TypeScript Classes
TypeScript adds types and visibility modifiers to JavaScript classes.
Members: Types
The members of a class (properties & methods) are typed using type annotations, similar to variables.
class Person { name: string; } const person = new Person(); person.name = "Jane"; Members: Visibility
Class members also be given special modifiers which affect visibility.
There are three main visibility modifiers in TypeScript.
- public – (default) allows access to the class member from anywhere
- private – only allows access to the class member from within the class
- protected – allows access to the class member from itself and any classes that inherit it, which is covered in the inheritance section below
class Person { private name: string; public constructor(name: string) { this.name = name; } getName(): string { return this.name; } } const person = new Person('Jane'); console.log(person.getName()); // console.log(person.name); person.name isn't accessible from outside the class since it's private Parameter Properties
TypeScript provides a convenient way to define class members in the constructor, by adding a visibility modifiers to the parameter.
class Person { public constructor(private name: string) { this.name = name; } getName(): string { return this.name; } } const person = new Person('Birusha'); console.log(person.getName()); // console.log(person.name); Property 'name' is private and only accessible within class 'Person' Readonly
Similar to arrays, the readonly keyword can prevent class members from being changed.
class Person { readonly name: string; public constructor(name: string) { this.name = name; } getName(): string { return this.name; } } const person = new Person('John'); console.log(person.getName()); // person.name = 'Jill'; // Cannot assign to 'name' because it is a read-only property. Inheritance: Implements
interface Shape { getArea: () => number; } class Rectangle implements Shape { public constructor ( private readonly width: number, private readonly height: number ) {} getArea(): number { return this.width * this.height; } } const r = new Rectangle(4, 3); console.log(r.getArea()); Inheritance: Extends
interface Shape { getArea: () => number; } class Rectangle implements Shape { public constructor( private readonly width: number, private readonly height: number ) {} getArea(): number { return this.width * this.height; } } class Square extends Rectangle { public constructor(width: number){ super(width, width); } // getArea function is inherited } const s = new Square(40); console.log(s.getArea()); Override
When a class extends another class, it can replace the members of the parent class with the same name.
Newer versions of TypeScript allow explicitly marking this with the override keyword.
interface Shape { getArea: () => number; } class Rectangle implements Shape { public constructor( public readonly width: number, private readonly heigth: number ) {} getArea(): number { return this.width * this.heigth; } public toString(): string { return `Rectangle[width=${this.width}, height=${this.heigth}]`; } } class Square extends Rectangle { public constructor(width: number) { super(width, width); } // get area function inherited here public override toString(): string { return `Square[width=${this.width}]`; } } const rectangle = new Rectangle(20, 4); const square = new Square(30); console.log(rectangle.toString()); console.log(square.toString()); Abstract Classes
Classes can be written in a way that allows them to be used as a base class for other classes without having to implement all the members. This is done by using the abstract keyword. Members that are left unimplemented also use the abstract keyword.
abstract class Polygon { public abstract getArea(): number; public toString(): string { return `Polygon[area=${this.getArea()}]`; } } class Rectangle extends Polygon { public constructor ( private readonly width: number, private readonly heigth: number ) { super(); } public getArea(): number { return this.width * this.heigth; } } const r = new Rectangle(43, 34); console.log(r.getArea());