Introduction to Express Review

## What Is Express.js? - **Express**: A minimal and flexible web framework built on top of Node.js that makes it easier to build web servers and APIs. - **Install Express**: `npm install express` - **What you can build with Express**: web apps, RESTful APIs, SPA backends, middleware-heavy services, and anything server-side with HTTP. - **Middleware**: Code that runs between receiving a request and sending a response. Used for error handling, input validation, modifying request or response data, and more. Multiple middleware functions can be chained together. - **Plugins**: External npm modules that extend Express, usually implemented as middleware. ## Creating a Basic Express App ```javascript const express = require("express"); const app = express(); const port = 3000; app.get("/", (req, res) => { res.send("Hello World!"); }); app.listen(port, () => { console.log(`Example app listening on port ${port}`); }); ``` - `require("express")` — loads the Express library. - `express()` — factory function that creates and returns a new Express application instance. - `app` — the main server object with access to methods like `.get()`, `.post()`, and `.listen()`. - `port` — the network endpoint the app listens on. Port `3000` is common for local development. - `app.listen(port, callback)` — starts the server and begins listening for incoming HTTP requests. ## Routing in Express - **Route**: A rule that tells the app how to respond when a client requests a specific URL with a specific HTTP method. - **Route methods**: `app.get()`, `app.post()`, `app.put()`, `app.delete()` — each handles requests with the matching HTTP method. - **Static route path**: A fixed URL like `/home` or `/menu/drinks`. - **Dynamic route path (route parameter)**: A placeholder in the URL that captures a value, prefixed with `:`, for example `/post/:postId`. - **`req.params`**: An object containing the dynamic values captured from the URL. For example, a request to `/post/42` gives `req.params.postId === "42"`. ### Route Handlers - A **route handler** is the callback function `(req, res) => { ... }` that runs when a request matches a route. - `req` — contains information about the incoming request (params, query strings, headers, body). - `res` — used to send a response back to the client. ## Response Methods | Method | What it does | | --- | --- | | `res.send()` | Sends a response — plain text, HTML, JSON, or binary data. | | `res.json()` | Sends a JSON response; automatically converts a JavaScript object. | | `res.status(code)` | Sets the HTTP status code; usually chained with `res.send()` or `res.json()`. | | `res.redirect(url)` | Redirects the client to a different URL. | | `res.render(view, data)` | Renders a template engine view and sends the resulting HTML. | ## Chainable Route Handlers with `app.route()` - `app.route(path)` groups multiple HTTP method handlers for the same path to avoid repeating it. ```javascript app .route("/user") .get((req, res) => { res.send("Fetching user data"); }) .post((req, res) => { res.send("Creating a user"); }) .put((req, res) => { res.send("Updating the user"); }) .delete((req, res) => { res.send("Deleting the user"); }); ``` ## Modular Routing with `express.Router()` - `express.Router()` creates a router object — like a mini Express app — that you can define routes on and then mount in the main app. - Mount a router with `app.use(path, router)`. Routes inside the router are relative to the mount path. - **Benefits**: separates concerns, keeps the main file clean, and makes routes reusable and easier to scale. ```javascript // userRoutes.js const express = require("express"); const router = express.Router(); router.get("/", (req, res) => { res.send("List of users"); }); router.get("/:id", (req, res) => { res.send(`User ${req.params.id}`); }); module.exports = router; // app.js app.use("/users", userRoutes); // mounts the router at /users ``` ## Serving Static Files - **Static files**: Assets like images, CSS, and client-side JavaScript that are served directly without server-side processing. - `express.static(directory)` — built-in middleware that serves files from a specified folder. - If a requested file is not found, Express automatically responds with a 404. - No route definitions are needed for individual static files. ```javascript app.use(express.static(path.join(__dirname, "public"))); ``` - You can serve from multiple directories by calling `app.use()` multiple times, optionally with a mount path: ```javascript app.use(express.static(path.join(__dirname, "public"))); app.use("/images", express.static(path.join(__dirname, "images"))); ```