Hey guys, today we're going to build an API using NodeJs and ExpressJS. This will be kind an 'Hello world' api but, maybe we do something more (challenging is always a good deal). Ok, so prepare the coffe and let's code!

Prepare the enviroment

To begin, you need to have the node installed (check the process here, to install it on linux, see this section), it will install the npm too during the process, the npm is the Node Package Manager, used to handle the node packages used on the project.

Now that we have the enviroment, we need to setup the project. For that we will create a folder to keep it and we're gonna call it as 'HelloUniverse' (The 'hello universe' is kind of my initiation rite, a step beyond the hello world. To start off on the right foot) and go into it. Now, let's start the project by running the command npm init -y on the terminal. The npm initcommand initializes the project, asking a few questions to fullfil the information about the project, when used with the -y flag, it creates a project with the basic information. Et voilà, now you can see a package.json file like this:

{
  "name": "HelloUniverse",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}

Yes, dev! Can you feel it? That feeling of "Fuck, is being born!!"?

Installing the packages

Ok, now that we have the project we're going to install the packages needed. For the basic step is just the Express, so we'll run the command npm install express -s. After installing it, you can see that it has created the folder node_modules/, on this folder the npm will store the packages installed and his dependencies, and the package-lock.json, this file controls all the dependencies installed and his dependencies (I install **A, and it has the dependencies **B* and C, this file controls this)*.
During the development we're going to use the nodemon package, for that we run the npm install nodemon -D. The -D or --dev flag tell the npm that is a dependency used for developing, not needed on production.

Creating the API

Ok, so, I'm gonna do this like I learned and feels cleaner. To this we're going to create a src folder, resulting on the following structure:

-HelloUniverse/
\_node_modules/
  _src/
  _package.json
  _package-lock.json

Ok, now we go to src/ and create the app.js file and start it importing the express package:

const express = require('express');

Then, we create the instance:

const app = express();

Almost done, now we're going to set the endpoints, or the routes, of our api. For this, we'll set a constant named routes that will handle them.

const routes = express.Router();

And now we create the basic route, the '/':

routes.get('/', (request, response) => {
    return response.send('hello');
})

So, every time it receives a request on '/', it answers with 'hello'. But our api wont hear it yet, we need 2 more steps for that:

app.use(routes) //So the api will use the routes we defined

app.listen(3333) //Here we setted the port the api will listen.

On the end, our code will look like this:

const express = require('express');

const app = express();

const routes = express.Router();

routes.get('/', (request, response) => {
    return response.send('hello');
})

app.use(routes);

app.listen('3333');

Let's add a few tips to make it better:

app.use(express.json()); //Add this before importing the routes (app.use(routes));

Starting the API

All ready, let's run! We're gonna do some tricks to help us, part of that was installing the nodemon. We're going to set npm commands. For this, we go to the package.json and add the following line on the scripts part:

    "start":"nodemon src/app.js"

This means that everytime we run the npm start command, it will call the nodemon and run the src/app.js file. The nodemon will keep watching all files with the .js,.mjs,.json extension. So, our scripts section will be like this:

"scripts": {
    "start":"nodemon src/app.js",
    "test": "echo \"Error: no test specified\" && exit 1"
  }

Now, go to http://localhost:3333 on your browser and see the magic!

Let's play!

Ok, I won't explain here all the kind of parameters you can send on a request because I know it, but don't know how to explain it. So, here is a little guide:

• http://localhost.com/:id You can have request params, that are params passed as part of the URL;
• http://localhost.com/?id=xxxx you can have query params, that are params passed as queries on the URL;
• You can have body params, that are informations sent trough the body of the request;
• Or header params, passed trough the header of the params.

The first challenge we're going to do is build a GET request with a request param to show us an especific message. For this, we add the following route:

routes.get('/user/:user', (request, response) => { 
    const user = request.params.user
    return response.send('hello ' + user);
})

So, when you go to http://localhost:3333/user/something, for example, it will return the hello something.