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From Express to Nexy

In Nexy an Express route becomes a Python file under src/routes/: app.get is a GET function, req turns into typed parameters, res.json into a return value, and app.use into config fields or a middleware.py.

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

A route

index.js
import express from "express"; const app = express(); app.get("/api/hello", (req, res) => {  res.json({ message: "Hello" });}); app.listen(3000);

The file path is the URL, the function name is the method, and the returned value is the JSON body:

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

req.params

index.js
app.get("/api/users/:id", (req, res) => {  const id = Number(req.params.id);  if (Number.isNaN(id)) return res.status(404).end();  res.json({ id });});

:id is a [id] segment, and the type annotation does the conversion and the 404:

src/routes/api/users/[id].py
async def GET(id: int):    return {"id": id}

A * wildcard is a [...rest].py file.

req.query

index.js
app.get("/api/search", (req, res) => {  const q = req.query.q ?? "";  const page = Number(req.query.page ?? 1);  res.json({ q, page });});
src/routes/api/search.py
async def GET(q: str = "", page: int = 1):    return {"q": q, "page": page}

?page=abc answers 422 with the parameter that failed, instead of NaN.

req.body and validation

index.js
import { z } from "zod"; app.use(express.json()); const NewItem = z.object({ name: z.string(), price: z.number().positive() }); app.post("/api/items", (req, res) => {  const parsed = NewItem.safeParse(req.body);  if (!parsed.success) return res.status(422).json(parsed.error);  res.status(201).json(parsed.data);});

The body is parsed and validated against a Pydantic model before the function runs:

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)

express.Router()

routes/admin.js
const admin = express.Router(); admin.get("/stats", (req, res) => res.json({ users: 3 })); app.use("/admin", admin);

The folder is the mount path:

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

Route middleware

routes/admin.js
function requireAdmin(req, res, next) {  if (req.get("x-role") !== "admin") return res.status(403).json({ error: "Admins only" });  next();} admin.use(requireAdmin);

A dependencies.py runs before every route of its folder. Raising stops the request, as not calling next() does:

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]

app.use middleware

index.js
app.use((req, res, next) => {  const start = Date.now();  res.on("finish", () => console.log(req.method, req.url, Date.now() - start));  next();});

A middleware.py at the routes root wraps every request; call_next is next():

src/routes/middleware.py
import time from nexy.http import Request  async def middleware(request: Request, call_next):    start = time.perf_counter()    response = await call_next(request)    response.headers["x-time-ms"] = f"{(time.perf_counter() - start) * 1000:.1f}"    return response

cors, compression, helmet-style hosts

index.js
import compression from "compression";import cors from "cors"; app.use(cors({ origin: "https://example.com" }));app.use(compression());
nexyconfig.py
from nexy.core.models import NexyConfigModel  class NexyConfig(NexyConfigModel):    useCORS = {"allow_origins": ["https://example.com"]}    useGZip = {"minimum_size": 1000}    useTrustedHost = {"allowed_hosts": ["example.com", "*.example.com"]}    useSession = {"secret_key": "change-me"}

Cookies

index.js
import cookieParser from "cookie-parser"; app.use(cookieParser()); app.post("/api/theme", (req, res) => {  res.cookie("theme", "dark").json({ theme: "dark" });}); app.get("/api/theme", (req, res) => res.json({ theme: req.cookies.theme ?? "light" }));

A cookie is a parameter to read, and a method of the response to write:

src/routes/api/theme.py
from nexy.http import Cookie, JSONResponse  async def GET(theme: str = Cookie("light")):    return {"theme": theme}  async def POST():    response = JSONResponse({"theme": "dark"})    response.set_cookie("theme", "dark")    return response

express-session

index.js
import session from "express-session"; app.use(session({ secret: "change-me" })); app.post("/api/login", (req, res) => {  req.session.user = req.body.name;  res.json({ user: req.session.user });});

useSession in nexyconfig.py (above) switches sessions on; request.session is a dict:

src/routes/api/login.py
from pydantic import BaseModel from nexy.http import Request  class Login(BaseModel):    name: str  async def POST(request: Request, body: Login):    request.session["user"] = body.name    return {"user": body.name}

res.redirect

index.js
app.get("/old", (req, res) => res.redirect(301, "/new"));
src/routes/old.py
from nexy.http import RedirectResponse  async def GET():    return RedirectResponse("/new", status_code=301)

File uploads with multer

index.js
import multer from "multer"; const upload = multer({ limits: { fileSize: 1_000_000 } }); app.post("/api/upload", upload.single("file"), (req, res) => {  res.json({ name: req.file.originalname, size: req.file.size });});
src/routes/api/upload.py
from nexy.http import HTTPException, UploadFile  async def POST(file: UploadFile):    data = await file.read()    if len(data) > 1_000_000:        raise HTTPException(status_code=413, detail="File too large")    return {"name": file.filename, "size": len(data)}

Views with EJS or Pug

index.js
app.set("view engine", "ejs"); app.get("/users", (req, res) => {  res.render("users", { users: [{ name: "Ada" }, { name: "Alan" }] });});
views/users.ejs
<ul>  <% users.forEach((user) => { %>  <li><%= user.name %></li>  <% }) %></ul>

A .nexy page is the route and the view at once:

src/routes/users.nexy
---users = [{"name": "Ada"}, {"name": "Alan"}]---<ul>  {% for user in users %}  <li>{{ user.name }}</li>  {% endfor %}</ul>

<%= %> escapes and {{ }} does not: escape what a visitor typed with {{ value | e }}.

express.static

index.js
app.use(express.static("public"));

Nothing to write: public/ is served at /.

The 404 handler and the error handler

index.js
app.use((req, res) => res.status(404).send("<h1>Not found</h1>")); app.use((err, req, res, next) => {  console.error(err);  res.status(500).send("<h1>Something broke</h1>");});

Two files at the routes root. API clients still get JSON, browsers get the page:

src/routes/notfound.nexy
<h1>Not found</h1>
src/routes/errors.nexy
<h1>Something broke</h1>

WebSockets with ws

index.js
import { WebSocketServer } from "ws"; const wss = new WebSocketServer({ server, path: "/ws/echo" });wss.on("connection", (socket) => socket.on("message", (data) => socket.send(data)));
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 Jest and Supertest

index.test.js
import request from "supertest";import { app } from "./app.js"; test("hello", async () => {  const res = await request(app).get("/api/hello");  expect(res.body).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

Express Nexy
npm init + npm i express nx new
nodemon index.js nx dev
NODE_ENV=production node index.js nx build, then nx start
npm test nx test
no API docs /docs, listing every .py route

Keeping your React front end

If the Express app served a React SPA, the components move to src/components/ and render as islands of a .nexy page, or the whole site moves to client routing with useCSR = True. See Islands and CSR.

Next steps

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