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

https://github.com/trekhleb/javascript-algorithms

Follow him on Twitter

https://twitter.com/Trekhleb

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

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 declarel 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

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

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

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

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 );

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: