TypeScript Tutorial · Classes and Enums · Lesson 14 of 25

TypeScript Classes

Typed fields, access modifiers, parameter properties, abstract classes and implements.

Typed classes

TypeScript adds types and access modifiers to JavaScript classes:

public (default), protected, private — checked at compile time.
readonly — assignable only in the constructor.
#private — JavaScript's runtime-private fields, also supported.

Parameter properties declare and assign a field straight from the constructor:

Example 1Try it yourself

class Account {
  private balance = 0;

  constructor(public readonly owner: string, private readonly currency = "USD") {}

  deposit(amount: number): this {
    if (amount <= 0) throw new Error("Amount must be positive");
    this.balance += amount;
    return this;
  }

  get summary(): string {
    return `${this.owner}: ${this.balance.toFixed(2)} ${this.currency}`;
  }
}

const acct = new Account("Ada").deposit(50).deposit(25);
console.log(acct.summary);
// @ts-expect-error — Property 'balance' is private and only accessible within class 'Account'.
console.log(acct.balance);  // still readable at runtime: use #balance for true privacy

Output

Ada: 75.00 USD
75

implements and abstract

implements checks that a class satisfies an interface; abstract classes can't be instantiated and may declare abstract methods:

Example 2Try it yourself

interface Shape {
  area(): number;
}

abstract class Base implements Shape {
  abstract area(): number;
  describe(): string {
    return `${this.constructor.name} with area ${this.area().toFixed(2)}`;
  }
}

class Circle extends Base {
  constructor(private r: number) { super(); }
  area() { return Math.PI * this.r ** 2; }
}
class Rect extends Base {
  constructor(private w: number, private h: number) { super(); }
  area() { return this.w * this.h; }
}

const shapes: Shape[] = [new Circle(1), new Rect(2, 3)];
for (const s of shapes) console.log((s as Base).describe());

Output

Circle with area 3.14
Rect with area 6.00

Test yourself

What does constructor(private name: string) do?

Only declares a parameter
Declares and assigns a private field named name
Makes the constructor private
Nothing — it's invalid

Show answer

Answer: Declares and assigns a private field named name