Fix 'no-undef' rule

Author: AlexKVal

.eslintrc

@@ -19,6 +19,7 @@
     "no-eq-null": 0,
     "no-param-reassign": 0,
     "no-underscore-dangle": 0,
+    "no-undef": 2,
     "no-unused-vars": [2, { "vars": "all", "args": "none" }],
     "no-var": 2,
     "babel/object-shorthand": 2,