v-model Breaking Changes from Vue 2 to Vue 3

Impact: HIGH - Vue 3 fundamentally changed how v-model works on custom components. Code using the Vue 2 pattern will silently fail - the component won't receive the prop value and changes won't propagate to the parent.

Task Checklist

Change prop name from value to modelValue
Change event name from input to update:modelValue
Replace .sync modifier with v-model:propName
Remove model component option (use defineModel or named v-model)
Update any v-bind.sync to v-model with named argument

Key Breaking Changes

Feature	Vue 2	Vue 3
Default prop	`value`	`modelValue`
Default event	`input`	`update:modelValue`
Custom name	`model: { prop, event }`	`v-model:customName`
Sync modifier	`v-bind:prop.sync`	`v-model:prop`
Multiple models	Not supported	Fully supported

Vue 2 Pattern (No longer works in Vue 3):

<!-- Vue 2 Child Component -->
<script>
export default {
  props: ['value'], // WRONG in Vue 3
  methods: {
    update(val) {
      this.$emit('input', val); // WRONG in Vue 3
    },
  },
};
</script>

<template>
  <input :value="value" @input="update($event.target.value)" />
</template>

Vue 3 Pattern (Options API):

<!-- Vue 3 Child Component -->
<script>
export default {
  props: ['modelValue'], // Changed from 'value'
  emits: ['update:modelValue'], // Declare emits
  methods: {
    update(val) {
      this.$emit('update:modelValue', val); // Changed from 'input'
    },
  },
};
</script>

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

Vue 3 Pattern (Composition API with defineModel):

<!-- Vue 3 Child Component - Recommended -->
<script setup>
const model = defineModel(); // Handles prop and emit automatically
</script>

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

Migrating the .sync Modifier

Vue 2's .sync modifier is removed. Use named v-model instead.

Vue 2:

<!-- Parent -->
<MyComponent :title.sync="pageTitle" />

<!-- Child -->
<script>
export default {
  props: ['title'],
  methods: {
    updateTitle(val) {
      this.$emit('update:title', val); // .sync pattern
    },
  },
};
</script>

Vue 3:

<!-- Parent -->
<MyComponent v-model:title="pageTitle" />

<!-- Child with defineModel -->
<script setup>
const title = defineModel('title');
</script>

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

<!-- Child with manual props/emits -->
<script setup>
const props = defineProps(['title']);
const emit = defineEmits(['update:title']);
</script>

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

Migrating Custom model Option

Vue 2's model component option is removed.

Vue 2:

<script>
export default {
  model: {
    prop: 'checked',
    event: 'change',
  },
  props: ['checked'],
};
</script>

Vue 3:

<!-- Use named v-model argument instead -->
<!-- Parent -->
<MyCheckbox v-model:checked="isChecked" />

<!-- Child -->
<script setup>
const checked = defineModel('checked');
</script>

Multiple v-model Bindings (New in Vue 3)

Vue 3 allows multiple v-model directives on a single component:

<!-- Parent -->
<UserForm v-model:firstName="first" v-model:lastName="last" v-model:email="email" />

<!-- Child -->
<script setup>
const firstName = defineModel('firstName');
const lastName = defineModel('lastName');
const email = defineModel('email');
</script>

<template>
  <input v-model="firstName" placeholder="First Name" />
  <input v-model="lastName" placeholder="Last Name" />
  <input v-model="email" type="email" placeholder="Email" />
</template>

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