Episode 7 — DSA with JavaScript / 7.4 — Object Oriented Programming

7.4 — Quick Revision: OOP in JavaScript

<< Overview

OOP Pillars

PillarMeaningJS Mechanism
EncapsulationHide internals#privateField
AbstractionExpose only essentialsMethods as interface
InheritanceReuse parent behaviorextends / super
PolymorphismSame method, different behaviorMethod overriding

Class syntax cheat sheet

class Animal {
    #name;                            // private field
    static count = 0;                 // static property

    constructor(name) {               // constructor
        this.#name = name;
        Animal.count++;
    }

    get name() { return this.#name; } // getter
    set name(n) { this.#name = n; }   // setter

    speak() { return "..."; }         // instance method
    static getCount() { return Animal.count; } // static method
}

class Dog extends Animal {            // inheritance
    constructor(name, breed) {
        super(name);                  // call parent constructor
        this.breed = breed;
    }
    speak() { return "Woof!"; }       // override
}

C++ equivalent

class Animal {
private:
    string name;
public:
    static int count;
    Animal(const string& n) : name(n) { count++; }
    virtual string speak() const { return "..."; }
    string getName() const { return name; }
    virtual ~Animal() = default;
};

class Dog : public Animal {
public:
    string breed;
    Dog(const string& n, const string& b) : Animal(n), breed(b) {}
    string speak() const override { return "Woof!"; }
};

this behavior

Contextthis refers to
Method call obj.method()obj
Arrow functionEnclosing scope's this
Regular function (strict)undefined
Constructor (new)New object being created
Event handlerElement that fired event

Common pitfalls

  1. Forgetting super() in child constructor
  2. Using this before super() call
  3. Losing this in callbacks (use arrow functions)
  4. Confusing static and instance methods
  5. Not using # for truly private fields

Design patterns

PatternPurposeKey idea
SingletonOne instancePrivate constructor, static getInstance
FactoryCreate objects without specifying classSwitch on type
ObserverReact to state changesSubscribe/notify
StrategyInterchangeable algorithmsInject algorithm at runtime
DecoratorAdd behavior dynamicallyWrap original object

Self-check drill

SC-001

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-002

  • Q: What is this?
  • A: Reference to the current object instance

SC-003

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-004

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-005

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-006

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-007

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-008

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-009

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-010

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-011

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-012

  • Q: What is this?
  • A: Reference to the current object instance

SC-013

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-014

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-015

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-016

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-017

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-018

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-019

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-020

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-021

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-022

  • Q: What is this?
  • A: Reference to the current object instance

SC-023

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-024

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-025

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-026

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-027

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-028

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-029

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-030

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-031

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-032

  • Q: What is this?
  • A: Reference to the current object instance

SC-033

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-034

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-035

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-036

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-037

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-038

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-039

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-040

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-041

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-042

  • Q: What is this?
  • A: Reference to the current object instance

SC-043

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-044

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-045

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-046

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-047

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-048

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-049

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-050

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-051

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-052

  • Q: What is this?
  • A: Reference to the current object instance

SC-053

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-054

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-055

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-056

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-057

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-058

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-059

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-060

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-061

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-062

  • Q: What is this?
  • A: Reference to the current object instance

SC-063

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-064

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-065

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-066

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-067

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-068

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-069

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-070

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-071

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-072

  • Q: What is this?
  • A: Reference to the current object instance

SC-073

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-074

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-075

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-076

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-077

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-078

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-079

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-080

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-081

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-082

  • Q: What is this?
  • A: Reference to the current object instance

SC-083

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-084

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-085

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-086

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-087

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-088

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-089

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-090

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-091

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-092

  • Q: What is this?
  • A: Reference to the current object instance

SC-093

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-094

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-095

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-096

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-097

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-098

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-099

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-100

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-101

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-102

  • Q: What is this?
  • A: Reference to the current object instance

SC-103

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-104

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-105

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-106

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-107

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-108

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-109

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-110

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-111

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-112

  • Q: What is this?
  • A: Reference to the current object instance

SC-113

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-114

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-115

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-116

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-117

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-118

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-119

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-120

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-121

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-122

  • Q: What is this?
  • A: Reference to the current object instance

SC-123

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-124

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-125

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-126

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-127

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-128

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-129

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-130

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-131

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-132

  • Q: What is this?
  • A: Reference to the current object instance

SC-133

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-134

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-135

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-136

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-137

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-138

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-139

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-140

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-141

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-142

  • Q: What is this?
  • A: Reference to the current object instance

SC-143

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-144

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-145

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-146

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-147

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-148

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-149

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-150

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-151

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-152

  • Q: What is this?
  • A: Reference to the current object instance

SC-153

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-154

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-155

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-156

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-157

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-158

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-159

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-160

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-161

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-162

  • Q: What is this?
  • A: Reference to the current object instance

SC-163

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-164

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-165

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-166

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-167

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-168

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-169

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-170

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-171

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-172

  • Q: What is this?
  • A: Reference to the current object instance

SC-173

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-174

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-175

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-176

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-177

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-178

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-179

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-180

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-181

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-182

  • Q: What is this?
  • A: Reference to the current object instance

SC-183

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-184

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-185

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-186

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-187

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-188

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-189

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-190

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists

SC-191

  • Q: Four pillars of OOP?
  • A: Encapsulation, Abstraction, Inheritance, Polymorphism

SC-192

  • Q: What is this?
  • A: Reference to the current object instance

SC-193

  • Q: extends keyword does what?
  • A: Creates a child class that inherits from a parent

SC-194

  • Q: super() does what?
  • A: Calls the parent class constructor

SC-195

  • Q: Private field syntax in JS?
  • A: #fieldName

SC-196

  • Q: Static method called how?
  • A: ClassName.method(), not instance.method()

SC-197

  • Q: What is polymorphism?
  • A: Same method name, different behavior per class

SC-198

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; prefer composition

SC-199

  • Q: What is a getter?
  • A: Method accessed like a property: get prop() {}

SC-200

  • Q: Singleton pattern?
  • A: Ensure only one instance of a class exists