[drostie]

I don't think TypeScript has a native GUID type, but if I am wrong about that please tell me as it will make my code more type-safe.

The `string` type here comes from a mapping that the router is using. That is, the router ultimately type-evaluates a `ValueOfType<{type: 'guid'}>` to `string`. But because it's a runtime object, the router can also, at runtime, validate that URL param, "did they actually give me a UUID?" -- and sanitize it, e.g. "convert all UUIDs to lowercase."

(In fact the benefit of having this TypeScript type at runtime is even bigger than that. With Express.js, the router can rewrite the route param so that the route doesn't even match if you don't provide a UUID, which matters because there is often a lot of accidental ambiguity in HTTP APIs -- but here you can embed the UUID regex into Express paths. The router can then also do some other trickery like confirm at initial load time that all params in URLs match params in this `params` dict, and it can convert all of its routes to OpenAPI/Swagger docs so that you can define another route which just gives you your OpenAPI JSON. Literally in what I have written the above would be a type error because the `Router` class would complain that `params` has the wrong type because the `objectguid` descriptor needs a key called `doc` which is a string for parameter documentation for OpenAPI.)