how to make an element show when the user scrolls down past code example

Example 1: when div is scrolled to it displays

window.addEventListener('scroll', function() {
	var element = document.querySelector('#main-container');
	var position = element.getBoundingClientRect();

	// checking whether fully visible
	if(position.top >= 0 && position.bottom <= window.innerHeight) {
		console.log('Element is fully visible in screen');
	}

	// checking for partial visibility
	if(position.top < window.innerHeight && position.bottom >= 0) {
		console.log('Element is partially visible in screen');
	}
});

Example 2: when div is scrolled to it displays

var observer = new IntersectionObserver(function(entries) {
	// isIntersecting is true when element and viewport are overlapping
	// isIntersecting is false when element and viewport don't overlap
	if(entries[0].isIntersecting === true)
		console.log('Element has just become visible in screen');
}, { threshold: [0] });

observer.observe(document.querySelector("#main-container"));