EN

DocsJavaScript backends

From Elysia to Nexy

In Nexy an Elysia route becomes a Python file under src/routes/: t.Object schemas become Pydantic models, .group becomes a folder, beforeHandle a dependencies.py, and Eden's typed client becomes typed actions.

Each section below shows an Elysia habit, then the same thing in Nexy.

A route

src/index.ts
import { Elysia } from "elysia"; new Elysia().get("/api/hello", () => ({ message: "Hello" })).listen(3000);

The file path is the URL, the function name is the method:

src/routes/api/hello.py
async def GET():    return {"message": "Hello"}

Params and query with t

src/index.ts
import { Elysia, t } from "elysia"; new Elysia().get("/api/users/:id", ({ params, query }) => ({ id: params.id, fields: query.fields }), {  params: t.Object({ id: t.Numeric() }),  query: t.Object({ fields: t.Optional(t.String()) }),});

The types go on the parameters, and :id is a [id] segment:

src/routes/api/users/[id].py
async def GET(id: int, fields: str | None = None):    return {"id": id, "fields": fields}

A body with t.Object

src/index.ts
app.post("/api/items", ({ body, set }) => {  set.status = 201;  return body;}, {  body: t.Object({ name: t.String(), price: t.Number({ exclusiveMinimum: 0 }) }),});
src/routes/api/items.py
from pydantic import BaseModel, Field from nexy.http import JSONResponse  class NewItem(BaseModel):    name: str    price: float = Field(gt=0)  async def POST(item: NewItem):    return JSONResponse(item.model_dump(), status_code=201)

response schemas

src/index.ts
app.get("/api/me", () => ({ name: "Ada" }), {  response: t.Object({ name: t.String() }),});

The return type is the response schema: undeclared fields are left out of the answer.

src/routes/api/me.py
from pydantic import BaseModel  class PublicUser(BaseModel):    name: str  async def GET() -> PublicUser:    return {"name": "Ada", "password_hash": "x"}

.group

src/index.ts
app.group("/admin", (admin) => admin.get("/stats", () => ({ users: 3 })));

The folder is the group:

src/routes/admin/stats.py
async def GET():    return {"users": 3}

beforeHandle and guard

src/index.ts
app.group("/admin", (admin) =>  admin    .guard({      beforeHandle({ headers, error }) {        if (headers["x-role"] !== "admin") return error(403, "Admins only");      },    })    .get("/stats", () => ({ users: 3 })),);

A dependencies.py guards every route of its folder:

src/routes/admin/dependencies.py
from nexy.http import Header, HTTPException  def require_admin(x_role: str = Header("")) -> None:    if x_role != "admin":        raise HTTPException(status_code=403, detail="Admins only")  dependencies = [require_admin]

derive, decorate and state

src/index.ts
app  .derive(({ headers }) => ({ token: headers.authorization?.replace("Bearer ", "") }))  .get("/api/whoami", ({ token }) => ({ token }));

What derive adds to the context is a dependency, asked for by the handler:

src/routes/api/whoami.py
from nexy.http import Depends, Header  def token(authorization: str = Header("")) -> str:    return authorization.removeprefix("Bearer ")  async def GET(token: str = Depends(token)):    return {"token": token}

decorate and state are plain module-level objects, imported where they are needed.

onAfterHandle and mapResponse

src/index.ts
app.onAfterHandle(({ set }) => {  set.headers["x-app"] = "demo";});
src/routes/middleware.py
from nexy.http import Request  async def middleware(request: Request, call_next):    response = await call_next(request)    response.headers["x-app"] = "demo"    return response

error() and onError

src/index.ts
app.get("/api/orders/:id", ({ params, error }) => {  if (Number(params.id) > 100) return error(404, "No such order");  return { id: params.id };});
src/routes/api/orders/[id].py
from nexy.http import HTTPException  async def GET(id: int):    if id > 100:        raise HTTPException(status_code=404, detail="No such order")    return {"id": id}

A notfound.nexy at the routes root answers browsers on an unknown URL.

Cookies

src/index.ts
app.get("/api/theme", ({ cookie: { theme } }) => ({ theme: theme.value ?? "light" }));
src/routes/api/theme.py
from nexy.http import Cookie  async def GET(theme: str = Cookie("light")):    return {"theme": theme}

Eden Treaty

src/index.ts
const app = new Elysia().post("/todos", ({ body }) => ({ title: body.title, done: false }), {  body: t.Object({ title: t.String() }),}); export type App = typeof app;
client.ts
import { treaty } from "@elysiajs/eden";import type { App } from "./src/index"; const api = treaty<App>("localhost:3000");const { data } = await api.todos.post({ title: "Write the docs" });

An action is the Python function and its typed client at once. Nexy generates the TypeScript types from the Python signature:

src/actions/todos.py
from pydantic import BaseModel from nexy.decorators import Action  class Todo(BaseModel):    title: str    done: bool = False  @Actionasync def add_todo(title: str) -> Todo:    return Todo(title=title)
src/components/AddTodo.tsx
import { useAction } from "@nexy/client";import { add_todo } from "@nexy/actions"; export function AddTodo() {  const { run, pending, data } = useAction(add_todo);  return (    <button disabled={pending} onClick={() => run({ title: "Write the docs" })}>      {data ? `Added ${data.title}` : "Add"}    </button>  );}

See Actions.

Plugins: cors, swagger, static

src/index.ts
import { cors } from "@elysiajs/cors";import { staticPlugin } from "@elysiajs/static";import { swagger } from "@elysiajs/swagger"; app.use(cors({ origin: "https://example.com" })).use(swagger()).use(staticPlugin());
Elysia plugin Nexy
@elysiajs/cors useCORS = {"allow_origins": [...]} in nexyconfig.py
@elysiajs/swagger nothing: /docs is on by default
@elysiajs/static nothing: public/ is served at /
nexyconfig.py
from nexy.core.models import NexyConfigModel  class NexyConfig(NexyConfigModel):    useCORS = {"allow_origins": ["https://example.com"]}

WebSockets

src/index.ts
app.ws("/ws/echo", {  message(ws, message) {    ws.send(message);  },});
src/routes/ws/echo.py
from fastapi import WebSocket  async def SOCKET(websocket: WebSocket):    await websocket.accept()    await websocket.send_text(await websocket.receive_text())

Tests with app.handle

test/index.test.ts
import { describe, expect, it } from "bun:test"; it("says hello", async () => {  const response = await app.handle(new Request("http://localhost/api/hello"));  expect(await response.json()).toEqual({ message: "Hello" });});
tests/test_hello.py
from fastapi.testclient import TestClient from nexy.app import app  def test_hello():    assert TestClient(app).get("/api/hello").json() == {"message": "Hello"}

Running

Elysia Nexy
bun create elysia app nx new
bun dev nx dev
bun build --compile nx build, then nx start
bun test nx test

Nexy runs on Python. Node is only needed when the project has client components: Vite builds them, and nx start runs without it.

Next steps

Edit on GitHub

Enter opens the result, Esc closes.