The principle of Vue is to pass data down through props and emit events up. It may seem straightforward, but it's crucial to remember when creating custom components.

Starting from Vue 2.2.0, you can utilize v-model with computed properties. This combination results in a simplified, neat, and consistent communication channel between components:

• Any props passed to your component remain reactive without the need for cloning or watch functions to track changes.
• Changes are automatically relayed back to the parent component.
• This setup works seamlessly across multiple levels of components.

A computed property allows you to define separate getter and setter methods. This enables you to redesign the 'Task' component as shown below:

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 specifies which prop corresponds to v-model and which event will be triggered upon changes. You can then use this component from the parent like so:

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

The 'listLocal' computed property offers a simple interface for getting and setting data within the component (similar to a private variable). In the '#task-template', you can display 'listLocal' and it will reactively update based on changes in 'parentList'. You can also modify 'listLocal' by calling the setter (e.g., this.listLocal = newList) which triggers an event emission to the parent component.

This pattern allows you to pass 'listLocal' to a child component of 'Task' using v-model, ensuring that changes made at lower levels cascade up to the top level component.

For instance, suppose we have a separate 'EditTask' component for editing task data. By employing the same v-model and computed properties methodology, we can send 'listLocal' to this component using v-model:

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

If 'EditTask' triggers a change, it will directly call set() on 'listLocal', thereby propagating the event to the top-level component. Similarly, 'EditTask' could further communicate with other child components (e.g., form elements) using v-model.

Vue provides a warning when you try to modify a prop within a component because any changes will be lost when the parent component re-renders. This can result in all your hard work being overwritten, making it risky behavior.

To avoid this issue, a better approach is to utilize a computed property:

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

When working with Lodash, it's a good practice to clone the prop before returning it. This approach comes in handy when you need to make changes to that prop both in the parent and child components.

Consider a scenario where we have a prop called data in the component named table.

Parent Component Code

<table :data.sync="data"></table>

Child Component Code

props: ['data'],
methods:{
    handleClickAction(item){
        let updatedData = _.clone(this.data)
        updatedData = _.uniq(updatedData) // Remove duplicates
        this.$emit('update:data', updatedData)
    }
}

In Vue's coding philosophy, props are passed down while events are sent up. This paradigm emphasizes that attempting to modify props received from a parent component will not have any effect as they will simply be overwritten by the parent repeatedly. The child component is limited to emitting events to trigger actions in the parent component. For those who find these limitations restrictive, VUEX can be utilized (although for more intricate component structures, utilizing VUEX is recommended)!

It is not recommended to alter the props' value within the child component. If necessary, you can utilize the .sync feature. Here's an example:

<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'
})

When working with VueJs 2.0, it's important to understand that props should not be mutated within the component itself. Props are meant to be managed and updated by their parent components. To work around this limitation, you can define variables in your data section with unique names and then watch for changes in the actual props. If the list prop is modified by a parent component, you can parse it and assign it to a mutableList variable. Below is a complete solution to handle this scenario:

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);
        }
    }
});

By using mutableList, you can safely render your template without directly manipulating the original list prop within the component.

Finding a solution to the problem is quite straightforward - you need to prevent direct prop mutation by storing the value in local component variables, such as data properties, computed with getters, setters, or watchers.

One effective approach is to utilize a watcher for this purpose.

<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>

This has proven to be an effective method for creating data input components, as it enables smooth communication between parent and child components. The data passed from the parent element through v-model will be monitored by the value watcher, which will then assign it to the input variable. Upon receiving the input data, we can trigger the emit input event to notify the parent component that new data has been submitted via the form element.

It is not recommended to directly alter props within components. If a change is necessary, create a new property in the following manner:

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

Then modify the value of newList instead.

I encountered a similar problem and found that the warning disappeared when I utilized the $on and $emit functions. It seems that using $on and $emit is advised for transmitting data from a child component to a parent component.

Vue3 offers a fantastic solution that took me hours to figure out, but it worked flawlessly.

In the parent template:

<user-name
  v-model:first-name="firstName"
  v-model:last-name="lastName"
></user-name>

The child component:

app.component('user-name', {
      props: {
      firstName: String,
      lastName: String
     },
template: `
   <input 
  type="text"
  :value="firstName"
  @input="$emit('update:firstName', 
 $event.target.value)">

<input
  type="text"
  :value="lastName"
   @input="$emit('update:lastName', 
    $event.target.value)">
  `
})

This was the only method I found for achieving two-way binding. While the first two answers suggested using SYNC and emitting update events, as well as compute property getters and setters, I found those methods too complicated and laborious.

When it comes to data flow in Vue.js, the one-way data flow concept is essential. As stated on https://v2.vuejs.org/v2/guide/components.html, components in Vue follow a one-way data flow mechanism.

This means that there is a unidirectional binding between child and parent properties - changes in the parent component will be passed down to the child component, but not vice versa. This prevents accidental mutation of parent properties by child components, ensuring a clear and understandable data flow within your app.

Furthermore, whenever the parent component updates, all props within child components are automatically refreshed with the latest values. It's important to note that attempting to mutate a prop inside a child component will result in a warning from Vue in the console.

There are instances where mutating a prop may seem necessary, such as when using it as an initial value or needing to transform its raw value. In such cases, it is recommended to:

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

Or define a computed property based on the prop's value:

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

Here is a helpful tip to streamline your code and reduce the use of watchers and computed properties:

Props are great for one-way communication.

If you have a modal that opens and closes with a prop, consider emitting an event instead:

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

Then, add a listener to the modal element:

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

(In this scenario, the prop show may not be necessary as you can simply use v-if="show" directly on the base-modal)

Make sure to include a computed method similar to the following:

app.vue

props: ['data'],
computed: {
    jsonData: function(){
        return JSON.parse(this.data);
    }
}

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

Perhaps this solution will fulfill your requirements.

To modify props, utilize an object.

<customComponent :data="app.data"></customComponent>

customComponent:

props: ['data'],
methods: {
  updateData: function() {
    this.data.value = "updated data";
  }
}

It is not recommended to mutate Vue.js props as it is considered an Anti-Pattern in Vue.

To work with the prop without mutating it, you can create a data property on your component that references the original prop property of list

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

Now you can reference and work with the list structure passed to the component through the data property of your component :-)

If you need to perform more complex operations on your list property, you can utilize the Vue component's computed property. This allows you to make 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 items, logs it out for laughs, and returns it.

Note: both data and computed properties are used in the template the same way e.g

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

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

Remember, a computed property (being a method) does not need to be called explicitly.

When TypeScript is the chosen language for development

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

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

// computed properties start with get followed by a function
get feesInLocale() {
    return this.fees;
}

instead of

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

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

Displayed below is a snack bar component. If the snackbar variable is directly given into v-model as shown, it will function properly; however, an error will be displayed in the console stating:

Avoid manipulating a prop directly since its value will be overridden whenever the parent component re-renders. Instead, utilize 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>

The correct method to avoid this mutation error is by using a 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>

Therefore, in the main component where this snack bar is loaded, you can simply add this code:

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

This solution successfully worked for me

Create a new variable and assign the properties to it.

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

Expanding on the top answer,

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

When setting props using an array, it is mainly for development purposes or prototyping. In a production environment, ensure to define prop types and set default values in case the prop is not provided by the parent component. Here's how you can do that:

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

By following this approach, you will receive an empty object in mutableList instead of encountering a JSON.parse error if the prop is undefined.

Absolutely, altering attributes in vue2 goes against the norm. HOWEVER... Challenge traditional conventions by embracing alternative approaches and moving ahead! The key is to incorporate the .sync modifier to your component attribute within the parent scope.

<your-awesome-components :custom-attribute-as-prob.sync="value" />

In Vue.js, it is frowned upon to directly manipulate props. For instance, setting a prop like this:

this.propsVal = 'new Value for Props'

To address this issue, you should instead pass the prop value into the data or computed property of a Vue component, as shown below:

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

By following this approach, you will be able to resolve the problem effectively.

Additionally, for those encountering the following issue:

"If the props value is not mandatory and therefore not always present, the passed data might return undefined (instead of being empty)". This can affect the default value of a <select> element. I resolved this by verifying if the value is defined in the beforeMount() hook (and initializing it if necessary) as shown below:

JavaScript:

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>

My recommendation is to always pass props to a computed property before mutating them. This way, you can easily track any prop mutations and determine if they are being mutated by another component as well. You can also use the watch functionality for this purpose.

In Vue, props facilitate one-way data flow to prevent inadvertent modifications to the parent's state by child components.

According to Vue's official documentation, there are two approaches to resolving this issue:

1. If a child component needs to utilize props as local data, it is recommended to define a local data property.

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

2. When the prop is received as a raw value requiring transformation, defining a computed property with the prop's value is advised:

     props: ['list'],
     computed: {
       localList: function() {
          return JSON.parse(this.list);
       },
       // For filtering purposes
       validList: function() {
          return this.list.filter(product => product.isValid === true)
       }
       // Any other transformations needed for the list
     }
   
   
   

It's important to refrain from mutating props in Vue, as well as other frameworks. A recommended approach is to create a copy of the prop in a separate variable.

For instance,

// Instead of directly altering the value of 'this.data', try using a new variable

this.new_variable = JSON.parse(this.data)