Access Modifiers & readonly

public / private / protected

public (default) is visible everywhere, private only inside the class, protected inside the class and its subclasses.

class Account {
  public owner: string;
  private balance: number;

  constructor(owner: string, balance: number) {
    this.owner = owner;
    this.balance = balance;
  }
}

const acc = new Account("Ada", 500);
console.log(acc.balance);
// Error: Property 'balance' is private and only accessible within class 'Account'.

Parameter properties

Adding a modifier directly to a constructor parameter declares and assigns the property in one step.

class Account {
  constructor(
    public owner: string,
    private balance: number,
    readonly openedOn: Date = new Date()
  ) {}
}
// equivalent to writing the fields + this.x = x manually

readonly on class fields

A readonly field can be set in the constructor but never reassigned afterward — good for IDs and creation timestamps.

Quick check: Which modifier allows access from a subclass but not from outside code?

• public
• private
• protected