Access Modifiers and Parameter Properties

TypeScript supports public, protected, and private modifiers, plus readonly. public is the default. protected allows access in the class and subclasses; TypeScript's private restricts access through the type system, while JavaScript's #private fields provide runtime privacy. Parameter properties let a constructor parameter declare and initialize a field in concise syntax.

Use the least permissive access that fits the design. Do not confuse compile-time access modifiers with security boundaries: client-side code and emitted JavaScript can be inspected, and TypeScript's private keyword alone is not a runtime security feature.

Parameter properties

class Account {
  constructor(
    public readonly id: string,
    private balance: number
  ) {}

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

const account = new Account("acct-1", 250);

💡 The constructor parameters create and initialize fields. id cannot be reassigned through this type; balance is only accessible inside the class according to TypeScript's access checking.

Key Takeaways

public is the default visibility.
TypeScript private is not the same as JavaScript #private runtime privacy.
Use readonly for fields that should not be reassigned after initialization.

Knowledge Check

Which feature provides JavaScript runtime private-field semantics?

Access Modifiers and Parameter Properties

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