Chapter 5 Cheatsheet: Classes

This cheatsheet summarizes the core mechanics of object-oriented classes in JavaScript covered in Chapter 5.

Basic Classes

Classes act as blueprints for generating multiple consistent object instances. The constructor runs automatically upon instantiation.

class User {
  constructor(name) {
    this.name = name;
  }
}

const u = new User("Alice");

Encapsulation (Private Properties)

By default, all properties are public. Prefix a property with a hash (#) to make it completely private and inaccessible from outside the class.

class Vault {
  #password; // Must be declared at the top level
  
  constructor(pw) {
    this.#password = pw;
  }
}

Static Members

static properties and methods belong to the Class definition itself, not the individual instances. Great for utility functions or global counters.

class MathUtils {
  static add(a, b) {
    return a + b;
  }
}

// Called directly on the class
MathUtils.add(5, 10); 

Getters and Setters

Act like standard properties from the outside, but execute function logic on the inside. Useful for data validation.

class User {
  set age(value) {
    if (value < 0) throw new Error("Invalid age");
    this._age = value;
  }
  get age() {
    return this._age;
  }
}

Inheritance (extends and super)

• extends: Allows a child class to inherit from a parent class.
• Method Overriding: Declaring a method in the child with the same name as the parent overrides the parent’s behavior.
• super(): Calls the parent constructor. Must be called before using this in a child constructor.
• super.methodName(): Calls the parent’s version of a method.

class Animal {
  constructor(name) { this.name = name; }
  speak() { return "Noise"; }
}

class Dog extends Animal {
  constructor(name, breed) {
    super(name); // Must be called first!
    this.breed = breed;
  }
  
  speak() {
    return super.speak() + " Woof!"; 
  }
}