title: Vue.js .$set is not a function
body: <p>I'm trying to modify the data that stored in vuejs by using <code>$set</code> function. But I got this error: <code>TypeError: app.messageBox.$set is not a function.</code></p>
  
  <p>Here is the code about how I define app and messageBox:</p>
  
  <pre><code>app = new Vue({
      el: '#app',
      data: {
          messageBox: [{
              id: 1,
              message: 'first'
          }, {
              id: 2,
              message: 'second'
          }]
      }
  });
  </code></pre>
  
  <p>and in another js file, I try to modify the data in messageBox:</p>
  
  <pre><code>function test() {
      app.messageBox.$set(0, {message: 'aaa'});
  }
  </code></pre>