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:
vue
<!-- Child.vue -->
<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
:
template
<!-- Parent.vue -->
<Child v-model="countModel" />
The value returned by defineModel()
is a ref. It can be accessed and mutated like any other ref, except that it acts as a two-way binding between a parent value and a local one:
- Its
.value
is synced with the value bound by the parentv-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:
vue
<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:
vue
<!-- Child.vue -->
<script setup>
const props = defineProps(['modelValue'])
const emit = defineEmits(['update:modelValue'])
</script>
<template>
<input
:value="props.modelValue"
@input="emit('update:modelValue', $event.target.value)"
/>
</template>
Then, v-model="foo"
in the parent component will be compiled to:
template
<!-- Parent.vue -->
<Child
:modelValue="foo"
@update:modelValue="$event => (foo = $event)"
/>
As you can see, it is quite a bit more verbose. However, it is helpful to understand what is happening under the hood.
Because defineModel
declares a prop, you can therefore declare the underlying prop's options by passing it to defineModel
:
js
// making the v-model required
const model = defineModel({ required: true })
// providing a default value
const model = defineModel({ default: 0 })
WARNING
If you have a default
value for defineModel
prop and you don't provide any value for this prop from the parent component, it can cause a de-synchronization between parent and child components. In the example below, the parent's myRef
is undefined, but the child's model
is 1:
js
// child component:
const model = defineModel({ default: 1 })
// parent component:
const myRef = ref()
html
<Child v-model="myRef"></Child>
v-model
arguments
v-model
on a component can also accept an argument:
template
<MyComponent v-model:title="bookTitle" />
In the child component, we can support the corresponding argument by passing a string to defineModel()
as its first argument:
vue
<!-- MyComponent.vue -->
<script setup>
const title = defineModel('title')
</script>
<template>
<input type="text" v-model="title" />
</template>
If prop options are also needed, they should be passed after the model name:
js
const title = defineModel('title', { required: true })
Pre 3.4 Usage
vue
<!-- MyComponent.vue -->
<script setup>
defineProps({
title: {
required: true
}
})
defineEmits(['update:title'])
</script>
<template>
<input
type="text"
:value="title"
@input="$emit('update:title', $event.target.value)"
/>
</template>
Multiple v-model
bindings
By leveraging the ability to target a particular prop and event as we learned before with v-model
arguments, we can now create multiple v-model
bindings on a single component instance.
Each v-model
will sync to a different prop, without the need for extra options in the component:
template
<UserName
v-model:first-name="first"
v-model:last-name="last"
/>
vue
<script setup>
const firstName = defineModel('firstName')
const lastName = defineModel('lastName')
</script>
<template>
<input type="text" v-model="firstName" />
<input type="text" v-model="lastName" />
</template>
Pre 3.4 Usage
vue
<script setup>
defineProps({
firstName: String,
lastName: String
})
defineEmits(['update:firstName', 'update:lastName'])
</script>
<template>
<input
type="text"
:value="firstName"
@input="$emit('update:firstName', $event.target.value)"
/>
<input
type="text"
:value="lastName"
@input="$emit('update:lastName', $event.target.value)"
/>
</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 an example custom modifier, capitalize
, that capitalizes the first letter of the string provided by the v-model
binding:
template
<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:
vue
<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:
vue
<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>
Pre 3.4 Usage
vue
<script setup>
const props = defineProps({
modelValue: String,
modelModifiers: { default: () => ({}) }
})
const emit = defineEmits(['update:modelValue'])
function emitValue(e) {
let value = e.target.value
if (props.modelModifiers.capitalize) {
value = value.charAt(0).toUpperCase() + value.slice(1)
}
emit('update:modelValue', value)
}
</script>
<template>
<input type="text" :value="props.modelValue" @input="emitValue" />
</template>
Modifiers for v-model
with arguments
Here's another example of using modifiers with multiple v-model
with different arguments:
template
<UserName
v-model:first-name.capitalize="first"
v-model:last-name.uppercase="last"
/>
vue
<script setup>
const [firstName, firstNameModifiers] = defineModel('firstName')
const [lastName, lastNameModifiers] = defineModel('lastName')
console.log(firstNameModifiers) // { capitalize: true }
console.log(lastNameModifiers) // { uppercase: true }
</script>
Pre 3.4 Usage
vue
<script setup>
const props = defineProps({
firstName: String,
lastName: String,
firstNameModifiers: { default: () => ({}) },
lastNameModifiers: { default: () => ({}) }
})
defineEmits(['update:firstName', 'update:lastName'])
console.log(props.firstNameModifiers) // { capitalize: true }
console.log(props.lastNameModifiers) // { uppercase: true }
</script>