How to convert TypeScript Interfaces and Types to JSON for Fastify Interoperability

Question

I am new to Fastify and I do not want to write JSON schema objects from scratch, but leverage TypeScript Interfaces and Types exposed from an existing Data Access Controller. How can I convert those TS Interfaces/Types into JSON schemas, either at dev-time or runtime, for consumption by Fastify to better support RESTful API validation? Thank you in advance.

Answer 1

According to the Fastify documentation you can achieve what you want by using type providers.

In the example below I used the Typebox to generate the TypeScript type and JSON schema-compatible Object to use in the runtime

import Fastify, { type RequestGenericInterface } from "fastify";
import { type TypeBoxTypeProvider } from "@fastify/type-provider-typebox";
import { type Static, Type } from "@sinclair/typebox";

const UserSchema = Type.Object({ username: Type.Optional(Type.String()) });

type UserType = Static<typeof UserSchema>;

const app = Fastify().withTypeProvider<TypeBoxTypeProvider>();

interface RequestPayloadType extends RequestGenericInterface {
  Querystring: UserType;
}

app.get<RequestPayloadType>(
  "/user",
  { schema: { querystring: UserSchema } },
  (req, _) => {
    const result = req.query.username ?? "no-name";
    return result;
  }
);