title: What does !: mean in Typescript?
body: <pre><code>$refs!: {
      helloComponent: Hello
  }
  </code></pre>
  
  <p><a href="https://github.com/vuejs/vue-class-component/blob/master/example/App.vue" rel="noreferrer">https://github.com/vuejs/vue-class-component/blob/master/example/App.vue</a></p>