Classes and Object-Oriented TypeScript

TypeScript enhances JavaScript's class syntax with access modifiers, abstract classes, interfaces, and other object-oriented features. Whilst TypeScript supports multiple paradigms, understanding OOP concepts is essential for working with many libraries and frameworks.

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Basic Classes

A TypeScript class looks similar to a JavaScript class, with the addition of type annotations:

class Person {
  name: string;
  age: number;

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

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

const alice = new Person('Alice', 30);
console.log(alice.greet());

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Access Modifiers

TypeScript provides three access modifiers that control visibility:

class BankAccount {
  public owner: string;        // accessible anywhere (default)
  protected accountType: string; // accessible in this class and subclasses
  private balance: number;      // accessible only in this class

  constructor(owner: string, initialBalance: number) {
    this.owner = owner;
    this.accountType = 'current';
    this.balance = initialBalance;
  }

  public deposit(amount: number): void {
    this.balance += amount;
  }

  public getBalance(): number {
    return this.balance;
  }
}

const account = new BankAccount('Alice', 1000);
account.deposit(500);
console.log(account.getBalance());  // 1500
// account.balance;                  // Error: Property 'balance' is private

The readonly Modifier