue.component('task', {
    template: '#task-template',
    props: ['list'],
    created() {
        this.$emit('update:list', JSON.parse(this.list))
    }
});
new Vue({
    el: '.container'
})
Answer

According to the VueJs 2.0, you should not mutate a prop inside the component. They are only mutated by their parents. Therefore, you should define variables in data with different names and keep them updated by watching actual props. In case the list prop is changed by a parent, you can parse it and assign it to mutableList. Here is a complete solution.

Vue.component('task', {
    template: ´<ul>
                  <li v-for="item in mutableList">
                      {{item.name}}
                  </li>
              </ul>´,
    props: ['list'],
    data: function () {
        return {
            mutableList = JSON.parse(this.list);
        }
    },
    watch:{
        list: function(){
            this.mutableList = JSON.parse(this.list);
        }
    }
});

It uses mutableList to render your template, thus you keep your list prop safe in the component.

Answer

do not change the props directly in components.if you need change it set a new property like this:

data () {
    return () {
        listClone: this.list
    }
}

And change the value of listClone.

Answer

The answer is simple, you should break the direct prop mutation by assigning the value to some local component variables(could be data property, computed with getters, setters, or watchers).

Here's a simple solution using the watcher.

<template>
  <input
    v-model="input"
    @input="updateInput" 
    @change="updateInput"
  />

</template>

<script>
  export default {
  props: {
    value: {
      type: String,
      default: '',
    },
  },
  data() {
    return {
      input: '',
    };
  },
  watch: {
    value: {
      handler(after) {
        this.input = after;
      },
      immediate: true,
    },
  },
  methods: {
    updateInput() {
      this.$emit('input', this.input);
    },
  },
};
</script>

It's what I use to create any data input components and it works just fine. Any new data sent(v-model(ed)) from parent will be watched by the value watcher and is assigned to the input variable and once the input is received, we can catch that action and emit input to parent suggesting that data is input from the form element.

Answer

If you want to mutate props - use object.

<component :model="global.price"></component>

component:

props: ['model'],
methods: {
  changeValue: function() {
    this.model.value = "new value";
  }
}
Answer

You need to add computed method like this

component.vue

props: ['list'],
computed: {
    listJson: function(){
        return JSON.parse(this.list);
    }
}
Answer

one-way Data Flow, according to https://vuejs.org/v2/guide/components.html, the component follow one-Way Data Flow, All props form a one-way-down binding between the child property and the parent one, when the parent property updates, it will flow down to the child but not the other way around, this prevents child components from accidentally mutating the parent's, which can make your app's data flow harder to understand.

In addition, every time the parent component is updates all props in the child components will be refreshed with the latest value. This means you should not attempt to mutate a prop inside a child component. If you do .vue will warn you in the console.

There are usually two cases where it’s tempting to mutate a prop: The prop is used to pass in an initial value; the child component wants to use it as a local data property afterwards. The prop is passed in as a raw value that needs to be transformed. The proper answer to these use cases are: Define a local data property that uses the prop’s initial value as its initial value:

props: ['initialCounter'],
data: function () {
  return { counter: this.initialCounter }
}

Define a computed property that is computed from the prop’s value:

props: ['size'],
computed: {
  normalizedSize: function () {
    return this.size.trim().toLowerCase()
  }
}
Answer

Vue.js props are not to be mutated as this is considered an Anti-Pattern in Vue.

The approach you will need to take is creating a data property on your component that references the original prop property of list

props: ['list'],
data: () {
  return {
    parsedList: JSON.parse(this.list)
  }
}

Now your list structure that is passed to the component is referenced and mutated via the data property of your component :-)

If you wish to do more than just parse your list property then make use of the Vue component' computed property. This allow you to make more in depth mutations to your props.

props: ['list'],
computed: {
  filteredJSONList: () => {
    let parsedList = JSON.parse(this.list)
    let filteredList = parsedList.filter(listItem => listItem.active)
    console.log(filteredList)
    return filteredList
  }
}

The example above parses your list prop and filters it down to only active list-tems, logs it out for schnitts and giggles and returns it.

note: both data & computed properties are referenced in the template the same e.g

<pre>{{parsedList}}</pre>

<pre>{{filteredJSONList}}</pre>

It can be easy to think that a computed property (being a method) needs to be called... it doesn't

Answer

Adding to the best answer,

Vue.component('task', {
    template: '#task-template',
    props: ['list'],
    data: function () {
        return {
            mutableList: JSON.parse(this.list);
        }
    }
});

Setting props by an array is meant for dev/prototyping, in production make sure to set prop types(https://vuejs.org/v2/guide/components-props.html) and set a default value in case the prop has not been populated by the parent, as so.

Vue.component('task', {
    template: '#task-template',
    props: {
      list: {
        type: String,
        default() {
          return '{}'
        }
      }
    },
    data: function () {
        return {
            mutableList: JSON.parse(this.list);
        }
    }
});

This way you atleast get an empty object in mutableList instead of a JSON.parse error if it is undefined.

Answer

Vue.js considers this an anti-pattern. For example, declaring and setting some props like

this.propsVal = 'new Props Value'

So to solve this issue you have to take in a value from the props to the data or the computed property of a Vue instance, like this:

props: ['propsVal'],
data: function() {
   return {
       propVal: this.propsVal
   };
},
methods: {
...
}

This will definitely work.

Answer

I want to give this answer which helps avoid using a lot of code, watchers and computed properties. In some cases this can be a good solution:

Props are designed to provide one-way communication.

When you have a modal show/hide button with a prop the best solution to me is to emit an event:

<button @click="$emit('close')">Close Modal</button>

Then add listener to modal element:

<modal :show="show" @close="show = false"></modal>

(In this case the prop show is probably unnecessary because you can use an easy v-if="show" directly on the base-modal)

Answer

I personally always suggest if you are in need to mutate the props, first pass them to computed property and return from there, thereafter one can mutate the props easily, even at that you can track the prop mutation , if those are being mutated from another component too or we can you watch also .

Answer

Because Vue props is one way data flow, This prevents child components from accidentally mutating the parent’s state.

From the official Vue document, we will find 2 ways to solve this problems

  1. if child component want use props as local data, it is best to define a local data property.

      props: ['list'],
      data: function() {
        return {
          localList: JSON.parse(this.list);
        }
      }
    
    
  2. The prop is passed in as a raw value that needs to be transformed. In this case, it’s best to define a computed property using the prop’s value:

      props: ['list'],
      computed: {
        localList: function() {
           return JSON.parse(this.list);
        },
        //eg: if you want to filter this list
        validList: function() {
           return this.list.filter(product => product.isValid === true)
        }
        //...whatever to transform the list
      }
    
    
    
Answer

YES!, mutating attributes in vue2 is anti-pattern. BUT... Just break the rules by using other rules, and go forward! What you need is adding .sync modifier to your component attribute in paret scope. <your-awesome-components :custom-attribute-as-prob.sync="value" />

???? It's simple we kill the batman ????

Answer

This has to do with the fact that mutating a prop locally is considered an anti-pattern in Vue 2

What you should do now, in case you want to mutate a prop locally, is to declare a field in your data that uses the props value as its initial value and then mutate the copy:

Vue.component('task', {
    template: '#task-template',
    props: ['list'],
    data: function () {
        return {
            mutableList: JSON.parse(this.list);
        }
    }
});

You can read more about this on Vue.js official guide


Note 1: Please note that you should not use the same name for your prop and data, i.e.:

data: function () { return { list: JSON.parse(this.list) } // WRONG!!

Note 2: Since I feel there is some confusion regarding props and reactivity, I suggest you to have a look on this thread

Answer

If you're using Lodash, you can clone the prop before returning it. This pattern is helpful if you modify that prop on both the parent and child.

Let's say we have prop list on component grid.

In Parent Component

<grid :list.sync="list"></grid>

In Child Component

props: ['list'],
methods:{
    doSomethingOnClick(entry){
        let modifiedList = _.clone(this.list)
        modifiedList = _.uniq(modifiedList) // Removes duplicates
        this.$emit('update:list', modifiedList)
    }
}
Answer

I faced this issue as well. The warning gone after i use $on and $emit. It's something like use $on and $emit recommended to sent data from child component to parent component.

Answer
Vue.component('task', {
    template: '#task-template',
    props: ['list'],
    computed: {
      middleData() {
        return this.list
      }
    },
    watch: {
      list(newVal, oldVal) {
        console.log(newVal)
        this.newList = newVal
      }
    },
    data() {
      return {
        newList: {}
      }
    }
});
new Vue({
    el: '.container'
})

Maybe this will meet your needs.

Answer

In addition to the above, for others having the following issue:

"If the props value is not required and thus not always returned, the passed data would return undefined (instead of empty)". Which could mess <select> default value, I solved it by checking if the value is set in beforeMount() (and set it if not) as follows:

JS:

export default {
        name: 'user_register',
        data: () => ({
            oldDobMonthMutated: this.oldDobMonth,
        }),
        props: [
            'oldDobMonth',
            'dobMonths', //Used for the select loop
        ],
        beforeMount() {
           if (!this.oldDobMonth) {
              this.oldDobMonthMutated = '';
           } else {
              this.oldDobMonthMutated = this.oldDobMonth
           }
        }
}

Html:

<select v-model="oldDobMonthMutated" id="dob_months" name="dob_month">

 <option selected="selected" disabled="disabled" hidden="hidden" value="">
 Select Month
 </option>

 <option v-for="dobMonth in dobMonths"
  :key="dobMonth.dob_month_slug"
  :value="dobMonth.dob_month_slug">
  {{ dobMonth.dob_month_name }}
 </option>

</select>
Answer

Below is a snack bar component, when I give the snackbar variable directly into v-model like this if will work but in the console, it will give an error as

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.

<template>
        <v-snackbar v-model="snackbar">
        {{ text }}
      </v-snackbar>
</template>

<script>
    export default {
        name: "loader",

        props: {
            snackbar: {type: Boolean, required: true},
            text: {type: String, required: false, default: ""},
        },

    }
</script>

Correct Way to get rid of this mutation error is use watcher

<template>
        <v-snackbar v-model="snackbarData">
        {{ text }}
      </v-snackbar>
</template>

<script>
/* eslint-disable */ 
    export default {
        name: "loader",
         data: () => ({
          snackbarData:false,
        }),
        props: {
            snackbar: {type: Boolean, required: true},
            text: {type: String, required: false, default: ""},
        },
        watch: { 
        snackbar: function(newVal, oldVal) { 
          this.snackbarData=!this.snackbarDatanewVal;
        }
      }
    }
</script>

So in the main component where you will load this snack bar you can just do this code

 <loader :snackbar="snackbarFlag" :text="snackText"></loader>

This Worked for me

Answer

For when TypeScript is your preferred lang. of development

<template>
<span class="someClassName">
      {{feesInLocale}}
</span>
</template>  



@Prop({default: 0}) fees: any;

// computed are declared with get before a function
get feesInLocale() {
    return this.fees;
}

and not

<template>
<span class="someClassName">
      {{feesInLocale}}
</span>
</template>  



@Prop() fees: any = 0;
get feesInLocale() {
    return this.fees;
}

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