Class Type Annotations

class ClassName {
  propertyName: Type;

  constructor(arg: Type) {
    this.propertyName = arg;
  }

  methodName(arg: Type): ReturnType {
    // ...
  }
}

class Person {
  readonly id: number;
  name: string;
  age: number;

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

  greet(): string {
    return `Hi, I'm ${this.name}, ${this.age} years old.`;
  }

  haveBirthday(): void {
    this.age++;
  }
}

const member = new Person(1, 'member_1', 28);
console.log(member.greet()); // Hi, I'm member_1, 28 years old.
member.haveBirthday();
console.log(member.age); // 29

// member.id = 99; // Error: cannot assign to a readonly property

// Shorthand for constructor parameters (combined with access modifiers)
class Point {
  constructor(
    public x: number,
    public y: number
  ) {}

  distance(): number {
    return Math.sqrt(this.x ** 2 + this.y ** 2);
  }
}

const p = new Point(3, 4);
console.log(p.distance()); // 5

Running the above produces the following output:

npx ts-node ts_class_type.ts
Hi, I'm member_1, 28 years old.
29
5

In TypeScript classes, you can either declare a property and then assign it, or use a shorthand syntax that declares and initializes a property at the same time by adding an access modifier (such as public or private) to a constructor parameter.

A property marked with readonly can only be assigned inside the constructor and cannot be changed afterward. This is a fundamental way to enforce immutability in an object.

A class also acts as a type. To apply an interface to a class, use implements; for inheritance, use JavaScript's standard extends keyword.