Classes and Objects

A class bundles data (properties) with the functions that operate on it (methods) — the same idea as in JavaScript, but TypeScript adds access modifiers that control where a property or method can be used from.

class Dog {
  name: string;
  private age: number;

  constructor(name: string, age: number) {
    this.name = name;
    this.age = age;
  }

  bark(): string {
    return `${this.name} says Woof! (age ${this.age})`;
  }
}

const rex = new Dog("Rex", 3);
console.log(rex.name);
console.log(rex.bark());

Rex
Rex says Woof! (age 3)

public, private, protected, and readonly

• public is the default — accessible from anywhere, including outside the class
• private is only accessible from inside the class itself — rex.age would be a compile error outside Dog
• protected is like private, but also accessible from subclasses
• readonly can be combined with any of the above, and can only be set once, usually in the constructor

Constructor shorthand

Writing this.name = name for every property gets repetitive — TypeScript lets you declare and assign a property directly in the constructor's parameter list instead.

class Point {
  constructor(public x: number, public y: number) {}

  toString(): string {
    return `(${this.x}, ${this.y})`;
  }
}

const p = new Point(3, 4);
console.log(p.toString());

(3, 4)

Next: generics — writing a function or type that works across many types, without giving up type safety.