How to get a typed output in a ResultHandler.handler?

[andremw]

I understand that the value output in the ResultHandler.handler (a FlatObject in this case) is the output of my endpoint handler (that receives an input and returns an output), whose type is defined by the output schema. But being a FlatObject, I can't access the properties with proper type safety.

Looking at the code, it seems that I cannot narrow the type of output in the ResultHandler (FlatObject). Is there any chance we can type that?

My use case is this:
I want to return different status codes depending on the result of my workflow, which might be of type type Result = { _type: "Success", data: ... } | {_type: "WorkflowError", error: ... }.
But defaultEndpointsFactory doesn't give me a low-level API to do that, so I thought that maybe I could just return from my handler (created with the default factory) and handle the output inside the ResultHandler, checking the _type and returning the appropriate response for each variant of my discriminated union.

I feel I'm misusing the lib tho, but I've tried to figure out by checking the docs and examples and couldn't spot what I'm missing.
At the same time, I think my idea of narrowing FlatObject makes sense, because once I use a ResultHandler to actually send the responses, I'm kinda ditching the output defined in the endpoint handler (in favor of positive/negative schemas), am I not?

Thank you! Love the library!

[RobinTail]

Hello @andremw ,

the purpose of ResultHandler is responding consistently for every Endpoint produced on EndpointsFactory having that ResultHandler attached to. Therefore, output can not have certain type, except the fact that we know that it is an object, because every output schema of Endpoint is required to be based on z.object().

However, you can still narrow it down to whatever you want by implementing Typescript conditions like this:

new ResultHandler({
  handler: ({ output }) => {
    if ("_type" in output && typeof output._type === "string") {
       // inside this block `output` has type { _type: string }
    }
  }
});

I must also say, that you should not return errors as a valid output from your Endpoint, like you mentioned:

{_type: "WorkflowError", error: ... }

this is the job of ResultHandler.

I recommend to throw from Endpoint, so that ResultHandler will receieve the error along with an empty output.
Consider using createHttpError() for that purpose, @andremw

factory.build({
  handler: async () => {
    if (something) {
       throw createHttpError(400, "WorkflowError ...");
    }
  }
});

new ResultHandler({
  handler: (output, error) => {
    if (error) {
      // handle error here
      return;
    }
    // handle no error here
  }
})

I hope it helps.
Let me know if it's unclear.

[andremw]

Yeah that's what I ended up doing. That makes sense, thanks!