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Creating Your Own ExpressJS from Scratch (Part 6) – Creating a Body-Parser Middleware

Creating Your Own ExpressJS from Scratch (Part 6) – Creating a Body-Parser Middleware

Welcome back! In this sixth part of our custom ExpressJS clone series, we'll implement one of the most important middleware components of any modern web server: the body-parser.

This middleware allows your application to parse incoming request bodies — like JSON and form data — and access them as JavaScript objects via req.body.

🧠 Why You Need a Body-Parser Middleware

By default, Node.js does not parse the body of incoming HTTP requests. That means if a client sends form data or JSON, you’ll only receive it as a stream of bytes.

A body-parser middleware helps:

  • Convert the incoming stream into usable text
  • Parse that text into a JavaScript object (e.g., from JSON or application/x-www-form-urlencoded)
  • Attach the parsed object to req.body so you can access it in your controllers

🛠️ Implementing the Middleware

src/body-parser.js

const BodyParser = async (req, res, next) => {
  let body = [];

  for await (const chunk of req) {
    body.push(chunk);
  }

  body = Buffer.concat(body).toString();

  if (req.headers['content-type'] === 'application/json') {
    req.body = JSON.parse(body);
  } else if (req.headers['content-type'] === 'application/x-www-form-urlencoded') {
    const params = new URLSearchParams(body);
    req.body = Object.fromEntries(params.entries());
  }

  next();
};

module.exports = BodyParser;

🔍 What’s Happening Here?

  1. Reading the Stream

    • We collect all the chunks of the request body into an array.

  2. Buffer Concatenation

    • Convert the chunks into a complete string using Buffer.concat.

  3. Content-Type Parsing

    • If it's application/json, we JSON.parse() the string.
    • If it's application/x-www-form-urlencoded, we use URLSearchParams to parse the form data.

After parsing, we attach the resulting object to req.body.

🧪 Using the Middleware

index.js

const App = require('./src/app');
const BodyParser = require('./src/body-parser');

const app = App();

app.useAll(BodyParser);

app.post('/body-parser', (req, res) => {
  res.send(`name: ${req.body.name}, age: ${req.body.age}`);
});

const start = async () => {
  app.run(3000);
};

start();

🚀 Testing the Middleware

Request

POST /body-parser
Content-Type: application/json

{
  "name": "Wesley Miranda",
  "age": 28
}

Response

name: Wesley Miranda, age: 28

You can also test with form data using x-www-form-urlencoded.

📦 What’s Next?

In the upcoming tutorials, we’ll implement:

  • ✅ CORS Middleware
  • ✅ Multer-style file upload Middleware

We’re getting closer to building a production-grade framework from scratch!

Thanks for following along! 💡