GKmeena Notes

Express Key Notes

`req` & `res` objects

req (request) and res (response) are objects automatically provided by express when handling HTTP requests.

req (Request Object): Contains details about the incoming request. - req.query – Query parameters (/user?id=1) - req.params – Route parameters (/user/:id) - req.body – Request body (POST/PUT) - req.headers – HTTP headers res (Response Object): Used to send data back to the client. - res.send() – Send text/HTML/JSON - res.json() – Send JSON data - res.sendFile() – Send a file - res.status() – Set HTTP status code

Express Middleware

Middleware in Express is a function that has access to the request (req) and response (res) objects, and the next function (next) in the application’s request-response cycle.

Middleware functions can modify the request, the response, or end the request-response cycle entirely.
Middleware can be global or route-specific and allows you to add functionality to the request-response cycle (such as authentication, logging, error handling, static file serving).

Middleware Functions

Access to Request and Response -> Middleware functions can read or modify the request object, response object, or both.
Next Function -> Middleware can pass control to the next middleware function using the next() function.

Order of Middleware

Middleware is executed in the order it is defined.
If a middleware doesn't call next(), the request-response cycle is considered complete, and no further middleware will be executed.

Types of Middleware

Application-Level Middleware: Applies to all routes or specific routes.

app.use((req, res, next) => {
  console.log('Request made!');
  next();  // Pass control to the next middleware
});

Route-Level Middleware: Bound to specific routes.

app.get('/about', (req, res, next) => {
  console.log('Accessing About Page');
  next(); // Pass control to the route handler
});

Error-Handling Middleware: This type of middleware is used to handle errors.

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something went wrong!');
});

Built-in Middleware: Express has built-in middleware like -express.json(), express.urlencoded(), express.static(), express.Router()

app.use(express.static('public'));  // Serve static files from the 'public' folder

Third-Party Middleware: Middleware from external packages (e.g., morgan for logging, cors for handling cross-origin requests).

Express Route Methods and Middleware

app.use() : Used to apply middleware functions that run on every request or specific routes.

app.use([path], middleware); // path is optinal

path is optional. specifies a path for the middleware. If omitted, it runs globally on all routes.
It works with all HTTP methods (GET, POST, PUT, etc.) unless specified otherwise.
Middleware can be chained together.

app.get() : Defines a route handler for GET requests at a specified URL path.

app.get(path, middleware, callback);

path: The URL or route where the GET request is made.
middleware is Optional middleware function(s) executed before the final handler.
callback: The final function that handles the GET request.

app.post() : Defines a route handler for POST requests at a specified URL path.

app.post(path, middleware, callback);

usually used for handling form submissions, API calls with POST data.

app.put() : Defines a route handler for PUT requests at a specified URL path.

app.put(path, middleware, callback);

app.delete() : Defines a route handler for DELETE requests at a specified URL path.

app.delete(path, middleware, callback);

Note: Square brackets [] are used in documentation or examples to indicate optional parameters, but in this case, the middleware itself is required to be a function.

Conclusion:

app.use() is used to define global middleware that works for all routes and HTTP methods.
app.get() and other methods like app.post() are used to define route-specific middleware for a particular route (e.g., /hello).

Difference Between Middleware and Route Handler

// Middleware
const midd = (req, res, next) => { ... };

// Route Handler (sends the response)
app.get('/', myMiddleware, (req, res) => {...}
);

Paths -> `'/'
Middleware -> const midd = (req, res,next) => {}
- next() Required to pass control to the next middleware.
Route Handler/Callback -> (req,res)=>{}
- next() Not needed (It ends the request-response cycle).

Note: In Express.js, the route handler is a callback function that processes a specific route.

Static(Public) vs Template Directory

static / public folder

The static (or public) -> used to serve static assets/files to the client
which are files that do not change in response to user input and are sent directly from the server to the client.
Images, Stylesheets, JavaScript, Font

template folder

The template folder (or templates in Flask) -> used to store HTML template files that your web application renders.
These templates are often dynamically generated with server-side data.
HTML

`express()` vs `express`

express() : creates an instance of an Express application.

it is a function call that initialize an Express app when you invoke express(), it returns an Express application object that can be used to define routes, middleware, and handle HTTP requests.
Function and Methods of Express Application Instance -> app.get(), app.post(), app.put(), app.delete(), app.use(), app.listen(), app.route(), app.set(), app.get(), app.all(), app.param()
example:

const app = express(); // Creates an Express 
 
app.use(...) // Use instance of `express()`

express : express module

It refers to the Express module itself, which is the imported package or function from the express library.
Function and Properties of Express Module -> express(), express.Router(), express.static(), express.json(), express.urlencoded(), express.raw(), express.text()
example:

const express = require('express');  // Import the Express module
 
express.static() // use 'static()' middleware of express module