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vue3 v-model component

// ChildComponent.vue

export default {
  props: {
    modelValue: String // раньше было `value: String`
  },
  emits: ['update:modelValue'],
  methods: {
    changePageTitle(title) {
      this.$emit('update:modelValue', title) // раньше было `this.$emit('input', title)`
    }
  }
}
1

vue v-model

<input v-model="message" placeholder="edit me">
<p>Message is: {{ message }}</p>

what does v-model do in vue

Although a bit magical, v-model is essentially syntax sugar for updating 
data on user input events, plus special care for some edge cases.