private

Learn class-internal members with private.

What is private?

private indicates that a member is accessible only inside the class itself. Trying to access it from outside causes a compile error. This is the core tool for encapsulation.

class Account {
  private balance: number;
  constructor(balance: number) {
    this.balance = balance;
  }
  getBalance(): number {
    return this.balance;
  }
}
const a = new Account(100);
// a.balance;  // error
console.log(a.getBalance());

Encapsulation

With private you hide internal state; from outside it is only accessed via methods. This preserves consistency inside the class and prevents external code from making direct changes.

class Account {
  private balance: number = 0;
  deposit(amount: number): void {
    this.balance += amount;
  }
}

Exercise Account class with a private field

Define an Account class with a private balance: number field. Add a constructor that takes balance and assigns it with this. Write a getBalance(): number method that returns this.balance. Create a new Account(100) and call getBalance().

Show example solution

class Account {
  private balance: number;
  constructor(balance: number) {
    this.balance = balance;
  }
  getBalance(): number {
    return this.balance;
  }
}

const a = new Account(100);
console.log(a.getBalance());

Test yourself

What happens if you try to access a private field from outside the class?
- Runtime error
- Returns undefined
- Compile error
- It works
Show answer

Compile error — Accessing a private field from outside the class causes a compile error. This enforces encapsulation at compile time; it does not wait until runtime.

Complete the private modifier

class Account {
  ____ balance: number = 0;
}

Show answer

class Account {
  private balance: number = 0;
}

private parts

private

Accessible only inside the class
this.balance

Access from inside the class
a.balance

External access (error)
getBalance()

Public accessor method

Next lesson: protected

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