Built-in Modules: require() function, user defined module (create and include), HTTP module

📖 Theory

The word you have typed 3 times unexplained

const http = require('http');

You have now used that line in 2 lessons on faith. And meanwhile app.js is quietly growing: server code, seat arithmetic, soon file logging: one file collecting everything, the exact disease BCA403's packages cured in Java.

Both stories end in the same place: Node's module system: how code is split into reusable files, how require() fetches them, and how a file decides what it shows the world.

📖 Theory

Modules and require(), formally

A module is a file of related code with a controlled public surface. require() loads one and returns its public object.

Two species, told apart by the argument's spelling:

• Built-in modules ship inside Node and load by bare name: require('http'), and friends fs (files: next lesson), url (query strings), path, os
• User-defined modules are your own .js files and load by relative path: require('./seats'): the ./ means this folder, and it is what tells Node "my file, not a package"

⌨️ Practical

seats.js + app.js: create and include

// ---------- seats.js: a user-defined module ----------
exports.seatsLeft = function (booked, total) {
    return total - booked;              // PUBLIC: attached to exports
};

function auditLog(msg) {                // PRIVATE: not exported,
    // internal bookkeeping...          // invisible outside this file
}

// ---------- app.js: including it ----------
// const seats = require('./seats');   // ./ = this folder
// console.log(seats.seatsLeft(310, 350));   // 40

// Simulated here so you can run the idea:
const seats = { seatsLeft: (booked, total) => total - booked };
console.log("Seats left: " + seats.seatsLeft(310, 350));   // 40

This example runs in Gri-Learn on the web, where you can edit it and see the output.

📖 Theory

exports: the module's public counter

Inside a module, the exports object is the counter window: whatever you attach to it (exports.seatsLeft = ...) is servable to customers; every other function in the file stays in the back room.

require('./seats') hands the caller exactly that exports object, so seats.seatsLeft(310, 350) works and seats.auditLog(...) does not exist.

Recognise the idea? It is BCA403's access control reborn: exported = public members, unexported = private helpers: encapsulation at file scale, with ./seats playing the import statement.

🧩 Quiz

seats.js sits in the same folder as app.js, which says: const seats = require('seats'); What happens?

1. Works: Node checks the current folder first
2. Error: Cannot find module 'seats': a bare name makes Node look for built-ins and installed packages, not your files
3. Works, but only functions attached to exports are loaded
4. Node asks interactively which seats module you meant

💭 Think first

Design the split

FestConnect's server file is bloating: seat arithmetic, a fee calculator, the http server code. Propose the module layout: which files, what each exports, and what stays private: then tap.

⚠️ Watch out

Module slips

The missing ./: the quiz bug; built-ins and packages by name, YOUR files by path.

Forgetting to export: a perfectly good function that was never attached to exports comes back undefined at the call site: check the module's counter window first when a require'd function is missing.

The .js is optional: require('./seats') and require('./seats.js') both work; convention omits it.

📖 Theory

The third telling of one story

Count the pattern's appearances in your BCA: Java's packages + import + public (BCA403), VB.NET's Friend/Public surfaces (BCA404), and now Node's modules + require + exports. Different spelling, same law: group related code, hide the plumbing, publish a small surface. Next lesson uses the built-in url module alongside http to finally READ what the visitor asked: query strings, split and served.

✅ Summary

Key takeaways

• A module is a reusable code file; require() loads it and returns its public object.
• Built-ins load by bare name: http, fs, url, path, os.
• Your own files load by relative path: require('./seats'): the ./ is compulsory.
• exports is the public counter: attached functions are callable outside; the rest stays private.
• Missing ./ gives Cannot find module; missing exports gives undefined functions.
• Same organising law as BCA403 packages: small public surface, hidden plumbing.
• Memory hook: bare name for the shop-bought, dot-slash for the homemade.