Combine CSS classes

const classNames = (...classes) => {
  return classes.filter(Boolean).join(' ')
}

How to use

The className function's central role is to join strings passed into the function, which the join() array method is doing. However, you may have also noticed this weird filter(Boolean) method. This filter boolean trick checks to see if the value passed in is null before joining.

You can see this function in use to change the styles of a navbar on scroll.