How to implements middlewares in Harmonia

The objective of this workshop is to practice the implementation of middleware otherwise called service within harmonia to validate data and exchange information between your different middlewares

Getting started

• Git clone this project
• Run npm install
• Setup your .env file in the server folder
• Run npm run db:migrate
• Run npm run dev:server

Project architecture

Diagram allowing you to visualize the architecture of the server folder, certain parts have been deliberately omitted in this diagram to concentrate on the essentials

The part that interests us here is found in the app/service folder, this is where we will declare all our middleware

server
├── database
│   ├── fixtures
│   │   └──  // ✅
│   ├── models
│   │   └──  // ✅
│   ├── client.js
│   ├── schema.sql // ✅
│   └── tables.js  // ✅
├── app
│   ├── routers
│   │   └──  // ✅
│   ├── controllers
│   │   └──  // ✅
│   ├── services # folder who contain others middlewares services (data validator, email service, etc..)
│   │   └──  // MAINTENANT
│   └── config.js
├── tests
│   └──  // bientôt
├── .env.sample
├── .gitignore
├── index.js // ✅
├── package-lock.json
└── package.json

Your mission

Data validation middlewares

You should implement a data validation middleware for an album table which contains at least the following information: title, genre, picture, artist To validate these fields, you must respect at least the following rules: the title must be at least 3 characters long, the genre must be one of the 3 genres mentioned (Rap, Rock, Electro) the case being important.

• To do this, start by creating a new albums.js file in the app/services folder.
• In your middleware validate the data using the rules above
• In case of error send a 400 status code with a message explaining the error
• In case of no error call the next method to pass the rest to the controller
• Apply your middleware to the POST route /albums just before your controller

Validation middleware example :

// Validation middleware
const validateData = (req, res, next) => {
  const { name, age } = req.body;

  if (!name || typeof name !== 'string') {
    return res.status(400).json({ error: 'Name must be a string with at least 3 characters.' });
  }

  if (!age || typeof age !== 'number' || age < 18) {
    return res.status(400).json({ error: 'Age must be a number and at least 18.' });
  }

  next();
};

Valid Request Example:

• URL: http://localhost:3000/albums
• Method: POST
• Body (JSON):

{
  "title": "Revolutionary",
  "genre": "Rock",
  "picture": "http://example.com/pic.jpg",
  "artist": "The Rockers"
}

Expected Response:

{
  "title": "Revolutionary",
  "genre": "Rock",
  "picture": "http://example.com/pic.jpg",
  "artist": "The Rockers"
}

Invalid Request Example (Title too short):

• URL: http://localhost:3000/albums
• Method: POST
• Body (JSON):

{
  "title": "Hi",
  "genre": "Rock",
  "picture": "http://example.com/pic.jpg",
  "artist": "The Rockers"
}

Expected Response:

{
  "error": "Title must be at least 3 characters long."
}

Data transfert betweens middlewares

In Express.js, middleware functions are pieces of code that execute during the lifecycle of a request to the server. They have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle. Middleware functions can perform a variety of tasks, such as executing code, modifying the request and response objects, ending the request-response cycle, and calling the next middleware function.

How Data Transfer Works Between Middlewares

Middleware functions are executed sequentially, in the order they are defined in the application. The primary way data is transferred between middleware functions is through the req and res objects.

Example of Middleware Data Transfer :

// Validation middleware
const validateData = (req, res, next) => {
  const { name, age } = req.body;

  if (!name || typeof name !== 'string' || name.length < 3) {
    return res.status(400).json({ error: 'Name must be a string with at least 3 characters.' });
  }

  if (!age || typeof age !== 'number' || age < 18) {
    return res.status(400).json({ error: 'Age must be a number and at least 18.' });
  }

  next();
};

// Transformation middleware
const transformData = (req, res, next) => {
  req.body.name = req.body.name.trim();
  req.isAdmin = true // we can also pass new data directly in the request object !
  next();
};

// Controller middlewares
const createUser = async (req, res, next) => {
  // call the model to insert data in the db & send response to the client
}

// Route with middleware
app.post('/user', validateData, transformData, createUser);

Now it's your turn, add in the albums file previously created in the service folder a middleware function which must add to the req object an isAdmin property set to false and move on to the next middleware using the next method

Then implement this middleware on the post /albums route before the validation middleware done above then modify the validation middleware so that it returns a 403 status in response in the case where the isAdmin property defined in the request object is false

Now test changing the value of the isAdmin property from false to true, you should now have a status 201 returned by your api

Bonus mission

To simplify and strengthen validation, modify your validation middleware to use a library like joi.

joi get started