Add class to each button

When I click on any button, I want to add a class to it. How can I do this?

Html file:

...
<button data-action="btn#addClass">btn 1</button>
...
<button data-action="btn#addClass">btn 2</button>
...
<button data-action="btn#addClass">btn 3</button>
...

Now how can I create my addClass() that when click on each, add class btnNewClass.