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Part 2

Initializing Our npm Package and Installing Dependencies

Navigate inside the directory which contains the files from the previous section and run this command

npm init

For the first three prompts, just hit Enter, for the next prompt

entry point: (index.js)

type in server.js. And then just hit Enter for the rest of the prompts.

This initializes our workspace as a Node.js package. This does two things: (1) It allows us to install packages through npm, and (2) it creates the package.json file, which handles our package's metadata and its dependencies. This package.json will look something like

{
  "name": "create_your_own_api",
  "version": "1.0.0",
  "description": "Welcome to this codelab on creating your own web api! Before getting started, let's make sure you have some things installed. \t* [node.js](nodejs.org/en/download)",
  "main": "server.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC"
}

Now, we want to install our dependencies

npm install --save express body-parser

If this worked, you should now see a package-lock.json file inside your directory, too. We won't need to mess with this.

Open up package.json again. There should now be a bit at the bottom that looks like

"dependencies": {
    "body-parser": "^1.18.2",
    "express": "^4.16.2"
}

Let's write some code!

Create a new file in your workspace directory called server.js (note that we use this specific name because it's specified with the main key in our package.json file)

Inside server.js write:

const express = require('express');

let app = express();

let port = 3000;
app.listen(port);
console.log("Listening on port " + port);

Go ahead and run this:

node server.js

and go to localhost:3000 in your browser. You should see the following message

Cannot GET /

So on the one hand, this is great news! The server is listening for requests from the browser (seriously, see what happens when you are not running the above command), but on the other, it's not getting any file or anything. Let's change that. Modify server.js to look like the following

...
let app = express();

app.get('/', (req, res, next) => {
	res.send('Hello');
});

let port = 3000;
...

Whoa, Whoa, Whoa, what did you just do?

Let's break it down.

app.get('/', (req, res, next) => {

This is telling the app to listen for a get request at location /. If you're not familiar with the concept, a get request is an HTTP Request to get something. Normally an HTML file or (in this case) just some text. The '/' bit tells it to listen for a get request at that address. If we had put '/apples' instead, it would listen for a get request at localhost:3000/apples

The next part (req, res, next) => is just a function definition in javascript. (req, res, next) is the argument list. This is a callback function for the get function. Meaning this function is called whenever we receive a get request at location /. I realize this is a lot to take in right now, let's see it in action.

Rerun server.js

node server.js

and once again navigate to localhost:3000 in your browser. What do you see?

Part 3