DJ Express

A minimal Express.js implementation for beginners to understand how Express works internally.

Perfect for learning: See how routing, middleware, and request handling work under the hood!

Why DJ Express?

• 🎓 Educational: Learn Express internals in ~100 lines of code
• 🔍 Transparent: See exactly how app.get(), next(), and res.send() work
• 🚀 Simple: No complex abstractions, just pure Node.js
• 📦 Lightweight: Zero dependencies

Installation

npm i dj-express

Quick Start

Option 1: Use as npm package

const { createApp } = require("dj-express");
const app = createApp();

app.get("/", (req, res) => {
  res.json({ message: "Hello from dj-express!" });
});

app.listen(3000);

Option 2: Run the example

git clone <repository-url>
cd mini-express
node example.js

How Express Works (Internally)

1. Routes are just an Array

const routes = [];

app.get = (path, handler) => {
  routes.push({ method: "GET", path, handler });
};

When you call app.get("/users", handler), it simply adds to an array!

2. Middlewares are also an Array

const middlewares = [];

app.use = (middleware) => {
  middlewares.push(middleware);
};

3. The next() Function (Magic!)

let i = 0;

function next() {
  const middleware = middlewares[i++];
  if (middleware) {
    middleware(req, res, next);
  } else {
    handleRoute();
  }
}

next() simply calls the next function in the array!

4. res.send() is just a wrapper

res.send = (data) => {
  if (typeof data === "object") {
    res.setHeader("Content-Type", "application/json");
    res.end(JSON.stringify(data));
  } else {
    res.end(data);
  }
};

5. Body Parsing (Reading POST data)

req.on("data", (chunk) => (body += chunk));
req.on("end", () => {
  req.body = JSON.parse(body);
});

Node.js sends data in chunks, we collect them and parse.

Usage

As npm package

const { createApp } = require("dj-express");

const app = createApp();

// Middleware
app.use((req, res, next) => {
  console.log(req.method, req.url);
  next();
});

// Routes
app.get("/", (req, res) => {
  res.json({ message: "Hello!" });
});

app.get("/users/:id", (req, res) => {
  res.json({ userId: req.params.id });
});

app.post("/data", (req, res) => {
  res.json(req.body);
});

app.listen(3000);

As local module

const { createApp } = require("./index");
// ... rest of the code same as above

Test Commands

# GET request
curl http://localhost:3000/

# Route params
curl http://localhost:3000/users/123

# Query params
curl "http://localhost:3000/search?q=express"

# POST with body
curl -X POST -H "Content-Type: application/json" -d '{"name":"John"}' http://localhost:3000/data

Features

| Feature | How it works | | -------------- | ------------------------------------------------ | | Routing | Routes stored in array, matched by path & method | | Middleware | Array of functions, next() calls the next one | | req.params | Extracted from /users/:id pattern | | req.query | Parsed from ?key=value | | req.body | Parsed from POST request body | | res.send() | Auto-detects JSON vs text | | res.json() | Sends JSON response | | res.status() | Sets status code |

Learn More

• 📦 npm package
• 🐛 Report issues
• 🤝 Contribute

License

MIT