2. Understanding Express.js

Installing Express.js

Install Express like you would any other package, using NPM:
```
npm install express
```
To ensure the install was successful, double check that express is now listed as a dependency in your package.json

Setting up a basic Express server doesn’t require very much code

const express = require('express');
const app = express();

app.listen(3000, () => {
    console.log('Server is running on port 3000');
});

This will create a new express server and tell it to listen on port 3000
This server doesn’t do very much, as we haven’t defined any routes yet, but it does get the barebones up and running
What is a route?
- A route is a defined way your express server will respond to a specific client request
- Defining a route requires a HTTP method (i.e. GET, POST, PUT, DELETE, etc.) and a URL path for the route (i.e. / or /search)
- It also requires a handler which is the function to execute when a request is sent to that particular route
- Using our example above, adding a route for the / path could look like:
  app.get('/', (request, response) => { response.send("Hello world!"); });
- This would display a simple page saying “Hello World” when the user attempted to navigate to /

Express.js works around a request and response flow:
- The client sends a request, usually from a browser
  - This request will contain the required parts of the route (the HTTP method and the URL) that express will need
- If Express has any middleware specified, that will run against the request to perform any required middleware operations
- Then, if Express knows the route the request is passed from the middleware to the request handler
- Finally, the response is sent from the handler to the client
An express app can really be broken down into three main components:
- Routes: Define the path and HTTP method (e.g., /users, GET).
- Middleware: Functions that modify the request/response or control the flow.
- Controller Logic: Functions tied to routes that perform application logic.

Example:

app.get('/products', (req, res) => {
    res.send('List of products');
});

Middleware functions can manipulate request and response objects before sending the final response.

Example:

app.use((req, res, next) => {
    console.log('Request received:', req.method, req.url);
    next();
});

Example: Error Handling Middleware

Error-handling middleware captures any errors and sends a response.

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