WTF.js

let nav_buttons = document.querySelectorAll('.nav');

function addNavButtonListeners() {
  Array.from(nav_buttons).forEach(button => {
    button.addEventListener('click', e => {
      console.log(button);
      e.preventDefault();

      if(button.classList.contains('next')){
        switchPanel();
        return;
      }

      switchPanel(-1);
    });
  });
}

addNavButtonListeners();

HTML is:

<button type="button" class=".nav .next">Next</button>
<button type="button" class=".nav .prev">Back</button>