In JavaScript, how to check if an element is in the viewport?

The following function returns true if the element el i s in the viewport:

// Return true if the element el is in viewport
function isInViewport(el) {
	var distance = el.getBoundingClientRect();
	return (
		distance.top >= 0 &&
		distance.left >= 0 &&
		distance.bottom <= (window.innerHeight || document.documentElement.clientHeight) &&
		distance.right <= (window.innerWidth || document.documentElement.clientWidth)
	);
};