title: ANGULAR - Disable submit button if form value not changed
body: <p>i have form for selected date. how to disable the submit button if nothing changed in date form?</p>
  
  <p>my html code</p>
  
  <pre><code>&lt;div class="col-xs-6"&gt;
     &lt;label&gt;&lt;i class="far fa-calendar-alt"&gt;&lt;/i&gt; Start &lt;span class="text-danger"&gt;*&lt;/span&gt;&lt;/label&gt;
       &lt;input id="startTrip1" name="NgbDate" data-provide="datepicker" ngbDatepicker #d="ngbDatepicker" [markDisabled]="markDisabled" [minDate]="minDates" type="text" class="form-control form-flat" [(ngModel)]="ad.start_date" (dateSelect)="onDateSelect($event, ad)" (blur)="validateInput()" (click)="d.toggle()" [ngModelOptions]="{standalone: true}" [disabled]="form.controls.tripduration.hasError('required')" &gt;
        &lt;div class="text-danger" *ngIf="(ad.start_date == '' || ad.start_date == undefined) &amp;&amp; ngForm.submitted"&gt;
           * This field is required
         &lt;/div&gt;
        &lt;div class="text-danger" *ngIf="form.controls.tripduration.hasError('required')"&gt;
        * Fill the durations first
        &lt;/div&gt;
   &lt;/div&gt;
  //submit button
  &lt;button class="col-xs-12 text-center text-strong pointer custom-trip-btn" (click)="publishTrip()"&gt;
  &lt;button class="custom-trip-btn"&gt;SUBMIT&lt;/button&gt;
  </code></pre>