NOTE

[[typescript|TypeScript]] Classes

authorgemini-cli aliasests-classes, class-inheritance, member-visibility, abstract-classes title[[typescript|TypeScript]] Classes statusactive date2026-04-24 typepermanent

TypeScript Classes

TypeScript offers full support for the ES2015 class keyword, adding type annotations and specific syntax for expressing relationships between classes and other types.

Class Members

  • Fields: Public writeable properties. Can have initializers and be marked as readonly.
  • Constructors: Similar to functions; can have parameters, defaults, and overloads. Note: They cannot have type parameters or return type annotations.
  • Methods: Functions defined on a class. Accessing fields/methods within a body requires this..
  • Accessors: get and set blocks. If get exists without set, the property is readonly.

Class Heritage

  • implements Clause: Ensures a class satisfies an interface. This is a check only and doesn't change the class's type or inference.
  • extends Clause: Inherit from a base class. Derived classes must be subtypes of their base classes.

Member Visibility

  • public (default): Accessible from anywhere.
  • protected: Accessible only within the class and its subclasses.
  • private: Accessible only within the class itself. Note: This is "soft private" and erased at runtime.
  • # (JS Private Fields): Hard private at runtime; not accessible via bracket notation or in subclasses.

Static Members

Belong to the class constructor object rather than instances. Certain names (like name, length, call) are reserved.

Generic Classes

Classes can have type parameters (e.g., class Box<T>). Static members cannot refer to the class's type parameters.

The this Type

  • this at Runtime: Use arrow functions to preserve this context or declare a this parameter in methods for static enforcement.
  • this Type: Dynamically refers to the type of the current class (useful for fluent APIs).
  • this-based Type Guards: Use this is Type as a return type for methods to perform narrowing on the instance.

Abstract Classes

Defined with the abstract keyword. Cannot be directly instantiated and may contain abstract methods/fields that must be implemented by concrete subclasses.

References