Core Concepts

Core Concepts

NextRush is a small functional core — one Application object that composes middleware and routes into a request handler — with runtimes built on top. There is no plugin framework bolted on the side; the whole request path is one composed pipeline of async middleware. Understand those three ideas and the rest of the framework is detail.

The Application: one composition root

createApp() from the nextrush meta-package gives you a single Application. It is the one object where a server declares everything it depends on before accepting a request: middleware, routes, and long-lived services (called Extensions, not plugins).

import { createApp, createRouter, listen } from 'nextrush';

const app = createApp();

Three methods build the pipeline, one freezes it:

• use(...middleware) pushes middleware onto a stack, in call order.
• route(path, router) mounts a router at a path prefix. A mounted router is middleware from the app's point of view.
• get/post/put/patch/delete/head/all delegate straight to the app-owned router (app.router).
• ready() boots every registered Extension's setup() once, mounts the app-owned router last, then freezes configuration — use(), route(), extend() all throw afterward. listen() calls ready() for you.

createApp() from nextrush wires a default router in, so app.get() works out of the box. The lower-level @nextrush/core createApp() has no router unless you pass one explicitly (createApp({ router: createRouter() })).

Context: one object per request

Every request gets a single ctx object, handed from the adapter through every middleware to your handler and back — never copied, never replaced. Handlers write through ctx; they never return a Response object. The adapter builds ctx from the platform's real request at the front and serializes it back at the end.

ctx has three faces:

Face	Members	Notes
Input	ctx.method, ctx.path, ctx.query, ctx.headers, ctx.ip, ctx.get(name), ctx.params, ctx.body	params is populated by the router during matching. body is undefined until a body-parser middleware fills it in.
Shared	ctx.state, ctx.next()	state is a plain object the whole pipeline reads and writes; next() passes control onward.
Output	ctx.status, ctx.json(), ctx.send(), ctx.html(), ctx.redirect(), ctx.set()	Set status, then send exactly once.

import { createApp, createRouter, listen } from 'nextrush';

const app = createApp();
const router = createRouter();

router.get('/users/:id', (ctx) => {
  const id = ctx.params.id;         // input: captured by the router
  const fields = ctx.query.fields;  // input: parsed query string
  ctx.json({ id, fields });         // output: JSON response
});

app.route('/', router);
await listen(app, 8080);

Everything the client sends arrives on ctx unvalidated. ctx guarantees a consistent shape across runtimes, never safe values — validating params/query/body is the handler's job. ctx.raw exposes the platform's native object and breaks cross-runtime portability; avoid it.

The handler signature

A handler (or middleware) is an async function of ctx that sends a response through ctx and returns undefined. No (req, res) pair, no return new Response(...):

router.get('/health', (ctx) => ctx.json({ ok: true }));

Middleware has two equivalent shapes — ctx.next() on the context, or next as the second argument:

import type { Middleware } from 'nextrush';

const a: Middleware = async (ctx) => { await ctx.next(); };
const b: Middleware = async (ctx, next) => { await next(); };

Both drive the same dispatch. See Middleware for the onion model and why order matters.

Middleware: the request path is one pipeline

Cross-cutting work (auth, logging, timing, body parsing) is registered once, in front of the handlers, instead of copy-pasted into every one. A middleware does its slice of work, calls await ctx.next(), and — because control comes back — can act after the response is produced too. See Middleware.

Two composable layers

NextRush exposes two paradigms over the same core:

• Functional core (nextrush) — createApp, createRouter, compose, listen, HTTP errors, and typed Middleware/Context. This is what this page uses.
• Class runtime (nextrush/class) — Controller, Get, @Service(), DI, guards. Built on the same Application; see Controllers-and-Decorators and Dependency-Injection.

Same application code runs unchanged on Node, Bun, Deno, and edge — only the adapter differs.

Minimal runnable app

import { createApp, createRouter, listen } from 'nextrush';

const app = createApp();
const router = createRouter();

router.get('/', (ctx) => ctx.json({ status: 'ok' }));

app.route('/', router);
await listen(app, 8080);

The server answers GET / on port 8080. From here: Routing, Middleware, and Request-Lifecycle walk the pieces in depth.