typefiles

Typefiles, in the software development sense, are files that describe the data types, interfaces, and API surfaces of code modules without containing executable implementation. They serve as a separate source of truth that type checkers and development tools can read to reason about code, provide autocompletion, and improve reliability across languages and ecosystems.

In practice, typefiles appear in several popular forms. TypeScript uses declaration files with the .d.ts extension to describe the types of JavaScript libraries and modules, enabling type checking and editor support for code that may be dynamic or untyped at runtime. Python uses stub files with a .pyi extension to describe the public interface of modules, especially for compiled or third-party libraries, allowing static type checkers such as mypy and Pyright to verify correctness. Other ecosystems rely on interface definitions or IDLs, such as Protocol Buffers or Thrift, which declare structured data types and message schemas shared across programming languages; these serve a similar purpose for data interchange, though they are often categorized as interface definitions rather than typefiles per se.

Creation and maintenance of typefiles can be manual or automatic. They may be authored by library maintainers or generated by tools that extract types from source code. Techniques such as stub generation or external type repositories (for example, a collection of community-maintained typings) support broader language ecosystems.

Benefits of typefiles include improved static analysis, safer integration of libraries, better IDE features, and clearer documentation of APIs. Drawbacks include the effort required to maintain them and the risk of mismatch between types and runtime behavior, which can lead to false positives or silent type errors if not managed carefully.