diff --git a/src/model/contents.ts b/src/model/contents.ts
index 823489e..ceca139 100644
--- a/src/model/contents.ts
+++ b/src/model/contents.ts
@@ -1,6 +1,10 @@
 import {TagAccessor} from './tag';
 import {check_type} from './../util/type_check'
 
+type JSONPrimitive = null|boolean|number|string;
+interface JSONMap extends Record<string, JSONType>{}
+interface JSONArray extends Array<JSONType>{};
+type JSONType = JSONMap|JSONPrimitive|JSONArray;
 
 export interface ContentContent{
     title           : string,
@@ -8,7 +12,7 @@ export interface ContentContent{
     basepath        : string,
     filename        : string,
     hash?           : string,
-    additional      : object,
+    additional      : JSONMap,
     tags            : string[],//eager loading 
 }