djbalin•3w ago

Narrowing function return type

Hi, consider the toy example below: passing true as an argument causes the function to return a number while passing false causes it to return a boolean. The inferred return type is however always number | boolean regardless of what argument is passed. I guess this is just a quirk with TS not narrowing function signature based on arguments passed. Is it possible to achieve this however in an elegant way? 🙂

export const returnNumOrBool = authQuery({
  args: {
    returnNumber: v.boolean(),
  },
  handler: async (ctx, { returnNumber }) => {
    if (returnNumber === true) {
      return 1
    } else {
      return false
    }
  },
})

export const returnNumOrBool = authQuery({
  args: {
    returnNumber: v.boolean(),
  },
  handler: async (ctx, { returnNumber }) => {
    if (returnNumber === true) {
      return 1
    } else {
      return false
    }
  },
})

6 Replies

Convex Bot•3w ago

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sshader•3w ago

Yeah taking out all the convex parts, I'm not sure how to get TS to do this sort of type narrowing for you. Playing around a little bit, I could only get this behavior by defining an overload. Let's see if discord will let me send this as a pretty link

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RJ•3w ago

You can also use generics, but I like Sarah's approach (overloads) better. However, both approaches aren't really type-safe in the sense that they don't enforce the type-level logic that you'd expect/want (playground) Regarding Convex specifically though (and honestly also in normal TypeScript code), I would recommend just splitting this into two different functions. It may not even end up being more code when you take into account the fancy TS type-level stuff you would need to do to get it to work with one, and it's safer and easier to understand

djbalinOP•3w ago

Thanks both of you! Haha yeah seems like quite a bit of hoop-jumping and arm-twisting has to be done to achieve this, and that you'll probably end up with even less peace of mind!! I think we'll end up writing two dedicated queries to get the intellisense and type safety in the frontend that we expect. If anyone's curious as to why I was interested in this, it was for basically this logic: getUnreadNotifications(userId, onlyReturnCount)

ballingt•3w ago

Two Convex queries sounds useful to me, remember you can write helpers functions so there's no code duplication The generated types https://docs.convex.dev/client/open-api for Convex functions can't include overloads or type parameters, in some languages these would not be expressible. The goal is that a Convex Query should be an API endpoint that can be expressed in any language; even though the TypeScript integration is good, you're defining an API and it's nice for that to return a consistent type.

lee•3w ago

I'm curious if you can do

export const returnNumOrBool = authQuery(...) as RegisteredQuery<"public", { returnNumber: true }, number> | RegisteredQuery<"public", { returnNumber: false }, boolean>

Narrowing function return type

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