how to find what button is pressed – Blancer.com Tutorials and projects

i am making a clicker game and need to find out which button is being clicked.
i am not sure how to do this.

this is my javascript

var multiplyer = 1
var carparts = 0;
document.getElementById("karpartz").innerHTML = carparts;
function onButtonClick() {
    carparts = carparts + multiplyer;
    document.getElementById("karpartz").innerHTML = carparts; 
}

const button = document.querySelector('button'); 
  button.addEventListener('click', onButtonClick);

and this is my html

<button type="submit">
  <img src= "example"
alt="buttonpng" border="0" />
</button>

<button>
  upgrade click power
</button>
<button>
  upgrade idle scrapping
</button>

<p id="karpartz"></p>

<html>

  <body>
    <h1>
      <script type="text/javascript">
        document.write(karpartz)

      </script>
    </h1>
  </body>

</html>

how do i find which one is clicked

i also cant find how to get rid of [object HTMLParagraphElement]