diff --git a/lib/parser/string-methods.js b/lib/parser/string-methods.js
index 43eb909..7d51fea 100644
--- a/lib/parser/string-methods.js
+++ b/lib/parser/string-methods.js
@@ -32,7 +32,8 @@ module.exports = {
       throw new Error(`${substr} is not a string.`);
     }
 
-    return str.indexOf(substr) + substr.length === str.length;
+    const strSuffix = str.substring(str.length - substr.length, str.length)
+    return strSuffix === substr
   },
 
   replace(str, substr, replacement) {