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Elysia is a TypeScript web framework built for Bun. It takes full advantage of Bun’s HTTP, file system, and hot reloading APIs, and is one of the fastest Bun HTTP frameworks.

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Quick start

Scaffold a new Elysia project with bun create: 
bun create elysia my-app
cd my-app
bun run dev

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Manual setup

1

Install Elysia

bun add elysia
2

Create a server

src/index.ts
import { Elysia } from "elysia";

const app = new Elysia()
  .get("/", () => "Hello Elysia")
  .listen(3000);

console.log(`Running at http://localhost:${app.server?.port}`);
3

Run the server

bun run src/index.ts

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Routing

Elysia uses a chainable API for defining routes: 
import { Elysia } from "elysia";

const app = new Elysia()
  .get("/", () => "Hello!")
  .get("/users/:id", ({ params }) => `User: ${params.id}`)
  .post("/users", ({ body }) => body)
  .put("/users/:id", ({ params, body }) => ({ id: params.id, ...body }))
  .delete("/users/:id", ({ params }) => `Deleted ${params.id}`)
  .listen(3000);

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Schema validation

Elysia provides end-to-end type safety through its t schema validation module. Schemas are validated at runtime and inferred as TypeScript types automatically: 
import { Elysia, t } from "elysia";

const app = new Elysia()
  .post("/sign-in", ({ body }) => body, {
    body: t.Object({
      username: t.String(),
      password: t.String({ minLength: 8 }),
    }),
  })
  .listen(3000);
If the request body does not match the schema, Elysia returns a 400 Bad Request with a descriptive error.

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Middleware (lifecycle hooks)

Use Elysia’s lifecycle hooks to run code before or after route handlers: 
import { Elysia } from "elysia";

const app = new Elysia()
  .onRequest(({ request }) => {
    console.log(`${request.method} ${request.url}`);
  })
  .onError(({ code, error }) => {
    if (code === "NOT_FOUND") return new Response("Not found", { status: 404 });
  })
  .get("/", () => "OK")
  .listen(3000);

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Plugins

Elysia has a plugin system for sharing routes, middleware, and state across your app: 
import { Elysia } from "elysia";

const auth = new Elysia({ name: "auth" })
  .derive({ as: "scoped" }, ({ headers }) => ({
    user: headers["x-user"] ?? "anonymous",
  }))
  .get("/me", ({ user }) => user);

const app = new Elysia()
  .use(auth)
  .get("/", () => "Hello!")
  .listen(3000);
Popular community plugins include:
  • @elysiajs/bearer — Bearer token extraction
  • @elysiajs/cors — CORS configuration
  • @elysiajs/jwt — JWT authentication
  • @elysiajs/swagger — OpenAPI / Swagger UI generation
  • @elysiajs/trpc — tRPC adapter
Install any of them with bun add: 
bun add @elysiajs/cors @elysiajs/swagger
See the Elysia documentation for the complete API reference, including WebSocket support, file uploads, and server-sent events.

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