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Test Plan: CI passes 🍏


Pull Request: #77

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Typed Fastify

This package adds strong TypeScript support to Fastify request handlers and enforces handlers to have typed schema which is used to validate request params and replies. From this schema it does two things:

  • static typechecking against TypeScript Schema
    • request.body
    • request.headers
    • request.querystring
    • request.params
    • route.path.params are also inferred and mapped to request.params, it is also not possible to make a typo in schema params
    • reply is always based on status, developer won't be able to use plain reply.send() but forced to explicitly set status first, based on which response type will be inferred
  • JSON schema generation from TS Schema (using typescript-json-schema with custom transforms, all @tjs annotations can be used to fine-tune output)
    • since we use typejescript-json-schema: all known limitations of lib are inherited:
      • Records are not transformed correctly, use { [k: string]: string } instead or hint with @tjs
  • Runtime validation using generated JSON schema (optional but strongly recommended as it brings extra safety to runtime and ensures that code assumptions about data are correct)


npm i @coobaha/typed-fastify

pnpm i @coobaha/typed-fastify

yarn add @coobaha/typed-fastify

Example of service we want to build

GET / => Hello ($ || world)

Simple implementation without schema generation will be following

import addSchema, { Schema } from '@coobaha/typed-fastify';
import fastify from 'fastify';

export interface ExampleSchema extends Schema {
  paths: {
    'GET /': {
      request: {
        querystring: {
          name?: string;
      response: {
        200: {
          content: string;

const exampleService: Service<ExampleSchema> = {
  'GET /': (req, reply) => {
    // typescript will infer correct types for us
    const name = ?? 'World';

    // Calling send directly is not allowed
    // reply.send(`Hello ${name}`)
    // Calling send with wrong payload will result in an error
    // reply.status(200).send(new Date())

    return reply.status(200).send(`Hello ${name}`);

const app = fastify();
addSchema(app, {
  // it is strongly recommended to generate json schema to guaruntee runtime validity
  jsonSchema: {},
  service: exampleService,

// Start listening.
app.listen(3000, (err: any) => {
  if (err) {

Complex examples can be found typescript tests and in integration.test.ts.

JSON schema generation

You can generate json schema from your TS types by using typed-fastify-schema or tfs bins

npx tfs gen
tfs gen [files]

Generates json schemas next to corresponding ts files

  files  glob pattern of files                               [string] [required]

  --help     Show help                                                 [boolean]
  --version  Show version number                                       [boolean]
# it will generate example_schema.gen.json next to file
npx tfs gen example_schema.ts

When schema is generated - just pass it to plugin to have runtime validations 🎉

import jsonSchema from './example_schema.gen.json';

// ...

addSchema(app, {

Writing service

  1. Handlers in one object Type inference will work nicely in this case, you just make TS happy and things are working 🥳

  2. Handlers in a different file or separate functions - you will need to hint TS with exact type of handler.

import { RequestHandler, Schema } from '@coobaha/typed-fastify';

interface MySchema extends Schema {}

const myHandler: RequestHandler<MySchema, 'GET /hello'>['AsRoute'] = (req, reply) => {};
  1. When you want to have complex shared handler for multiple endpoints that intersect (share same props)
import { RequestHandler, Schema } from '@coobaha/typed-fastify';

interface MySchema extends Schema {}

const myHandlers: RequestHandler<MySchema, 'GET /hello' | `GET /hello2`>['AsRoute'] = (req, reply) => {};
  1. Sometimes properties won't be the same (for instance GET never has body and POST will). In this case you will probably be asked to add types to function params
import { RequestHandler, Schema } from '@coobaha/typed-fastify';

interface MySchema extends Schema {}

type MyHandlers = RequestHandler<MySchema, 'GET /hello' | `POST /hello`>;
const myHandlers = (req: MyHandlers['Request'], reply: MyHandlers['Reply']): MyHandlers['Return'] => {};

// if handler is async/await
const myHandlersAsync = async (req: MyHandlers['Request'], reply: MyHandlers['Reply']): MyHandlers['ReturnAsync'] => {};

addSchema(app, {
  jsonSchema: {},
  service: {
    'GET /hello': myHandlers,
    'GET /hello2': myHandlers,

It might be that TS can't infer exact type of complex handler when passed to addSchema so you'll need to do it manually

addSchema(app, {
  jsonSchema: {},
  service: {
    'GET /hello': myHandlers,
    'GET /hello2': myHandlers as RequestHandler<ExtendedSchema, 'GET /hello2'>['AsRoute'],

Annotating types

This library is using typescript-json-schema with custom transforms for schema generation. All @tjs annotations can be used to fine-tune schema output

  • @type can be used to specify end type after using toJSON, toString methods of objects like ObjectID from MogoDB

  • since we use typejescript-json-schema: all known limitations are also inherited: - Records are not transformed correctly, use { [k: string]: string } instead or hint with @tjs