Easy switch focus between two buttons

switchFocus.js

const buttons = [...document.getElementsByClassName('button')];
let focusedButtonIndex = 0;

buttons[focusedButtonIndex].focus();

window.addEventListener('keydown', event => {
  if ( buttons.some(button => button === event.target) ) {
    if ( event.code === 'ArrowDown' || event.code === 'ArrowUp' ) {
      buttons[focusedButtonIndex ^= 1].focus();
    }
  }
});