TypeScript private modifier vs ECMAScript private fields runtime visibility

Encapsulation Strategy in TypeScript

TypeScript offers two primary mechanisms for restricting access to class members: the private modifier and ECMAScript private fields (using the # prefix). While both prevent access during development, they differ fundamentally in their implementation during transpilation to JavaScript.

The private keyword provides soft privacy, meaning it is enforced by the TypeScript compiler but erased in the resulting JavaScript output. In contrast, #private fields provide hard privacy enforced by the JavaScript engine at runtime, preventing access even when casting to any or using bracket notation.

This creates a discrepancy when designing libraries where strict data encapsulation is required to prevent external modification of internal state in production environments.

• How does the choice between private and # impact the serialization of class instances?
• What are the implications for reflection and debugging when using hard runtime privacy?