Classes & Objects in JavaScript

Why classes?

As programs grow, you'll have many things that share the same shape and behavior: users, products, carts, orders. A class is a blueprint for them. Each object made from it (an instance) has its own data.

class Player {
  constructor(name) {
    this.name = name;
    this.score = 0;
  }

  addPoints(n) {
    this.score += n;
  }

  describe() {
    return `${this.name} has ${this.score} points`;
  }
}

const ada = new Player("Ada");
const bo = new Player("Bo");
ada.addPoints(10);
console.log(ada.describe());
console.log(bo.describe()); // Bo's score is separate

• constructor runs once, when you write new Player(...)
• this means "the object this method was called on"
• Methods are functions that belong to the object

Classes vs plain objects

For a single config object, a plain { } is fine. Use a class when you'll create many objects with the same behavior, or when the data and the code that changes it belong together.

Static methods

Some functions belong to the class, not to any one object:

class Temperature {
  constructor(celsius) {
    this.celsius = celsius;
  }
  static fromFahrenheit(f) {
    return new Temperature(((f - 32) * 5) / 9);
  }
}

console.log(Temperature.fromFahrenheit(212).celsius);

You've seen Python classes already. JavaScript's work the same way with different syntax: constructor instead of __init__, and this instead of self.