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Hello Express

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Hello, Express!

In this lesson you will create a trivial web application in Express and deploy it to Heroku, where it will be visible to everyone on the Internet.

Hello, File!

  • In your Terminal, make a directory called hello-express with mkdir hello-express

  • Immediately enter the directory with cd hello-express

  • run npm init -y to create a fresh package.json file

  • run npm install express to load the express package into this project

  • Launch the Visual Studio Code text editor and open this hello-express directory

  • Inside this directory make a file named server.js and fill it with the following code:

const express = require('express')
const app = express()
const port = process.env.PORT || 5000

app.get('/', (request, response) => {
  response.send('Hello, World!');
})

app.listen(port, () => console.log(`Listening on port ${port}!`))

Hello, Localhost!

  • Go back to the Terminal and run the app with node server.js

  • Visit http://localhost:5000/ to see it running on your own computer.

Hello, Package!

A package file contains information about how to build and run your app.

In Code, open the file named package.json, it will have a section like this:

"scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },

Add a start script like this:

"scripts": {
    "start": "node server.js",
    "test": "echo \"Error: no test specified\" && exit 1"
  },

Note that the code after start is exactly what you typed in the shell to run the app locally.

You can test this locally by running npm start and refreshing your browser.

Hello, Git!

Now make a git repo for your app.

Remember to press CTRL-C to stop the server

Make sure you are in the correct directory with pwd

git init

git add .

git commit -m "initial commit"

Hello, Heroku!

Heroku uses git for its deploys. Whenever you push a new version of your git repo to Heroku, it automatically deploys the app to the cloud.

heroku create

git push heroku master

If all goes well, you will see a URL on your console, something like this:

remote: https://damp-retreat-99529.herokuapp.com/ deployed to Heroku

Visit this URL in a web browser using copy-and-paste, or use this handy shortcut from the console:

heroku open

High Five!

Give yourself a high five.

high five

You deserve it!

image by Pandark (CC-BY-SA)

Lab: Hello, You!

Now go back to VSCode, and modify the server code so instead of saying "Hello, World!" it says something clever and personalized.

Once you've made the change...

  • test it locally
  • add the changed file to git and commit the change
  • re-deploy to Heroku
  • reload the web page and read your new message
  • give yourself a high five!

Parameters in Express

The special character : means "this is a path parameter"

Example:
Path: /hello/Gandalf
Route: /hello/:friend
Params: {friend: 'Gandalf'}

Express will grab the value from the path itself, and assign it to request.params for you to use later.

Lab: Visualize It

In your server.js file, set up a route /:key, that when visited, prints request.params to the command line.

Then visit http://localhost:5000/value

You should see the params printing to the terminal

Solution
```javascript app.get('/:key', (request,response)=>{ console.log(request.params) // prints {key: 'value'} }) ```

Lab: Hello, Query Friend!

Now change your server.js code so that when you visit a path containing someone's name, it greets the user by that name.

e.g. visiting http://localhost:5000/Gandalf prints "Hello, Gandalf!" on the web page.

Hint You can set up a path parameter in express with a colon `:` followed by the variable name e.g. `/:friend` provides a variable in the `params` object with the key of `friend`
Solution Add the following route to your `server.js` file
```js app.get('/:friend', (req, res) => { res.send('Hello, ' + req.params.friend + "!") }) ```

Resources

Some other Node/Express tutorials: