Classes

Syntax

class Name {
  constructor(params) { }
  method() { }
}

Examples

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  greet() {
    console.log(`Hi, I am ${this.name}, age ${this.age}`);
  }
}

const alice = new Person("Alice", 25);
alice.greet(); // "Hi, I am Alice, age 25"

class Animal {
  constructor(name) {
    this.name = name;
  }
  speak() {
    console.log(`${this.name} makes a sound`);
  }
}

class Dog extends Animal {
  constructor(name, breed) {
    super(name); // calls the parent constructor
    this.breed = breed;
  }
  speak() {
    console.log(`${this.name} barks`); // overrides the parent method
  }
}

const dog = new Dog("Rex", "Labrador");
dog.speak(); // "Rex barks"

class Counter {
  #count = 0; // private field, inaccessible from outside the class

  increment() {
    this.#count++;
    return this.#count;
  }

  static describe() {
    return "A simple counter class";
  }
}

const counter = new Counter();
console.log(counter.increment()); // 1
console.log(counter.increment()); // 2
// console.log(counter.#count); // SyntaxError - private field

console.log(Counter.describe()); // called on the class itself, not an instance

Best practices

• Always call super() in a subclass constructor before accessing this
• Use private fields (#field) for internal state that should not be accessed or modified from outside the class
• Prefer composition over deep inheritance chains - favor small, focused classes over sprawling hierarchies
• Use static methods for functionality related to the class itself rather than any particular instance

At a glance

Purpose

Application logic and interaction

File extension

.js · .mjs

Runs in

Browsers and JavaScript runtimes

Usually used with

HTML, CSS and Web APIs

Specifications & further reading

• ECMAScript specification

Related JavaScript documentation