diff --git a/tests/dev-test.js b/tests/dev-test.js
index f640e14c..f8adc108 100644
--- a/tests/dev-test.js
+++ b/tests/dev-test.js
@@ -1,3 +1,5 @@
+'use strict';
+
 let JavaScriptObfuscator = require('../dist/index');
 
 let obfuscatedCode = JavaScriptObfuscator.obfuscate(