Rename d.ts. to .ts to be able to be able to use Serializable type in other projects also + add documentation

Author: Gijs Nieuwenhuis

README.md

@@ -36,6 +36,8 @@ A library containing javascript utilities that we until now often copy between p
     -   [`cookieStorage`](#cookiestorage)
 -   [Cache](#cache)
     -   [`createInMemoryCache`](#createinmemorycache)
+-   [Types](#types)
+    -   [`Serializable`](#serializable)
 
 ## Utilities
 
@@ -528,6 +530,24 @@ cache.clear();
 cache.count();
 ```
 
+## Types
+
+### `Serializable`
+
+An interface that can be used to only allow properties that are serializable. Usable for JSON serialization or as a validation for Redux global state data.
+
+Usage:
+
+```typescript
+import { Serializable } from '@freshheads/javascript-essentials/build/types/utility';
+
+const toJson = (values: Serializable): string => JSON.stringify(values);
+
+class SomeClass {}
+
+toJson({ value: new SomeClass() }); // = typescript error
+```
+
 # Todo
 
 -   [Money formatting](https://github.com/freshheads/013/blob/develop/assets/frontend/src/js/utility/numberUtilities.ts)

src/types/utility.d.ts src/types/utility.tssrc/types/utility.d.ts renamed to src/types/utility.ts

@@ -1,3 +1,5 @@
+// Not named as utility.d.ts because it will then not be outputted into the build dir
+
 export type Serializable =
     | null
     | boolean