// Intialise variable.
	var x = window.matchMedia( "(max-width: 767px)" )
	myFunction( x ) // Call listener function at run time.
	x.addListener( myFunction )

function myFunction(x) {
	if (x.matches) { // If media query matches.
		// jQuery element variables.
		var $hamburgerMenuBtn,
			$slideNav,
			$closeBtn,
			$main;

		// focus management variables.
		var $focusableInNav,
			$firstFocusableElement,
			$lastFocusableElement;

		jQuery( document ).ready(
			function() {
				$hamburgerMenuBtn = jQuery( "#hamburger-menu" ),
				$slideNav         = jQuery( "#slide-nav" ),
				$closeBtn         = jQuery( "#close" ),
				$main             = jQuery( "main" ),
				$focusableInNav   = jQuery( '#slide-nav button, #slide-nav [href], #slide-nav input, #slide-nav select, #slide-nav textarea, #slide-nav [tabindex]:not([tabindex="-1"])' );
				if ($focusableInNav.length) {
					$firstFocusableElement = $focusableInNav.first();
					$lastFocusableElement  = $focusableInNav.last();
				}
				addEventListeners();
			}
		);

		function addEventListeners() {
			$hamburgerMenuBtn.click( openNav );
			$closeBtn.click( closeNav );
			$slideNav.on( "keyup", closeNav );
			$firstFocusableElement.on( "keydown", moveFocusToBottom );
			$lastFocusableElement.on( "keydown", moveFocusToTop );
		}

		function openNav() {
			$slideNav.addClass( "visible active" );
			setTimeout(
				function() {
					$firstFocusableElement.focus()
				},
				1
			);
			$main.attr( "aria-hidden", "true" );
			$hamburgerMenuBtn.attr( "aria-hidden", "true" );
		}

		function closeNav(e) {
			if (e.type === "keyup" && e.key !== "Escape") {
				return;
			}

			$slideNav.removeClass( "active" );
			$main.removeAttr( "aria-hidden" );
			$hamburgerMenuBtn.removeAttr( "aria-hidden" );
			setTimeout(
				function() {
					$hamburgerMenuBtn.focus()
				},
				1
			);
			setTimeout(
				function() {
					$slideNav.removeClass( "visible" )
				},
				501
			);
		}

		function moveFocusToTop(e) {
			if (e.key === "Tab" && ! e.shiftKey) {
				e.preventDefault();
				$firstFocusableElement.focus();
			}
		}

		function moveFocusToBottom(e) {
			if (e.key === "Tab" && e.shiftKey) {
				e.preventDefault();
				$lastFocusableElement.focus()
			}
		}
	}
}