Getting started

Requirements

• Bun 1.3.6 or later
• TypeScript 5.9 or later

Installation

bun add @rouret/lyn

Your first server

Create server.ts:

import { Lyn, z } from "@rouret/lyn";

await new Lyn()
  .get("/", () => "Hello World")
  .get(
    "/hello/:name",
    ({ params }) => ({ message: `Hello ${params.name}` }),
    { params: z.object({ name: z.string() }) }
  )
  .listen(3000);

Start it. NODE_ENV is required: Lyn refuses to start without it.

NODE_ENV=development bun --hot run server.ts

curl localhost:3000/hello/Ada
# {"message":"Hello Ada"}

Lyn prints its logo, then one log line per request.

Accepting data

Add a route that receives JSON. The body is validated before your handler runs, and body is typed from the schema:

.post(
  "/users",
  ({ body }) => ({ id: crypto.randomUUID(), ...body }),
  {
    body: z.object({
      name: z.string().min(1),
      email: z.email(),
    }),
  }
)

curl -X POST localhost:3000/users \
  -H "Content-Type: application/json" \
  -d '{"name":"Ada"}'

{
  "code": "VALIDATION",
  "message": "Bad Request",
  "cause": [{ "expected": "string", "code": "invalid_type", "path": ["email"], "message": "Invalid input: expected string, received undefined" }]
}

Send a valid email and you get a 201 Created with the new user.

Next steps

Pick the next step that matches what you are building:

You want to	Read
Organise a growing app in several files	Project structure
Understand paths, parameters and priority	Routing
Validate bodies, params and query strings	Validation
Store data in Postgres	Database
Test your routes	Testing
Ship fast without shipping badly	Best practices
Deploy with Docker	Production

Everything Lyn exports is listed in the API reference.