title: Angular form validation: compare two fields
body: <p>In an Angular 4 application, how can I validate two fields of a form doing a comparison?</p>
  
  <p>For example, let's suppose that my form has a <code>startDate</code> and an <code>endDate</code> date fields and I want to make sure that the <code>endDate</code> must be bigger than the <code>startDate</code>.</p>