Vue two way prop binding

Question

Below is my current structure (which doesn't work).

Parent component:

<template>
<field-input ref="title" :field.sync="title" />
</template>

<script>
import Field from './input/Field'
export default {
  components: {
    'field-input': Field
  },
  data() {
    return {
      title: {
        value: '',
        warn: false
      }
    }
  }
}
</script>

Child component:

<template>
<div>
  <input type="text" v-model="field.value">
  <p v-bind:class="{ 'is-invisible' : !field.warn }">Some text</p>
</div>
</template>

<script>
export default {
  props: ['field']
}
</script>

The requirements are:

• If parent's data title.warn value changes in parent, the child's class bind should be updated (field.warn).
• If the child's <input> is updated (field.value), then the parent's title.value should be updated.

What's the cleanest working solution to achieve this?

Answer 1

Don't bind the child component's <input> to the parent's title.value (like <input type="text" v-model="field.value">). This is a known bad practice, capable of making your app's data flow much harder to understand.

The requirements are:

• If parent's data title.warn value changes in parent, the child's class bind should be updated (field.warn).

This is simple, just create a warn prop and pass it from parent to child.

Parent (passing the prop to the child):

<field-input ref="title" :warn="title.warn" />

Child/template (using the prop -- reading, only):

<p v-bind:class="{ 'is-invisible' : !warn }">Some text</p>

Child/JavaScript (declaring the prop and its expected type):

export default {
  props: {warn: Boolean}
}

Notice that in the template it is !warn, not !title.warn. Also, you should declare warn as a Boolean prop because if you don't the parent may use a string (e.g. <field-input warn="false" />) which would yield unexpected results (!"false" is actually false, not true).

• If the child's <input> is updated (field.value), then the parent's title.value should be updated.

You have a couple of possible options here (like using .sync in a prop), but I'd argue the cleanest solution in this case is to create a value prop and use v-model on the parent.

Parent (binding the prop using v-model):

<field-input ref="title" v-model="title.value" />

Child/template (using the prop as initial value and emitting input events when it changes):

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

Child/JavaScript (declaring the prop and its expected type):

export default {
  props: {value: String}
}

Click here for a working DEMO of those two solutions together.

Answer 2

There are several ways of doing it, and some are mentioned in other answers:

1. Use props on components

2. Use v-model attribute

3. Use the sync modifier (for Vue 2.0)

4. Use v-model arguments (for Vue 3.0)

5. Use Pinia

Here are some details to the methods that are available:

1.) Use props on components

Props should ideally only be used to pass data down into a component and events should pass data back up. This is the way the system was intended. (Use either v-model or sync modifier as "shorthands")

Props and events are easy to use and are the ideal way to solve most common problems.

Using props for two-way binding is not usually advised but possible, by passing an object or array you can change a property of that object and it will be observed in both child and parent without Vue printing a warning in the console.

Because of how Vue observes changes all properties need to be available on an object or they will not be reactive. If any properties are added after Vue has finished making them observable 'set' will have to be used.

 //Normal usage
 Vue.set(aVariable, 'aNewProp', 42);
 //This is how to use it in Nuxt
 this.$set(this.historyEntry, 'date', new Date());

The object will be reactive for both component and the parent:

I you pass an object/array as a prop, it's two-way syncing automatically - change data in the child, it is changed in the parent.

If you pass simple values (strings, numbers) via props, you have to explicitly use the .sync modifier

As quoted from --> https://stackoverflow.com/a/35723888/1087372

2.) Use v-model attribute

The v-model attribute is syntactic sugar that enables easy two-way binding between parent and child. It does the same thing as the sync modifier does only it uses a specific prop and a specific event for the binding

This:

 <input v-model="searchText">

is the same as this:

 <input
   v-bind:value="searchText"
   v-on:input="searchText = $event.target.value"
 >

Where the prop must be value and the event must be input

3.) Use the sync modifier (for Vue 2.0)

The sync modifier is also syntactic sugar and does the same as v-model, just that the prop and event names are set by whatever is being used.

In the parent it can be used as follows:

 <text-document v-bind:title.sync="doc.title"></text-document>

From the child an event can be emitted to notify the parent of any changes:

 this.$emit('update:title', newTitle)

4.) Use v-model arguments (for Vue 3.0)

In Vue 3.x the sync modifier was removed.

Instead you can use v-model arguments which solve the same problem

 <ChildComponent v-model:title="pageTitle" />

<!-- would be shorthand for: -->

<ChildComponent :title="pageTitle" @update:title="pageTitle = $event" />

5.) Use Pinia (or Vuex)

As of now Pinia is the official recommended state manager/data store

Pinia is a store library for Vue, it allows you to share a state across components/pages.

By using the Pinia store it is easier to see the flow of data mutations and they are explicitly defined. By using the vue developer tools it is easy to debug and rollback changes that were made.

This approach needs a bit more boilerplate, but if used throughout a project it becomes a much cleaner way to define how changes are made and from where.

Take a look at their getting started section

---

**In case of legacy projects** :

If your project already uses Vuex, you can keep on using it.

Vuex 3 and 4 will still be maintained. However, it's unlikely to add new functionalities to it. Vuex and Pinia can be installed in the same project. If you're migrating existing Vuex app to Pinia, it might be a suitable option. However, if you're planning to start a new project, we highly recommend using Pinia instead.