TypeScript private vs # Fields: Choosing the Right Encapsulation in 2026

TypeScript offers two distinct encapsulation models — the compile-time private modifier and ECMAScript # hard-private fields — which are fundamentally incompatible and not interchangeable. The private keyword provides no runtime protection, as it disappears after transpilation and leaves fields accessible as plain JavaScript properties. ECMAScript # fields enforce true runtime privacy through WeakMap storage, making fields inaccessible to any external code even after compilation. Developers are advised to use private for type safety within controlled TypeScript-only codebases, and # when building libraries, handling sensitive data, or operating in untrusted environments. Mixing both patterns without clear conventions can silently break encapsulation and cause reflection-based tooling to fail.
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