Providing a kind of complete answer to your question,
Do: <input class='form-control' [ngClass]="{'inputExtraClass': true}" placeholder="Not working">
and if you want more than one class or switch between classes you can also do something like
<input class='form-control' [ngClass]="{'inputExtraClass': true, 'notInputExtraClass': !true }" placeholder="Not working">
this way above, it will be either one class or the other
You can also aply any other variation you like using this, or make a property in your component like this one:
toggleClass: boolean = true;
and in your html:
<input class='form-control' [ngClass]="{ 'inputExtraClass': toggleClass, 'notInputExtraClass': !toggleClass }" placeholder="Not working">
same idea, and then you could create a method and change the toggleClass property or whatever :) hope it helped