curriculum/challenges/english/blocks/lecture-working-with-express/69a6f5e1264af50c407aecfe.md
In the previous lesson, you created a project folder and installed Express:
mkdir my-app
cd my-app
npm init -y
npm install express
As a reminder, these commands create a new project folder, cd into the project folder, initialize the project and create a basic package.json file, and install Express.
Next, open up a code editor, create a file named index.js, and write your first Express server:
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}`);
});
You can run your code with the following command in the terminal:
node index.js
Open your browser to http://localhost:3000 and you'll see the message "Hello World!"
That’s how fast it is to get started with Express! Now let’s walk through the code step-by-step.
In the first line of code, you are loading the Express library and assigning that to a variable called express:
const express = require("express");
The express variable is a factory function. When you call it, it creates and returns a new Express application instance:
const app = express();
app now has access to all the methods provided by Express, such as .get(), .post(), .listen(), and so on. This is your main server object.
The next line defines the port variable. A port is a network endpoint that your application listens on, and 3000 is a commonly used port for local development:
const port = 3000;
Now, let’s take a look at the next part here:
app.get("/", (req, res) => {
res.send("Hello World!");
});
This block of code defines a route and tells the server how to respond to requests. app.get() registers a handler for HTTP GET requests. A GET request is a commonly used HTTP method for retrieving data.
The "/" argument inside app.get() represents the route path. In this case, it points to the root URL of the application.
The (req, res) => { ... } part is a callback function that runs when a request matches this route. The req object contains information about the incoming request, and the res object is used to send a response back to the client.
Finally, res.send("Hello World!") sends a response to the client. In this example, the response is the plain text "Hello World!".
Now, let’s take a look at the last piece of code here:
app.listen(port, () => {
console.log(`Example app listening on port ${port}`);
});
The app.listen() method starts the Express application and tells Node to listen for incoming HTTP requests on the specified port, in this case, 3000.
Once the server starts, the callback function executes and the message Example app listening on port 3000 is logged to the console, confirming that the Express app has started successfully and is ready to receive requests.
What does this Express route do?
app.get("/", (req, res) => {
res.send("Hello World!");
});
It deletes the homepage.
What happens when a user visits /?
It creates a new database.
What happens when a user visits /?
It sends a response to the root URL.
It starts the server.
What happens when a user visits /?
3
What does app.listen() do?
app.listen(port, () => {
console.log(`Example app listening on port ${port}`);
});
It stops the app.
Think about what must happen before you can visit http://localhost:3000 in a browser.
It sends a 404 code back to the client.
Think about what must happen before you can visit http://localhost:3000 in a browser.
It defines the port for the app.
Think about what must happen before you can visit http://localhost:3000 in a browser.
It starts the server and listens for incoming requests on a port.
4
Which of these is a root URL?
http://localhost:3000/posts
Recall that the root URL does not include any additional path segments.
http://localhost:3000/posts?id=34
Recall that the root URL does not include any additional path segments.
http://localhost:3000
http://localhost:3000/posts+comments
Recall that the root URL does not include any additional path segments.
3