hide_nav_on_scroll.js

var header = document.getElementsByClassName("yourHeaderClass")[0];
// Use prevScroll value to determine scroll direction
var prevScroll = window.pageYOffset;

window.onscroll = function() {
	var scroll = window.pageYOffset;
	// Add class after scrolling 300px from top, remove class when scrolling up
	// Remove (scroll > prevScroll) to hide nav both on scroll Up and scroll down
  	if ((scroll > 300 ) && (scroll > prevScroll)) {
		header.classList.add("hideNav");
  	} else {
		header.classList.remove("hideNav");
	}
	prevScroll = scroll;
}