Parameter Properties

TypeScript offers a convenient way to define class members directly in the constructor by adding visibility modifiers to the constructor parameters.

Example

class Person {
  // name is a private member variable
  public constructor(private name: string) {}

  public getName(): string {
    return this.name;
  }
}

const person = new Person(“Jane”);
console.log(person.getName());

Readonly

Similar to arrays, the readonly keyword can be used to prevent class members from being modified.

Example

class Person {
  private readonly name: string;

  public constructor(name: string) {
    // name cannot be changed after this initial definition, which has to be either at it’s declaration or in the constructor.
    this.name = name;
  }

  public getName(): string {
    return this.name;
  }
}

const person = new Person(“Jane”);
console.log(person.getName());

Inheritance: Implements

Interfaces (discussed here) can define the type that a class must adhere to using the implements keyword.

Example

interface Shape {
  getArea: () => number;
}

class Rectangle implements Shape {
  public constructor(protected readonly width: number, protected readonly height: number) {}

  public getArea(): number {
    return this.width * this.height;
  }
}

A class can implement multiple interfaces by listing them after implements, separated by commas, like this: class Rectangle implements Shape, Colored.