jsonSerializer does not pass through correct event type

[parkan]

Describe the bug

Hello it's me again 🙂

jsonSerializer (and I think some of the other middlewares as well) don't respect the pattern of generically passing the event from the previous middleware described in the README so the type information about the modified event is lost, replaced with base APIGatewayEvent

To Reproduce

compose(
  errorHandler(),
  jwtAuth(...),
  jsonSerializer()
)(handler) // the handler gets APIGatewayEvent without AuthorizedEvent

Expected behavior

Type information is preserved in the chain

Additional context

This should be a simple fix, replace

export const jsonSerializer = () => (
  handler: PromiseHandler<APIGatewayEvent, JSONObject | undefined>
) => 
...

with

export const jsonSerializer = () => (
  <E extends APIGatewayProxyEvent>(handler: PromiseHandler<APIGatewayEvent, JSONObject | undefined>): PromiseHandler<E, APIGatewayProxyResult> =>
...

happy to PR this!

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[parkan]

seems that this exact configuration is used in https://github.com/dbartholomae/lambda-middleware/blob/main/internal/e2e-tests/examples/helloWorld.ts#L52 but the handler doesn't actually use anything other than body on the event, I can add a test for .auth/explicit event type expectation

[dbartholomae]

Unfortunately there is a limit to how TypeScript is implemented that currently makes it impossible to have a full 100% passthrough of events. This was discussed in detail in #36. Please feel free to play around with it a bit, maybe there is a way to improve it I didn't find :)

[parkan]

gotcha, gonna close this for now and mess around with it a bit