What are access modifiers in TypeScript?

Access modifiers control where a class member can be used. TypeScript has three, and a member is public unless you say otherwise.

• public members are accessible everywhere.
• private members are accessible only inside the class that declares them.
• protected members are accessible inside the class and its subclasses, but not from outside.

class BankAccount {
  private balance: number;
  protected accountType: string;
  constructor(accountNumber: string, initialBalance: number) {
    this.balance = initialBalance;
    this.accountType = "savings";
  }
}
class PremiumAccount extends BankAccount {
  constructor(accountNumber: string, initialBalance: number) {
    super(accountNumber, initialBalance);
    // this.balance; // Error: private property
    console.log(this.accountType); // OK: protected property
  }
}

The compiler checks the modifiers only at compile time. The emitted JavaScript has ordinary properties, so private data can still be reached at runtime with bracket access or plain JavaScript. For privacy the runtime enforces, use a # private field such as #balance.