> For the complete documentation index, see [llms.txt](https://ga0-2.gitbook.io/seifxr10anz-content/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ga0-2.gitbook.io/seifxr10anz-content/week-14/day-1-promises-and-intro-to-express/intro-to-express.md).
# Intro to Express
#### Create a New Express Project
```bash
mkdir starlight
cd starlight
touch app.js
npm init -y
npm i express
```
#### Set Up a Basic Express Server
In `app.js`:
```javascript
const express = require('express')
const app = express()
const port = 3000
app.listen(port, () => {
console.log(`Server listening on port ${port}`)
})
```
Then start the server with:
```bash
node app.js
```
This starts a server on port 3000 and… does nothing.
#### GET Routes
To respond to GET requests, use the `app.get` method:
```javascript
app.get('/', () => {
console.log('Index route / hit')
})
```
Web servers are long-running processes. If you make any changes to the code, you'll need to stop the server with `Control-c` and then start it again with `node app.js`.
Once you're restarted the server, visit `localhost:3000` in your web browser, you should see the text `"Index route / hit"` in your terminal but nothing in your web browser.
To send a response back to the browser, you can use `res.send`:
```javascript
app.get('/', (req, res) => {
console.log('Index route / hit')
res.send('Hello world')
})
```
Or return some HTML:
```javascript
app.get('/', (req, res) => {
console.log('Index route / hit')
res.send('Hello world
')
})
```
Restart the server and reload `localhost:3000`. You should now see the text `Hello world` in the web browser.
**NOTE** You can only send one response for every request. If you try to send multiple responses you'll get an error.
#### `req` & `res`
* `req` is an object containing information about the HTTP request (HTTP method, request body, ip address, query params, etc).
* `res` is an object representing the HTTP response, which is used to send something back to the client.
#### POST Routes
To handle form submissions, you have to tell Express to parse the incoming form data using the `urlencoded` middleware. In the POST route, the form data is available in `req.body`.
```javascript
app.use(express.urlencoded({ extended: true }))
app.post('/', (req, res) => {
res.send(req.body)
})
```
#### Route Parameters
To capture the value of the dynamic path segment of the URL, define a GET route with a route parameter (note the `:` in `:bookID`). The value of the route parameter can be accessed in the `req.params` object.
```javascript
app.get('/books/:bookID', (req, res) => {
console.log(req.params.bookID)
})
```
You can define as many route params as you want as long as they're named differently:
```javascript
app.get('/hello/:firstname/:lastname', (req, res) => {
console.log(req.params)
res.send(`Hello ${req.params.firstname} ${req.params.lastname}!`)
})
```
In your browser, go to `localhost:3000/hello/taylor/hebert`.
#### Query Parameters
To get the values in a query string, use `req.query`.
```javascript
// localhost:3000/books?page=2&limit=12
app.get('/books', (req, res) => {
console.log(req.query)
})
```
#### nodemon
Restarting the server manually any time you want to test changes to the code is tedious. Using an npm package called `nodemon`, you can automate this process and streamline your workflow.
```bash
npm i -g nodemon
```
The `-g/--global` flag installs an npm package so that any project can use it. **NOTE** If you're using `nvm`, globally installed packages will only be available for the version of node you were using when you installed the package.
Start your server with:
```bash
nodemon
```
Nodemon will look for the `"main"` property in your package.json and execute that .js file. If you have `"app.js"` as `"main"` in package.json then the above command is equivalent to:
```bash
nodemon app.js
```
Nodemon will watch for any changes you to make to your code and restart the server automatically.
---
# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.
## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.
Perform an HTTP GET request on the current page URL with the `ask` query parameter:
```
GET https://ga0-2.gitbook.io/seifxr10anz-content/week-14/day-1-promises-and-intro-to-express/intro-to-express.md?ask=
```
The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.
Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.