OOP Best Practices in JavaScript

OOP Best Practices
Haim Michael
April 11th
, 2022
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to the respective owners.
JavaScript
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life michael

© 2008 Haim Michael 20150805
About Me
 My name is Haim Michael. I was born in Israel, and I live in
Tel-Aviv. One of the best cities in the world.
 I am a father for two. Jonathan (6 years old), and Shira (4
years old).
 I love sport, and I am addicted to Snowboarding.
 I have passion for learning programming, and teaching.

© 2008 Haim Michael 20150805
About Me
 I have more than 25 years of experience in computer
programming.
CC++ (30+ Years), Java (25+ Years), JavaScript (25+
Years), PHP (17+ Years),C# (16+ Years), Python (14+
Years), Scala (14+ Years), Kotlin (8+ Years), TypeScript (8+
Years), and Swift (8+ Years).

© 2008 Haim Michael 20150805
Introduction
 OOP in JavaScript is based on prototypical inheritance. As of
ECMAScript 2016 (AKA ES6) we can define classes, and we
can define classes that extend others.
 With the emergence of C++ nearly 40 years ago, as well as
with the emergence of many other programming languages
that were based on C++, best practices have evolved.

© 2008 Haim Michael 20150805
Best Practice
 Apart of the SOLID design principles, during the last four
decades, we can identify best practices that continuously
evolve in various object oriented programming languages,
such as Java, Scala, C#, Swift, Kotlin, Python, and others.
 Some of these best practices are based on the SOLID
design principles.

© 2008 Haim Michael 20150805
Code Sample
class Person {
#id
#name
constructor(id,name) {
console.log("inside Person constructor");
this.id = id;
this.name = name;
}
set id(val){
console.log("setting id")
if (val>0) {
this.#id = val;
} else {
this.#id = 1000000000;
}
}

© 2008 Haim Michael 20150805
Code Sample
get id(){
console.log("getting id");
return this.#id;
}
set name(val){
console.log("setting name")
if (val!=="") {
this.#name = val;
} else {
this.#name = 'noname';
}
}
get name(){
console.log("getting name");
return this.#name;
}
toString() {
return "id="+this.id+" name="+this.name;
}
}

© 2008 Haim Michael 20150805
Code Sample
class Student extends Person {
#average
constructor(id,name,average) {
console.log("inside Student constructor");
super(id,name);
this.average = average;
}
set average(val){
console.log("setting average")
if (val>=0 && val<=100) {
this.#average = val;
} else {
this.#average = 0;
}
}

© 2008 Haim Michael 20150805
Code Sample
get average(){
console.log("getting average");
return this.#average;
}
toString() {
return super.toString() + " average=" + this.average;
}
}
let ob = new Student(123123123,"mosh",92);
console.log(ob.toString());

© 2008 Haim Michael 20150805
The Private Access Modifier
 In most cases, the variables we define in a class should be
private. Doing so, we will actually force the use of the
setter method. The setter method can include a validation
check for the assigned value.
There are very few cases where it might be useful to avoid
the use of the private accessibility.

© 2008 Haim Michael 20150805
Constructors Chain of Responsibility
 When defining a class that extends another class, the
constructor in our class should include a call to the
constructor in the parent class (using the super keyword).
 In addition to the invocation of the constructor that was
defined in our class, the other constructors (in the classes
that our class extends from) should do their part as well.
Doing so we will be in accordance with the single
responsibility principle.

© 2008 Haim Michael 20150805
The toString Method
 Whenever we define a new class, it would be best to
include the definition for the toString method.
This way when creating log messages it would be simpler
to include a textual representation for specific objects.

© 2008 Haim Michael 20150805
Exceptions Classes
 When using the try & catch exceptions handling
mechanism it would be useful to define a new exception
class specific for our project.

© 2008 Haim Michael 20150805
The SOLID Design Principles
 SOLID is an acronym that stands for five key design
principles in OOP. Following these principles will contribute
to the quality of our code by improving its maintainability, its
clarity and its flexibility for changes.

© 2008 Haim Michael 20150805
Single Responsibility
"A class should have only one reason to change."
(Robert Martin)
https://blog.cleancoder.com/uncle-bob/2014/05/08/SingleReponsibilityPrinciple.html

© 2008 Haim Michael 20150805
 Every class should have one single responsibility, and it
shouldn't change unless it is in order to fulfill its responsibility.
When having a class with only one single responsibility the
testing will be simpler, the class will have less dependencies,
the code will be characterized with lower coupling, and the
clarity of the entire code will be improved.

© 2008 Haim Michael 20150805
class Car {
#engineVolume
#engineHP
#brand
#color
//...
}

© 2008 Haim Michael 20150805
class Engine {
#volume
#hp
}
class Car {
#engine
#brand
#color
//...
}

© 2008 Haim Michael 20150805
Open Closed
"software entities (classes, modules, functions, etc.) should
be open for extension, but closed for modification"
(Bertnard Meir)

© 2008 Haim Michael 20150805
Open Closed
 Classes should be open for extension but closed for
modification.
When following this principle we will avoid modifying
existing code, and by doing so we will avoid potential bugs
in the rest of the code. The only exception for this
principle would be fixing bugs.

© 2008 Haim Michael 20150805
Open Closed
class CheckingAccount {
#balance
//...
getBalance() {
return this.#balance;
}
}
class SavingAccount {
#balance
#interest
//...
getValue() {
return this.#balance * (1+this.#interest);
}
}

© 2008 Haim Michael 20150805
Open Closed
class Customer {
#accounts
//...
getCustomerValue() {
var total = 0;
for(let i=0; i<this.#accounts.length; i++) {
if (accounts[i] instanceof CheckingAccount) {
total += this.#accounts[i].getBalance();
} else if(accounts[i] instanceof SavingAccount) {
total += this.#accounts[i].getValue();
}
}
return total;
}
}

© 2008 Haim Michael 20150805
Open Closed
class BankExceptoion {
#message
constructor(text) {
this.#message = text;
}
}
class BankAccount {
getValue() {
throw new BankException("not implemented");
}
}

© 2008 Haim Michael 20150805
Open Closed
class CheckingAccount extends BankAccount {
#balance
getValue() {
//...
//return ...
}
}
class SavingAccount extends BankAccount {
#balance
#interest
getValue() {
//...
//return ...
}
}

© 2008 Haim Michael 20150805
Open Closed
class Customer {
#accounts
//...
getCustomerValue() {
var total = 0;
for(let i=0; i<this.#accounts.length; i++) {
total += this.#accounts[i].getValue();
}
return total;
}
}

© 2008 Haim Michael 20150805
Liskov Substitution
“Let Φ(x) be a property provable about objects x of type T.
Then Φ(y) should be true for objects y of type S where S is a
subtype of T.”
(Barbara Liskov)

© 2008 Haim Michael 20150805
Liskov Substitution
 When having superclass object it should be possible to
replace it with a subclass object without breaking the
functionality.
When defining a new class that extends a class that already
exists we better avoid overriding a method with a new version
that throws exception when calling it (as a way for showing
that it still wasn't implemented).

© 2008 Haim Michael 20150805
Liskov Substitution
class BankAccount {
#balance
constructor(sum) {
this.#balance = sum;
}
//...
}
//...
}
var ob = new CheckingAccount();

© 2008 Haim Michael 20150805
Liskov Substitution
class BankAccount {
#balance
constructor(sum) {
this.#balance = sum;
}
//...
}
constructor(sum) {
super(sum);
}
}
var ob = new CheckingAccount(100);

© 2008 Haim Michael 20150805
Interface Segregation
 We better split large interfaces into smaller ones. This
way, the classes we define will implement just the relevant
interfaces.
Splitting an interface into multiple ones will improve the
clarity of our code.

© 2008 Haim Michael 20150805
Interface Segragation
“Many client specific interfaces are better than one general
purpose interface”
(Robert C.Martin)

© 2008 Haim Michael 20150805
class WebBrowser {
browse(url) {
throw new OperationException("browse is not supported");
}
back() {
throw new OperationException("back is not supported");
}
forward() {
throw new OperationException("forward is not supported");
}
executeJS(code) {
throw new OperationException("executeJS is not supported");
}
}

© 2008 Haim Michael 20150805
class WebBrowser {
browse(url) {
throw new OperationException("browse is not supported");
}
back() {
throw new OperationException("back is not supported");
}
forward() {
throw new OperationException("forward is not supported");
}
}
class JavaScriptEngine {
executeJS(code) {
throw new OperationException("executeJS is not supported");
}
}

© 2008 Haim Michael 20150805
Dependency Inversion
“ High-level modules should not import anything from low-
level modules. Both should depend on abstractions (e.g.,
interfaces). Abstractions should not depend on details.
Details (concrete implementations) should depend on
abstractions.”
(Robert C.Martin)

© 2008 Haim Michael 20150805
 Instead of having high level modules depending on low
level modules, both should depend on abstraction.
Following this principle, we will decouple the
dependencies in our code and improve its flexibility for
changes.

© 2008 Haim Michael 20150805
class Engine {
#volume
#hp
}
class Car {
#engine
#brand
#color
constructor() {
this.#engine = new Engine();
}
}

© 2008 Haim Michael 20150805
class Engine {
#volume
#hp
}
class Car {
#engine
#brand
#color
constructor(engine) {
this.#engine = engine;
}
}

© 2008 Haim Michael 20150805
Conclusions
 Instead of sticking with JavaScript let's shift to TypeScript.
The use of types and the additional capabilities, such as
the possibility to define interfaces, and the possibility to
use generics will provide us with a better foundation and
the quality of our code will improve.

© 2008 Haim Michael 20150805
Conclusions
 When the need for simplicity governs, let's keep it simple
by using the simple JavaScript object notation.
{name:”Mosh”, id:3452344, average:98}

OOP Best Practices in JavaScript

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