@types/type-is

This package provides TypeScript definitions for the `type-is` library, which is essential for validating Content-Type headers in Express and Node.js applications. The type definitions are straightforward and accurately represent the runtime behavior - you get proper IntelliSense for checking request content types against expected MIME types, which is critical for input validation.

In practice, these types integrate seamlessly with Express middleware patterns. The definitions correctly type the return values (string | false | null) which forces you to handle cases where content-type validation fails - this is actually a security benefit as it prevents assumptions about request types. The types for checking arrays of MIME types work well for scenarios where you need to accept multiple content types.

One limitation is that the types don't provide stricter literal types for common MIME types, so typos in strings like 'application/json' won't be caught at compile time. However, this is a minor inconvenience given that `type-is` itself works with runtime strings. The definitions stay current with the underlying library and don't introduce breaking changes unnecessarily.

@types/type-is provides TypeScript definitions for the type-is library, which is essential for validating HTTP content types in Express and other Node.js applications. The type definitions are straightforward and accurately represent the underlying JavaScript API. In practice, you'll use this primarily when implementing middleware that needs to handle different content types securely—checking if a request is JSON, multipart, or other MIME types before parsing.

The types work well with input validation pipelines, letting you validate content-type headers before passing data to parsers. This is critical for preventing content-type confusion attacks where malicious payloads exploit parser mismatches. The type signatures correctly model the function overloads, making it easy to check against single types or arrays of acceptable types.

One minor drawback is that the types don't encode MIME type strings as literal types, so you won't get autocomplete for common values like 'application/json'. However, this reflects the runtime library's flexibility and doesn't impact security—you should still validate against a whitelist of expected types in your application logic.

This is a straightforward TypeScript definitions package for the `type-is` library, which checks request content types in Express/Connect applications. The types are accurate and cover the main use cases you'll encounter: checking if a request matches specific MIME types. The API surface is small - mainly the `is()` function and `hasBody()` helper - so there's virtually no learning curve if you already know the JavaScript version.

In day-to-day usage, the type definitions work seamlessly with Express request objects. IntelliSense properly suggests string arrays for MIME types, and the return types (string | false | null) are correctly defined, helping you write proper type guards. Error messages are standard TypeScript fare - nothing special, but nothing problematic either. The package stays in sync with type-is updates reasonably well.

The main limitation is that it's a types-only package, so you won't find examples or documentation here - you need to reference the main type-is library docs. For a utility this simple though, that's rarely an issue. It just works without fanfare.