diff --git a/progress-nav/script.js b/progress-nav/script.js
index 2c1117d..4abc7d3 100644
--- a/progress-nav/script.js
+++ b/progress-nav/script.js
@@ -1,128 +1,92 @@
 window.onload = function() {
 
-	var toc = document.querySelector( '.toc' );
-	var tocPath = document.querySelector( '.toc-marker path' );
-	var tocItems;
+    const toc = document.querySelector('.toc');
+    const tocPath = document.querySelector('.toc-marker path');
+    let tocItems = [];
 
-	// Factor of screen size that the element must cross
-	// before it's considered visible
-	var TOP_MARGIN = 0.1,
-		BOTTOM_MARGIN = 0.2;
+    const TOP_MARGIN = 0.1,
+          BOTTOM_MARGIN = 0.2;
 
-	var pathLength;
+    let pathLength, lastPathStart, lastPathEnd;
 
-	var lastPathStart,
-		lastPathEnd;
+    window.addEventListener('resize', drawPath);
+    window.addEventListener('scroll', sync);
 
-	window.addEventListener( 'resize', drawPath, false );
-	window.addEventListener( 'scroll', sync, false );
+    drawPath();
 
-	drawPath();
+    function drawPath() {
+        tocItems = Array.from(toc.querySelectorAll('li')).map(item => {
+            const anchor = item.querySelector('a');
+            const target = document.getElementById(anchor.getAttribute('href').slice(1));
 
-	function drawPath() {
+            return { listItem: item, anchor, target };
+        }).filter(item => item.target);
 
-		tocItems = [].slice.call( toc.querySelectorAll( 'li' ) );
+        let path = [];
+        let pathIndent;
 
-		// Cache element references and measurements
-		tocItems = tocItems.map( function( item ) {
-			var anchor = item.querySelector( 'a' );
-			var target = document.getElementById( anchor.getAttribute( 'href' ).slice( 1 ) );
+        tocItems.forEach((item, i) => {
+            const x = item.anchor.offsetLeft - 5,
+                  y = item.anchor.offsetTop,
+                  height = item.anchor.offsetHeight;
 
-			return {
-				listItem: item,
-				anchor: anchor,
-				target: target
-			};
-		} );
+            if (i === 0) {
+                path.push(`M${x},${y}L${x},${y + height}`);
+                item.pathStart = 0;
+            } else {
+                if (pathIndent !== x) path.push(`L${pathIndent},${y}`);
 
-		// Remove missing targets
-		tocItems = tocItems.filter( function( item ) {
-			return !!item.target;
-		} );
+                path.push(`L${x},${y}`);
 
-		var path = [];
-		var pathIndent;
+                tocPath.setAttribute('d', path.join(' '));
+                item.pathStart = tocPath.getTotalLength() || 0;
 
-		tocItems.forEach( function( item, i ) {
+                path.push(`L${x},${y + height}`);
+            }
 
-			var x = item.anchor.offsetLeft - 5,
-				y = item.anchor.offsetTop,
-				height = item.anchor.offsetHeight;
+            pathIndent = x;
 
-			if( i === 0 ) {
-				path.push( 'M', x, y, 'L', x, y + height );
-				item.pathStart = 0;
-			}
-			else {
-				// Draw an additional line when there's a change in
-				// indent levels
-				if( pathIndent !== x ) path.push( 'L', pathIndent, y );
+            tocPath.setAttribute('d', path.join(' '));
+            item.pathEnd = tocPath.getTotalLength();
+        });
 
-				path.push( 'L', x, y );
+        pathLength = tocPath.getTotalLength();
+        sync();
+    }
 
-				// Set the current path so that we can measure it
-				tocPath.setAttribute( 'd', path.join( ' ' ) );
-				item.pathStart = tocPath.getTotalLength() || 0;
+    function sync() {
+        const windowHeight = window.innerHeight;
 
-				path.push( 'L', x, y + height );
-			}
+        let pathStart = pathLength, 
+            pathEnd = 0,
+            visibleItems = 0;
 
-			pathIndent = x;
+        tocItems.forEach(item => {
+            const { top, bottom } = item.target.getBoundingClientRect();
 
-			tocPath.setAttribute( 'd', path.join( ' ' ) );
-			item.pathEnd = tocPath.getTotalLength();
+            if (bottom > windowHeight * TOP_MARGIN && top < windowHeight * (1 - BOTTOM_MARGIN)) {
+                pathStart = Math.min(item.pathStart, pathStart);
+                pathEnd = Math.max(item.pathEnd, pathEnd);
+                visibleItems++;
 
-		} );
+                item.listItem.classList.add('visible');
+            } else {
+                item.listItem.classList.remove('visible');
+            }
+        });
 
-		pathLength = tocPath.getTotalLength();
+        if (visibleItems > 0 && pathStart < pathEnd) {
+            if (pathStart !== lastPathStart || pathEnd !== lastPathEnd) {
+                tocPath.setAttribute('stroke-dashoffset', '1');
+                tocPath.setAttribute('stroke-dasharray', `1, ${pathStart}, ${pathEnd - pathStart}, ${pathLength}`);
+                tocPath.setAttribute('opacity', 1);
+            }
+        } else {
+            tocPath.setAttribute('opacity', 0);
+        }
 
-		sync();
+        lastPathStart = pathStart;
+        lastPathEnd = pathEnd;
+    }
 
-	}
-
-	function sync() {
-
-		var windowHeight = window.innerHeight;
-
-		var pathStart = pathLength,
-			pathEnd = 0;
-
-		var visibleItems = 0;
-
-		tocItems.forEach( function( item ) {
-
-			var targetBounds = item.target.getBoundingClientRect();
-
-			if( targetBounds.bottom > windowHeight * TOP_MARGIN && targetBounds.top < windowHeight * ( 1 - BOTTOM_MARGIN ) ) {
-				pathStart = Math.min( item.pathStart, pathStart );
-				pathEnd = Math.max( item.pathEnd, pathEnd );
-
-				visibleItems += 1;
-
-				item.listItem.classList.add( 'visible' );
-			}
-			else {
-				item.listItem.classList.remove( 'visible' );
-			}
-
-		} );
-
-		// Specify the visible path or hide the path altogether
-		// if there are no visible items
-		if( visibleItems > 0 && pathStart < pathEnd ) {
-			if( pathStart !== lastPathStart || pathEnd !== lastPathEnd ) {
-				tocPath.setAttribute( 'stroke-dashoffset', '1' );
-				tocPath.setAttribute( 'stroke-dasharray', '1, '+ pathStart +', '+ ( pathEnd - pathStart ) +', ' + pathLength );
-				tocPath.setAttribute( 'opacity', 1 );
-			}
-		}
-		else {
-			tocPath.setAttribute( 'opacity', 0 );
-		}
-
-		lastPathStart = pathStart;
-		lastPathEnd = pathEnd;
-
-	}
-
-};
\ No newline at end of file
+};