Write-Type-Provider
How to write your own type provider
Things to keep in mind when implementing a custom type provider:
Type Contravariance
Whereas exhaustive type narrowing checks normally rely on never
to represent
an unreachable state, reduction in type provider interfaces should only be done
up to unknown
.
The reasoning is that certain methods of FastifyInstance
are
contravariant on TypeProvider
, which can lead to TypeScript surfacing
assignability issues unless the custom type provider interface is
substitutable with FastifyTypeProviderDefault
.
For example, FastifyTypeProviderDefault
will not be assignable to the following:
export interface NotSubstitutableTypeProvider extends FastifyTypeProvider {
// bad, nothing is assignable to `never` (except for itself)
validator: this['schema'] extends /** custom check here**/ ? /** narrowed type here **/ : never;
serializer: this['schema'] extends /** custom check here**/ ? /** narrowed type here **/ : never;
}
Unless changed to:
export interface SubstitutableTypeProvider extends FastifyTypeProvider {
// good, anything can be assigned to `unknown`
validator: this['schema'] extends /** custom check here**/ ? /** narrowed type here **/ : unknown;
serializer: this['schema'] extends /** custom check here**/ ? /** narrowed type here **/ : unknown;
}