What are best practices for optional parameters?

Consider this code. It’s a redux reducer, but you don’t need to know redux to help me with this issue. I was trying to follow this doc, and am running into problems.

// Types
// Types
type InitAction = { +type: typeof INIT | typeof INIT_SUCCESS };
type ErrorAction = { +type: typeof INIT_ERROR, +error: Error };
type Action = InitAction | ErrorAction;  // flow doesn't seem to be doing 

type InitState = {
  +error?: Error
};

export default function InitReducer(
  state: InitState = {},
  { type, error }: Action = {}  //  i need this param or jest complains
): InitState {
  switch (type) {
    case INIT_ERROR:
      Raven.captureException(error, { state }); // flow complains, error doesn't always exist
      return {
        ...state,
        error
      };
    case INIT_SUCCESS:
    case INIT:
    default:
      return state;
  }
}

Raven.captureException(error, { state }); expects error to be of type Error. Whenever the case is INIT_ERROR, error will always be defined. Is there a way to imply this in flow? Originally I thought the flow-types on the actions imply this, but flow doesn’t seem to like this. To fix it, I’ve gone with a more-generic Action type.

type Action = {
  +type: string,
  +error?: Error
};

however even with this Action-type i still need this line

error && Raven.captureException(error, { state });   // flow is now happy, but this seems wrong

I thought maybe flow would be smart enough to know when INIT_ERROR is the type, an error will always exist. Another way I’ve found to get flow to stop complaining, change Raven.captureException to allow Error | void, but this seems wrong too.

Am i doing something completely wrong? Did I find a bug? What should I do?

Apologies in advance - I’m very new to flow

[mackbrowne]

@GAntoine great idea, however part of my question relates to whether the flow-typed definitions for raven.js should be changed. But thank you, I’ll post the question there

[mackbrowne]

@GAntoine - Thank you. When i get an answer on any channel I’ve posted on, I’ll be sure to report back here and close myself.