title: angular form disable submit button if field is empty
body: <p>Sounds so simple but I've tried quite a few things and none work. </p>
  
  <p>I'm using Angular 4 and my form is template-driven:</p>
  
  <pre><code>&lt;form #form="ngForm" novalidate&gt;
      &lt;label for="insz"&gt;{{ 'SEARCH_PAGE.searchInszNumber' | translate }}&lt;/label&gt;
      &lt;input type="text" name="insz" [placeholder]="'SEARCH_PAGE.searchInszNumber' | translate" #input required&gt;
      &lt;button (click)="onSearch(input.value)" &gt;&lt;span&gt;{{'SEARCH_PAGE.search' | translate }}&lt;/span&gt;&lt;/button&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>I want to disable the button when the (one and only) input field is empty. </p>