Vue.js show

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.0.3/vue.min.js"></script>
<div id="app">
    <form>
        <span class="error" v-show="!message">
      You must enter a message
    </span>
        <textarea v-model="message"></textarea>
        <button type="submit" >
            dddd
        </button>
        <button type="submit" v-show="message">
            Send Message
        </button>
    </form>
    <pre>
    {{ $data | json }}
  </pre>
</div>

JavaScript

new Vue({

    el: '#app',
    data: {

        message: ''

    }

});