Component V Model

Basic Usage

v-model can be used on a component to implement a two-way binding.

Starting in Vue 3.4, the recommended approach to achieve this is using the defineModel() macro:

<script setup>
const model = defineModel()
function update() {
  model.value++
}
</script>

<template>
  <div>Parent bound v-model is: {{ model }}</div>
  <button @click="update">Increment</button>
</template>

The parent can then bind a value with v-model:

<Child v-model="countModel" />

The value returned by defineModel() is a ref. It can be accessed and mutated like any other ref.

Its .value is synced with the value bound by the parent v-model;

When it is mutated by the child, it causes the parent bound value to be updated as well.

This means you can also bind this ref to a native input element with v-model, making it straightforward to wrap native input elements while providing the same v-model usage:

<script setup>
const model = defineModel()
</script>

<template>
  <input v-model="model" />
</template>

Under the Hood

defineModel is a convenience macro. The compiler expands it to the following:

A prop named modelValue, which the local ref’s value is synced with;
An event named update:modelValue, which is emitted when the local ref’s value is mutated.

This is how you would implement the same child component shown above prior to 3.4:

<script setup>
const props = defineProps(['modelValue'])
const emit = defineEmits(['update:modelValue'])
</script>

<template>
  <input
    :value="props.modelValue"
    @input="emit('update:modelValue', $event.target.value)"
  />
</template>

Then, v-model="foo" in the parent component will be compiled to:

<Child
  :modelValue="foo"
  @update:modelValue="$event => (foo = $event)"
/>

As you can see, it is quite a bit more verbose. However, it is helpful to understand what is happening under the hood.

Because defineModel declares a prop, you can therefore declare the underlying prop’s options by passing it to defineModel:

// making the v-model required
const model = defineModel({ required: true })

// providing a default value
const model = defineModel({ default: 0 })

Warning

If you have a default value for defineModel prop and you don’t provide any value for this prop from the parent component, it can cause a de-synchronization between parent and child components. In the example below, the parent’s myRef is undefined, but the child’s model is 1:

Child component:

const model = defineModel({ default: 1 })

Parent component:

<Child v-model="myRef"></Child>

v-model Arguments

v-model on a component can also accept an argument:

<MyComponent v-model:title="bookTitle" />

In the child component, we can support the corresponding argument by passing a string to defineModel() as its first argument:

<script setup>
const title = defineModel('title')
</script>

<template>
  <input type="text" v-model="title" />
</template>

If prop options are also needed, they should be passed after the model name:

const title = defineModel('title', { required: true })

Multiple v-model Bindings

By leveraging the ability to target a particular prop and event as we learned before with v-model arguments, we can now create multiple v-model bindings on a single component instance.

Each v-model will sync to a different prop, without the need for extra options in the component:

<UserName
  v-model:first-name="first"
  v-model:last-name="last"
/>

<script setup>
const firstName = defineModel('firstName')
const lastName = defineModel('lastName')
</script>

<template>
  <input type="text" v-model="firstName" />
  <input type="text" v-model="lastName" />
</template>

Handling v-model Modifiers

When we were learning about form input bindings, we saw that v-model has built-in modifiers - .trim, .number and .lazy. In some cases, you might also want the v-model on your custom input component to support custom modifiers.

Let’s create a custom modifier, capitalize, that capitalizes the first letter of the string provided by the v-model binding:

<MyComponent v-model.capitalize="myText" />

Modifiers added to a component v-model can be accessed in the child component by destructuring the defineModel() return value like this:

<script setup>
const [model, modifiers] = defineModel()

console.log(modifiers) // { capitalize: true }
</script>

<template>
  <input type="text" v-model="model" />
</template>

To conditionally adjust how the value should be read / written based on modifiers, we can pass get and set options to defineModel(). These two options receive the value on get / set of the model ref and should return a transformed value. This is how we can use the set option to implement the capitalize modifier:

<script setup>
const [model, modifiers] = defineModel({
  set(value) {
    if (modifiers.capitalize) {
      return value.charAt(0).toUpperCase() + value.slice(1)
    }
    return value
  }
})
</script>

<template>
  <input type="text" v-model="model" />
</template>

Modifiers for v-model with Arguments

Here’s another example of using modifiers with multiple v-model with different arguments:

<UserName
  v-model:first-name.capitalize="first"
  v-model:last-name.uppercase="last"
/>

<script setup>
const [firstName, firstNameModifiers] = defineModel('firstName')
const [lastName, lastNameModifiers] = defineModel('lastName')

console.log(firstNameModifiers) // { capitalize: true }
console.log(lastNameModifiers) // { uppercase: true }
</script>