Move boolean array definitions outside of function

src/lib/isBoolean.js

@@ -1,16 +1,15 @@
 import assertString from './util/assertString';
 
 const defaultOptions = { loose: false };
+const strictBooleans = ['true', 'false', '1', '0'];
+const looseBooleans = [...strictBooleans, 'yes', 'no'];
 
 export default function isBoolean(str, options = defaultOptions) {
   assertString(str);
 
-  const strictBooleans = ['true', 'false', '1', '0'];
-  const looseBooleans = [...strictBooleans, 'yes', 'no'];
-
   if (options.loose) {
-    return (looseBooleans.indexOf(str.toLowerCase()) >= 0);
+    return looseBooleans.includes(str.toLowerCase());
   }
 
-  return (strictBooleans.indexOf(str) >= 0);
+  return strictBooleans.includes(str);
 }