RESTful API

A RESTful API is a method to expose data and perform CRUD operations on top of the HTTP protocol.
In this exercise we will be creating a web server using NodeJS and the ExpressJS framework. Data will be stored in MySQL and we will be using Sequelize to model the tables and relations.
1. How to initialize a NodeJS app?
In order to initialize a node app open a directory where you want to create the app and  execute the following command in the terminal
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npm init
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You will be prompted to fill in the following details:
Name
Version
Description
Entry point
Test command
Git repository
Keywords
Author
License
Before completing this step you need to confirm the data is correct:
If everything went well a pagckage.json file is generated
TODO: check if the package.json exists
TODO: create a new file called server.js
2. How to build a HTTP server using ExpressJS?
ExpressJS is a lightweight, but powerful framework for building web applications or web services.
To include this dependency in your project run the following npm command
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npm install express --save
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Write the code in the server.js file
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const express = require('express')
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Next create a constant calledapp
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const app = express()
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Use the static files middleware to serve your webpage assets express.static
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app.use('/', express.static('frontend'))
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The first parameter is the path where you want the files to be served. The second parameter is the middleware configured with the directory where the files are located.
Configure the server to listen to 8080 port for HTTP.
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app.listen(8080)
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TODO: create a directory called frontend
TODO: add a new file called index.html
TODO: open the app in the browser (http://localhost:8080) - make sure to replace localhost with your IP if you work in the cloud
3. How to install MySQL and create the database?
If you haven't done this already, follow the tutorial published here.
Login into the console and create a database called profile:
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create database profile;
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TODO: check if the database was created by executing show databases;
TODO: exit the console by typing exitand pressing enter
4. How to connect your database from NodeJS using Sequelize?
Sequelize is an object oriented library that provides a high level mapping between the database and your code. In practice we call this pattern object-relational mapping (ORM). It allows us to represent tables from the database by creating models and relationships. On each model basic CRUD (Create, Read, Update, Delete) operations are exposed. 
Sequelize is a wrapper on multiple database drivers. In our example we will use mysql2. Both the library and the drivers will be installed using npm.
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npm install --save sequelize
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npm install --save mysql2
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Next edit the server.js file and create an instance of the Sequelize class
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const Sequelize = require('sequelize')
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​
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const sequelize = new Sequelize('profile', 'root', '', {
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    dialect: "mysql",
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    host: "localhost"
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})
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Notice the difference between sequelize as an object and Sequelize as a class. 
The first parameter of the constructor is the database name, the second parameter is the username and the third is the password. Further options are passed as an object in the fourth parameter.
In order to perform the database connection we call the  authenticate() method.
As many Sequelize methods, the authentication will be asynchronous, thus it will return a promise.  Promise. When the promise is fulfilled the function passed as a parameter on the then() method is called. If the promise gets rejected the function on the catch() method will be called.
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sequelize.authenticate().then(() => {
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    console.log("Connected to database")
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}).catch((err) => {
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    console.log(err)
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    console.log("Unable to connect to database")
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})
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TODO: run node server.js
TODO: check the console for the message: Connected to database
5. How to define models for my messages table?
The fist parameter is the name of the table. As a convention is preferred to define the tables using English language and use the plural version. The second parameter is an object that describes the structure of the table as key value pairs.
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const Messages = sequelize.define('messages', {
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    subject: Sequelize.STRING,
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    name: Sequelize.STRING,
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    message: Sequelize.TEXT
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})
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Extended documentation is here - http://docs.sequelizejs.com/manual/tutorial/models-definition.html​
Supported data type - http://docs.sequelizejs.com/manual/tutorial/models-definition.html#data-types​
6. How to create the tables in the database?
Using sync() all the models will be created in your database
Note that by adding {force: true} existent tables will be recreated from scratch
To create the tables we are exposing a GET /createdb endpoint
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app.get('/createdb', (request, response) => {
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    sequelize.sync({force:true}).then(() => {
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        response.status(200).send('tables created')
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    }).catch((err) => {
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        console.log(err)
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        response.status(200).send('could not create tables')
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    })
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})
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TODO: open the browser and navigate to http://localhost:8080/createdb
TODO: login to the mysql console type use profile and show tables; to check if the tables were created
HTTP Methods
Now that we have the model let's implement the Create, Read, Update and Delete operations.
metode http
7. How to create resources using POST?
We consider the following endpoint
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POST /messages
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The client will perform a HTTP request containing data in json or urlencoded. To parse the data we need to add two middleware functions. The endpoint is defined using app.post
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app.use(express.json())
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app.use(express.urlencoded())
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​
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//definire endpoint POST /messages
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app.post('/messages', (request, response) => {
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    Messages.create(request.body).then((result) => {
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        response.status(201).json(result)
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    }).catch((err) => {
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        response.status(500).send("resource not created")
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    })
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})
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 create ---> INSERT INTO messages (`subject`, `name`, `message`) VALUES ('test','test','test').
postman post method
1.Choose POST
2.Type the URL of the resource
3.Choose raw a body type
4.Choose application/json content type
5.Press Send
8. How to expose data from a table using GET?
Considering the following endpoints. The first one will retrieve all the entries in the table. The second will retrieved one entry identified by the ID 1.
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GET /messages
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GET /messages/1
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Details on data queries: http://docs.sequelizejs.com/manual/tutorial/querying.html​
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app.get('/messages', (request, response) => {
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    Messages.findAll().then((results) => {
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        response.status(200).json(results)
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    })
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})
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​
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app.get('/messages/:id', (request, response) => {
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    Messages.findByPk(request.params.id).then((result) => {
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        if(result) {
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            response.status(200).json(result)
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        } else {
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            response.status(404).send('resource not found')
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        }
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    }).catch((err) => {
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        console.log(err)
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        response.status(500).send('database error')
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    })
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})
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TODO: test the endpoint in POSTMAN
9. How to update a resource using PUT?
Considering the following endpoint
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PUT /messages/1
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First, the resource is retrieved by ID. If the resource was not found the server's response is 404 NOT FOUND
If the resource was fount we call the update() method with the payload sent by the client in the body.
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app.put('/messages/:id', (request, response) => {
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    Messages.findByPk(request.params.id).then((message) => {
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        if(message) {
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            message.update(request.body).then((result) => {
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                response.status(201).json(result)
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            }).catch((err) => {
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                console.log(err)
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                response.status(500).send('database error')
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            })
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        } else {
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            response.status(404).send('resource not found')
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        }
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    }).catch((err) => {
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        console.log(err)
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        response.status(500).send('database error')
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    })
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})
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TODO: test the endpoint in POSTMAN
postman put
10. How to remove resources using delete DELETE?
Considering the following endpoint
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DELETE /messages/1
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First, the resources is retrieved by id. If the resource was found we are calling the destroy method to send the delete instruction. This will return a Promise that needs to be treated accordingly. If the delete was successful the server's response is 204 NO CONTENT
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app.delete('/messages/:id', (request, response) => {
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    Messages.findByPk(request.params.id).then((message) => {
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        if(message) {
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            message.destroy().then((result) => {
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                response.status(204).send()
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            }).catch((err) => {
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                console.log(err)
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                response.status(500).send('database error')
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            })
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        } else {
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            response.status(404).send('resource not found')
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        }
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    }).catch((err) => {
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        console.log(err)
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        response.status(500).send('database error')
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    })
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})
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TODO: test the endpoint in POSTMAN
11. Putting it all together (server.js)
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const express = require('express')
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const Sequelize = require('sequelize')
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​
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const sequelize = new Sequelize('profile', 'root', '', {
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    dialect: "mysql",
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    host: "localhost"
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})
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​
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sequelize.authenticate().then(() => {
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    console.log("Connected to database")
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}).catch((err) => {
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    console.log(err)
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    console.log("Unable to connect to database")
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})
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​
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const Messages = sequelize.define('messages', {
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    subject: Sequelize.STRING,
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    name: Sequelize.STRING,
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    message: Sequelize.TEXT
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})
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​
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const app = express()
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app.use('/', express.static('frontend'))
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​
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app.use(express.json())
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app.use(express.urlencoded())
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​
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app.get('/createdb', (request, response) => {
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    sequelize.sync({force:true}).then(() => {
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        response.status(200).send('tables created')
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    }).catch((err) => {
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        console.log(err)
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        response.status(200).send('could not create tables')
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    })
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})
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​
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//definire endpoint POST /messages
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app.post('/messages', (request, response) => {
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    Messages.create(request.body).then((result) => {
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        response.status(201).json(result)
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    }).catch((err) => {
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        response.status(500).send("resource not created")
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    })
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})
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​
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​
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app.get('/messages', (request, response) => {
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    Messages.findAll().then((results) => {
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        response.status(200).json(results)
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    })
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})
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​
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app.get('/messages/:id', (request, response) => {
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    Messages.findByPk(request.params.id).then((result) => {
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        if(result) {
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            response.status(200).json(result)
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        } else {
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            response.status(404).send('resource not found')
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        }
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    }).catch((err) => {
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        console.log(err)
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        response.status(500).send('database error')
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    })
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})
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​
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app.put('/messages/:id', (request, response) => {
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    Messages.findByPk(request.params.id).then((message) => {
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        if(message) {
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            message.update(request.body).then((result) => {
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                response.status(201).json(result)
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            }).catch((err) => {
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                console.log(err)
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                response.status(500).send('database error')
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            })
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        } else {
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            response.status(404).send('resource not found')
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        }
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    }).catch((err) => {
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        console.log(err)
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        response.status(500).send('database error')
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    })
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})
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​
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​
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app.delete('/messages/:id', (request, response) => {
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    Messages.findByPk(request.params.id).then((message) => {
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        if(message) {
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            message.destroy().then((result) => {
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                response.status(204).send()
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            }).catch((err) => {
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                console.log(err)
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                response.status(500).send('database error')
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            })
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        } else {
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            response.status(404).send('resource not found')
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        }
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    }).catch((err) => {
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        console.log(err)
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        response.status(500).send('database error')
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    })
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})
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​
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​
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app.listen(8080)
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12. Using async/await keywords to simplify the code
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const express = require('express')
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const Sequelize = require('sequelize')
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​
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const sequelize = new Sequelize('profile', 'root', '', {
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    dialect: "mysql",
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    host: "localhost"
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})
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​
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sequelize.authenticate().then(() => {
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    console.log("Connected to database")
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}).catch((err) => {
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    console.log(err)
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    console.log("Unable to connect to database")
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})
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​
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const Messages = sequelize.define('messages', {
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    subject: Sequelize.STRING,
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    name: Sequelize.STRING,
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    message: Sequelize.TEXT
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})
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​
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const app = express()
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app.use('/', express.static('frontend'))
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​
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app.use(express.json())
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app.use(express.urlencoded())
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​
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app.get('/createdb', async (request, response) => {
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    try {
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        await sequelize.sync({force:true})
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        response.status(200).send('tables created')
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    } catch(err) {
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        console.log(err)
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        response.status(200).send('could not create tables')
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    }
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})
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​
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//definire endpoint POST /messages
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app.post('/messages', async (request, response) => {
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    try {
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        let result = await Messages.create(request.body)
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        response.status(201).json(result)
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    } catch(err) {
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        console.log(err)
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        response.status(500).send("resource not created")
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    }
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})
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​
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​
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app.get('/messages', async (request, response) => {
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    try {
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        let results = await Messages.findAll()
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        response.status(201).json(results)
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    } catch(err) {
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        console.log(err)
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        response.status(500).send("server error")
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    }
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})
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​
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app.get('/messages/:id', async (request, response) => {
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    try {
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        let result = await Messages.findByPk(request.params.id)
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        if(result) {
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            response.status(200).json(result)
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        } else {
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            response.status(404).send('resource not found')
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        }
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    } catch(err) {
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        console.log(err)
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        response.status(500).send('database error')
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    }
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})
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​
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app.put('/messages/:id', async (request, response) => {
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    try {
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        let message = await Messages.findByPk(request.params.id)
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        if(message) {
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            let result = await message.update(request.body)
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            response.status(201).json(result)
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        } else {
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            response.status(404).send('resource not found')
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        }
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    } catch(err) {
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        console.log(err)
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        response.status(500).send('database error')
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    }
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})
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​
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​
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app.delete('/messages/:id', async (request, response) => {
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    try {
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        let message = await Messages.findByPk(request.params.id)
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        if(message) {
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            await message.destroy()
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            response.status(204).send()
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        } else {
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            response.status(404).send('resource not found')
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        }
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    } catch(err) {
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        console.log(err)
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        response.status(500).send('database error')
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    }
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})
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​
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​
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app.listen(8080)
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Next steps...
Get more in depth knowledge by reading this articles
​https://www.restapitutorial.com/​
​https://medium.com/pixelpoint/oh-man-look-at-your-api-22f330ab80d5​