fade a background color on header over course of scrolling with jQuery

header-transition.js

var scroller = function() {
	var scrollPosition = $(window).scrollTop();
	if ( scrollPosition < 90 ) {
		var bgColor = 'rgba(0, 0, 0, ' + (scrollPosition / 100) + ')';
		$('.navbar').css('background-color', bgColor);
	} else {
		$('.navbar').css('background-color', 'rgba(0, 0, 0, 0.9)');
	}
}

scroller(); // call on load in case someone refreshes halway down the page or hits a hash link
$(window).on('scroll touchmove', function(){ // scroll for mouse/trackpad, touchmove for immediate touch response
	scroller();
});