title: Dynamic validation in Angular reactive forms
body: <p>I have a reactive form in Angular. How can I perform a required validation on <code>tax</code> only if <code>openDate</code> and <code>closeDate</code> have been populated?</p>
  <pre><code>this.employerBasicsForm = this.fb.group({
    tax: ['', [Validators.required],
    openDate: [''],
    closeDate: [''],
  });
  </code></pre>