To enable or disable the input field based on the value of select component in angular

You have to do some editings. in your Html file

<mat-form-field class="id-name">
        <mat-select placeholder="ID Card" formControlName="IDproof" (ngModelChange)="onChange($event)">
            <mat-option *ngFor="let IDproof of  IDproofs" [value]="IDproof.value">
                {{IDproof.viewValue}}
            </mat-option>
        <mat-option  [value]="'disabled'">
                {{'disabled'}}
            </mat-option>
        </mat-select>
    </mat-form-field>

And in your ts file

onChange(data) {
    if(data && data != 'disabled'){
      this.myForm.get('idNum').enable()
    }else{
      this.myForm.get('idNum').disable()
    }
  }