Sync in Vue 1.x

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/1.0.28/vue.js"></script>

<div id="app">
  <p>
    Value in parent: {{value}}
  </p>
  <custom-component :value.sync="value"></custom-component>
  <button v-on:click="value = 'Changed From Parent!'">CHANGE FROM PARENT</button>
</div>

<template id="custom-component">
  <div>
    <p>
      Value in child: {{value}}
    </p>
    <button v-on:click="value = 'Changed From Child!'">CHANGE FROM CHILD</button>
  </div>
</template>

JavaScript

Vue.component('custom-component', {
  template: '#custom-component',
  props: ['value'],
});

var app = new Vue({
    el: '#app',
    data: {
        value: 'Original Value',
    }
});
alert(1)