Simple focus trap script

focus-trap.js

containerElement.addEventListener('blur', (evt) => {
  const { target: previousFocusEl, relatedTarget: nextFocusEl, currentTarget: container } = evt;

  // Do nothing if the next focus remains inside our container:
  if (container.contains(nextFocusEl)) return;

  // Otherwise, stop exiting the container:
  evt.preventDefault();

  // Find all focusables (adjust to your own liking) and filter out `display: none` elements:
  const focusables = [...container.querySelectorAll('button, a[href], input, select, textarea, [tabindex]:not([tabindex^="-"])')].filter(el => el.offsetParent);

  // We want either the first or the last one, depending on the previous one:
  const newFocus = previousFocusEl == focusables[0] ? focusables.pop() : focusables.shift();

  newFocus.focus();
}, true) // Capture child events

Example:

https://codepen.io/jelmerdemaat/pen/xbVrodm