title: !: (bang colon) notation in Angular
body: <p>I found <code>!:</code> notation being used in <a href="https://angular.io/guide/deprecations#dependency-on-a-reflect-metadata-polyfill-in-jit-mode" rel="noreferrer">Angular Deprecation docs</a>. </p>
  
  <pre><code>@Input() tpl !: TemplateRef&lt;any&gt;;
  @ContentChild(TemplateRef) inlineTemplate !: TemplateRef&lt;any&gt;;
  </code></pre>
  
  <p>Found the same <a href="https://angular.io/api/core/ViewChild#usage-notes" rel="noreferrer">here as well</a>.</p>
  
  <pre><code>@Input() id !: string;
  </code></pre>
  
  <p>I'm not sure what is the terminology (or concept behind) for <code>!:</code> notation in Angular. Googling didn't help me much. Tried <a href="http://symbolhound.com/?q=%21%3A+angular" rel="noreferrer">SymbolHound</a> and Angular Docs for the same, but all in vain.</p>
  
  <p>It'd be helpful if someone could shed some light on it, like how does it function, or share the doc link at the very least.</p>