Fix readonly nullable properties (#1036)

The current logic generates invalid TypeScript code with the
`immutableTypes` option for nullable properties which include at least
one union (potentially nested):

Simple example:

  type: array
  nullable: true
  items:
    oneOf:
      - type: number
      - type: string

Generated code before and after this PR:

  readonly before: readonly ((number | string)[]) | null; // Invalid
  readonly after: (readonly (number | string)[]) | null; // OK

Author: mtth

src/transform/schema-object.ts

@@ -154,8 +154,8 @@ export function defaultSchemaObjectTransform(
         }
       }
       itemType = tsArrayOf(itemType);
-      if (schemaObject.nullable) itemType = tsUnionOf(itemType, "null");
-      return ctx.immutableTypes || schemaObject.readOnly ? tsReadonly(itemType) : itemType;
+      itemType = ctx.immutableTypes || schemaObject.readOnly ? tsReadonly(itemType) : itemType;
+      return schemaObject.nullable ? tsUnionOf(itemType, "null") : itemType;
     }
   }
 

test/schema-object.test.ts

@@ -447,6 +447,33 @@ describe("Schema Object", () => {
         );
       });
     });
+
+    describe("immutableTypes", () => {
+      test("nullable array of records property", () => {
+        const schema: SchemaObject = {
+          type: "object",
+          properties: {
+            array: {
+              type: "array",
+              nullable: true,
+              items: {
+                type: "object",
+                additionalProperties: true,
+              },
+            },
+          },
+        };
+        const generated = transformSchemaObject(schema, {
+          ...options,
+          ctx: { ...options.ctx, immutableTypes: true },
+        });
+        expect(generated).toBe(`{
+  readonly array?: (readonly ({
+      [key: string]: unknown | undefined;
+    })[]) | null;
+}`);
+      });
+    });
   });
 });