Francesco CiullaThe 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:
O: Open/Close
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);
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
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
That's all. If you have any question, drop a comment below.
