Static & Private Class Fields

To improve performance
To prevent external access to class internals
Private class fields serve several important purposes: 1) They provide true encapsulation by preventing access from outside the class, 2) They help enforce information hiding principles, 3) They create a clear boundary between public API and internal implementation, 4) They prevent name collisions with subclass fields, 5) They enable better refactoring by guaranteeing field access is only within the class, 6) They make JavaScript's privacy model more robust compared to convention-based approaches like underscore prefixes.
To save memory
To make code shorter