vue 2 two way binding prop

vue.js

// ChildComponent.vue

<script>
  export default {
    props: {
      title: {
        type: String,
        default: 'Default title'
      }
    },
    methods: {
      changeTitle() {
        this.$emit('update:title', 'new title')
      }    
    }
  }
</script>

// ParentComponent.vue

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