diff --git a/perf/perf.js b/perf/perf.js
index baee142b9..3588c7dc3 100644
--- a/perf/perf.js
+++ b/perf/perf.js
@@ -1,4 +1,5 @@
 ;(function() {
+  'use strict';
 
   /** Used to access the Firebug Lite panel (set by `run`). */
   var fbPanel;
diff --git a/test/asset/worker.js b/test/asset/worker.js
index 8ccffa873..95e3ab8a8 100644
--- a/test/asset/worker.js
+++ b/test/asset/worker.js
@@ -1,3 +1,5 @@
+'use strict';
+
 self.console || (self.console = { 'log': function() {} });
 
 addEventListener('message', function(e) {
diff --git a/test/test-fp.js b/test/test-fp.js
index 05839e6ba..ca5391461 100644
--- a/test/test-fp.js
+++ b/test/test-fp.js
@@ -1,4 +1,5 @@
 ;(function() {
+  'use strict';
 
   /** Used as a safe reference for `undefined` in pre-ES5 environments. */
   var undefined;