S.O.L.I.D. Principles around You, in JavaScript

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The SOLID principles are a set of software design principles, that help us to understand how we can structure our code in order to be robust, maintainable, flexible as much as possible

Here come the S.O.L.I.D. principles:


S - Single Responsibility Principle

Alt Text

Any function must be responsible for doing only ONE thing. Only one potential change in the software’s specification should be able to affect the specification of the class.

Example : Let's say we want to validate a form, then create a user in a DB

NO

/* A function with such a name is a symptom of ignoring the Single Responsibility Principle
*  Validation and Specific implementation of the user creation is strongly coupled.
*  That's not good
*/ 
validateAndCreatePostgresUser = (name, password, email) => {   

  //Call an external function to validate the user form
  const isFormValid = testForm(name, password, email); 

  //Form is Valid
  if(isFormValid){
    CreateUser(name, password, email) //Specific implementation of the user creation!
  }
}

YES

//Only Validate
validateRequest = (req) => {

  //Call an external function to validate the user form
  const isFormValid = testForm(name, password, email); 

  //Form is Valid
  if(isFormValid){
    createUser(req); // implemented in another function/module 
  }
}

//Only Create User in the Database
createUser = (req) => CreateUser(req.name, req.password, req.email)

/*A further step is to declare this function in another file
* and import it into this one.
*/

This seems a pretty little change, but decouples the logic of validation from the user creation, which could change in the future, for many reasons!


O - Open-Closed Principle

Alt Text

Software systems must be allowed to change their behavior by adding new code rather than changing the existing code.

Open for extension, but Closed to modification

If we have something like this:

const roles = ["ADMIN", "USER"]
checkRole = (user) => {
  if(roles.includes(user.role)){
    return true; 
  }else{
    return false
  }
}

//Test role
checkRole("ADMIN"); //true
checkRole("Foo"); //false

And we want to add a superuser, for any reason, instead of modifying the existing code (or maybe we just can't modify it), we could do it in another function.

//UNTOUCHABLE CODE!!!
const roles = ["ADMIN", "USER"]
checkRole = (user) => {
  if(roles.includes(user.role)){
    return true; 
  }else{
    return false
  }
}
//UNTOUCHABLE CODE!!!

//We can define a function to add a new role with this function
addRole(role){
  roles.push(role)
}

//Call the function with the new role to add to the existing ones
addRole("SUPERUSER");

//Test role
checkRole("ADMIN"); //true
checkRole("Foo"); //false
checkRole("SUPERUSER"); //true


L - Liskov Substitution Principle

Alt Text

Build software systems from interchangeable parts.

Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.

class Job {
  constructor(customer) {
    this.customer = customer;
    this.calculateFee = function () {
      console.log("calculate price"); //Add price logic
    };
  }
  Simple(customer) {
    this.calculateFee(customer);
  }
  Pro(customer) {
    this.calculateFee(customer);
    console.log("Add pro services"); //additional functionalities
  }
}



const a = new Job("Francesco");
a.Simple(); 
//Output:
//calculate price


a.Pro();
//Output: 
//calculate price 
//Add pro services...


I - Interface Segregation Principle

Alt Text

Many client-specific interfaces are better than one general-purpose interface.

We don't have interfaces in Javascript, but let's see this example

NO

//Validate in any case
class User {
  constructor(username, password) {
    this.username = username;
    this.password = password;
    this.initiateUser();
  }
  initiateUser() {
    this.username = this.username;
    this.validateUser()
  }

  validateUser = (user, pass) => {
    console.log("validating..."); //insert validation logic here!
  }
}
const user = new User("Francesco", "123456");
console.log(user);
// validating...
// User {
//   validateUser: [Function: validateUser],
//   username: 'Francesco',
//   password: '123456'
// }

YES

//ISP: Validate only if it is necessary
class UserISP {
  constructor(username, password, validate) {
    this.username = username;
    this.password = password;
    this.validate = validate;

    if (validate) {
      this.initiateUser(username, password);
    } else {
      console.log("no validation required"); 
    }
  }

  initiateUser() {
    this.validateUser(this.username, this.password);
  }

  validateUser = (username, password) => {
    console.log("validating...");
  }
}

//User with validation required
console.log(new UserISP("Francesco", "123456", true));
// validating...
// UserISP {
//   validateUser: [Function: validateUser],
//   username: 'Francesco',
//   password: '123456',
//   validate: true
// }


//User with no validation required
console.log(new UserISP("guest", "guest", false));
// no validation required
// UserISP {
//   validateUser: [Function: validateUser],
//   username: 'guest',
//   password: 'guest',
//   validate: false
// }


D - Dependency Inversion Principle

Alt Text

Abstractions must not depend on details.

Details must depend on abstractions.

NO

//The Http Request depends on the setState function, which is a detail
http.get("http://address/api/examples", (res) => {
 this.setState({
  key1: res.value1,
  key2: res.value2,
  key3: res.value3
 });
});

YES

//Http request
const httpRequest = (url, setState) => {
 http.get(url, (res) => setState.setValues(res))
};

//State set in another function
const setState = {
 setValues: (res) => {
  this.setState({
    key1: res.value1,
    key2: res.value2,
    key3: res.value3
  })
 }
}

//Http request, state set in a different function
httpRequest("http://address/api/examples", setState);

I would like to thank my friend Oleksii Trekhleb for the contribution to this article.

Oleksii is the original author of this legendary GitHub repository github.com/trekhleb/javascript-algorithms

Follow him on Twitter twitter.com/Trekhleb


In Conclusion...

The main goal of the SOLID principles is that any software should tolerate change and should be easy to understand.

The S.O.L.I.D. principles can be very useful to write code:

  • Easy to understand
  • Where things are where they're supposed to be
  • Where classes do what they were intended to do
  • That can be easily adjusted and extended without bugs
  • That separates the abstraction from the implementation
  • That allows to easily swap implementation (Db, Api, frameworks, ...)
  • Easily testable

Sunil Kumar's photo

Great article Francesco!

Francesco Ciulla's photo

Thank you Sunil and thanks for the following!

Mohd Shad Mirza's photo

Loved it.

Francesco Ciulla's photo

Thank you Mohd, really appreciated!

Apoorv Tyagi's photo

Great Article Francesco Ciulla!!! Good to see you here :)

Mohit Sehgal's photo

What a start .. Loved it

Francesco Ciulla's photo

Thank you Mohit! I will check your articles!

Gyen Abubakar's photo

I once took a course on Design Principles, dumped it before I could finish off the second module, which was about the SOLID principle.

Wish I had seen this. Amazing write-up, Francesco!

Francesco Ciulla's photo

Thank you Gyen! I hope it was useful as a refresher!

Maximilian Berkmann's photo

Lovely article!

Francesco Ciulla's photo

Thank you Maximilian!

Chris Bongers's photo

Awesome write-up Francesco! Love SOLID principles

Francesco Ciulla's photo

Thank you Chris!

Tapas Adhikary's photo

Francesco Ciulla, Awesome writeup, and great content.

Question: Are the images snaps taken from the white-board or using a tool? They look awesome and very relevant to each of the sections. Nice work!

Edidiong Asikpo's photo

Fantastic and insightful article Francesco Ciulla. Thanks for sharing.

Francesco Ciulla's photo

Hey Edidiong! You have a different picture on Twitter but I recognize you! :)

Thank you so much!

Your DevOps Guy's photo

The visuals definitely help people get the gist of every principle. Then, coding becomes easy.

Good article, Francesco Ciulla!

Salil Kapoor's photo

Well, Explained! I read them many times but I wasn't clear about the last three. It's really well explained. Many thanks!

Peter Thaleikis's photo

Awesome article! Thanks!