Document serialization/deserialization of custom scalars

website/docs/04-docblock-tags/08-scalars.mdx

@@ -32,3 +32,48 @@ class Math {
   }
 }
 ```
+
+## Serialization and Parsing of Custom Scalars
+
+Grats does not ([yet](https://github.com/captbaritone/grats/issues/66)) support a first-class way to define serialization and parsing logic for custom scalars. However, you can do this manually by modifying the schema after it is generated.
+
+For example if you had a `Date` type in your schema:
+
+```ts title="scalars.ts"
+/** @gqlScalar Date */
+export type GqlDate = Date;
+```
+
+To define a custom `serialize/parseValue/parseLiteral` transform for this type, which serialized the data as a Unix timestamp, you could do the following:
+
+```ts title="server.ts"
+import { getSchema} from "./schema"; // Generated by Grats
+import { GqlDate } from "./scalars"
+
+const schema = getSchema(); 
+
+const date = schema.getType("Date") as GraphQLScalarType<GqlDate, number>;
+
+date.serialize = (value) => {
+  if (!(value instanceof Date)) {
+    throw new Error("Date.serialize: value is not a Date object");
+  }
+  return value.getTime();
+};
+date.parseValue = (value) => {
+  if (typeof value !== "number") {
+    throw new Error("Date.parseValue: value is not a number");
+  }
+  return new Date(value);
+};
+date.parseLiteral = (ast) => {
+  if (!(ast.kind === "IntValue" || ast.kind === "StringValue")) {
+    throw new Error(
+      "Date.parseLiteral: ast.kind is not IntValue or StringValue",
+    );
+  }
+  return new Date(Number(ast.value));
+};
+
+// ... Continue on, using the schema to create a GraphQL server
+```