Q:

Class change of button via jquery is changed back, when clicking another button from same class

I want to change the color of the buttons on my white ('btn-secondary') pagination from bootstrap to yellow ('btn-warning'), when they are clicked.
This works with my code below, but If I click the next button on the same pagination, the previously clicked button turns to the previous color. 
The HTML:
<button class="showAufgabe btn btn-secondary btn-sm">01</button>
<button class="showAufgabe btn btn-secondary btn-sm">02</button>
...
<button class="showAufgabe btn btn-secondary btn-sm">39</button>
<button class="showAufgabe btn btn-secondary btn-sm">40</button>

The jQuery:
$(document).ready(function() {
    var showAufgabe = $('.showAufgabe');
    $(document).on('click', showAufgabe, clickedAufgabe);

    function clickedAufgabe() {
        $(this).addClass('btn-warning').removeClass('btn-secondary');
    }
});

Thank you in advance! :-) 
Here's a gif of how it looks: 
Simulation.gif
I want the first one to stay yellow!

A:

When you are setting .showAufgabe to a variable, you are actually selecting all elements that share that same class name - in this case all buttons.
var showAufgabe = $('.showAufgabe');

Instead you could call $(this) when a button is clicked within your function. It would then toggle .btn-warning only on that specific button.

$(".showAufgabe").on('click', function() {
  $(this).toggleClass('btn-warning');
});
.btn-warning {
  background-color: yellow;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<button class="showAufgabe btn btn-secondary btn-sm">01</button>
<button class="showAufgabe btn btn-secondary btn-sm">02</button>
...
<button class="showAufgabe btn btn-secondary btn-sm">39</button>
<button class="showAufgabe btn btn-secondary btn-sm">40</button>

Side note: it may depend on your specific requirement, but you wouldn't have to remove the previous class, as .btn-warning appears further down in the bootstrap style sheet than .btn-secondary, applying it to the button class would overwrite the previous style applied via .btn-secondary.

