classList toggle

classList.toggle accepts a second boolean parameter to add or remove a class.
This technique saves an if statement.

// add/remove with if
if (checkbox.checked) {
  document.body.classList.add('menu-open')
} else {
  document.body.classList.remove('menu-open')
}

// toggle with 2nd parameter
document.body.classList.toggle('menu-open', checkbox.checked)

Thanks to the legendary David DeSandro for the trick.