diff --git a/tools/doc/html.js b/tools/doc/html.js
index 0c15162c4c3..22a793c3394 100644
--- a/tools/doc/html.js
+++ b/tools/doc/html.js
@@ -36,7 +36,7 @@ const DOC_CREATED_REG_EXP = /<!--\s*introduced_in\s*=\s*v([0-9]+)\.([0-9]+)\.([0
 // customized heading without id attribute
 const renderer = new marked.Renderer();
 renderer.heading = function(text, level) {
-  return '<h' + level + '>' + text + '</h' + level + '>\n';
+  return `<h${level}>${text}</h${level}>\n`;
 };
 marked.setOptions({
   renderer: renderer
@@ -107,7 +107,7 @@ function loadGtoc(cb) {
       if (err) return cb(err);
 
       data = marked(data).replace(/<a href="(.*?)"/gm, function(a, m) {
-        return '<a class="nav-' + toID(m) + '" href="' + m + '"';
+        return `<a class="nav-${toID(m)}" href="${m}"`;
       });
       return cb(null, data);
     });
@@ -152,7 +152,7 @@ function render(opts, cb) {
     template = template.replace(/__TOC__/g, toc);
     template = template.replace(
       /__GTOC__/g,
-      gtocData.replace('class="nav-' + id, 'class="nav-' + id + ' active')
+      gtocData.replace(`class="nav-${id}`, `class="nav-${id} active`)
     );
 
     if (opts.analytics) {
@@ -500,8 +500,8 @@ function buildToc(lexed, filename, cb) {
     const realFilename = path.basename(realFilenames[0], '.md');
     const id = getId(realFilename + '_' + tok.text.trim());
     toc.push(new Array((depth - 1) * 2 + 1).join(' ') +
-             '* <span class="stability_' + tok.stability + '">' +
-             '<a href="#' + id + '">' + tok.text + '</a></span>');
+             `* <span class="stability_${tok.stability}">` +
+             `<a href="#${id}">${tok.text}</a></span>`);
     tok.text += '<span><a class="mark" href="#' + id + '" ' +
                 'id="' + id + '">#</a></span>';
   });