diff --git a/src/librustdoc/html/static/playpen.js b/src/librustdoc/html/static/playpen.js
index ef8bdf5e2ce..6b96a285992 100644
--- a/src/librustdoc/html/static/playpen.js
+++ b/src/librustdoc/html/static/playpen.js
@@ -12,6 +12,8 @@
 /*globals $: true, rootPath: true */
 
 document.addEventListener('DOMContentLoaded', function() {
+    'use strict';
+
     if (!window.playgroundUrl) {
         return;
     }