Nuxt•2mo ago

Losing types when returning data from server API handler

Hey folks! 👋 I'm working on a Nuxt 3 project and running into a TypeScript issue with one of my server API handlers. I have this setup:

import { useApiClient } from '~~/server/lib/apiClient';

export default defineEventHandler(async (event) => {
  const api = useApiClient(event);
  const { data, meta } = await api<IndexApiResponse<Tags>>('bo/tags');

  return {
    tags: data,
    meta: meta,
  };
});

import { useApiClient } from '~~/server/lib/apiClient';

export default defineEventHandler(async (event) => {
  const api = useApiClient(event);
  const { data, meta } = await api<IndexApiResponse<Tags>>('bo/tags');

  return {
    tags: data,
    meta: meta,
  };
});

This works fine at runtime, but when I use useFetch('/api/tags') on the client, the response is typed as

const data: globalThis.Ref<{} | undefined, {} | undefined>

const data: globalThis.Ref<{} | undefined, {} | undefined>

, and I lose all the type info. Strangely, if I return something simple like { hello: 'foo' }, types are inferred just fine. Any idea how to get full type inference working for the return value so useFetch() is typed correctly?? Thanks in advance! 🙏

3 Replies

kapa.ai•2mo ago

Hi @EatSleepGames. I'm kapa.ai, your friendly support bot. I will try to answer your question by searching through the knowledge base. :wave: ---------------------- You're encountering a common limitation in Nuxt 3's type inference for API routes, especially when returning complex or generic data structures from your server API handlers. Why does this happen? Nuxt tries to infer the return type of your API endpoints for useFetch by analyzing the return value of your handler. However, when you use generics or more complex types (like your IndexApiResponse<Tags>), Nuxt's type generation may not be able to extract the correct type information, resulting in a fallback to {} or any on the client side. In contrast, simple literal objects (like { hello: 'foo' }) are easy for the type system to infer, so you get proper types in those cases. This is a known issue and has been discussed in the Nuxt community. The type inference works well for static, non-generic return types, but struggles with dynamic or generic ones, especially when route parameters or generics are involved. There is currently no built-in way to fully infer types from dynamic or generic API responses in all cases. See the discussion and examples in this GitHub issue. Workarounds 1. Manual Typing on the Client
The most reliable workaround is to manually specify the expected type when calling useFetch:

   const { data } = await useFetch<{ tags: Tag[]; meta: Meta }>('/api/tags')

   const { data } = await useFetch<{ tags: Tag[]; meta: Meta }>('/api/tags')

This ensures you get proper type safety in your components, even if Nuxt can't infer it automatically. 2. Custom Serializer (Advanced)
If your API returns complex objects (like Dates or Maps), you can use a custom toJSON method or a library like superjson to control serialization and help with type inference, but this doesn't solve the generic type inference issue directly. See the Nuxt docs on custom serialization. 3. Type Inference Improvements (Ongoing)
The Nuxt team is aware of these limitations and there are ongoing discussions and proposals to improve type inference for dynamic routes and generics, but as of now, a fully automatic solution does not exist. See the proposal and community feedback here. Summary - This is a known limitation with Nuxt's type inference for API routes, especially with generics. - The recommended approach is to manually specify the expected type in your useFetch call. - Keep an eye on Nuxt's updates for future improvements in this area. For now, manual typing is the best way to ensure type safety for complex API responses in Nuxt 3. ---------------------- Note: you can ask me a follow up question by @ mentioning me again :speech_balloon: ----------------------

kapa.ai•2mo ago

<@140278910406361088> Kapa.ai is still learning and improving, please let me know how I did by reacting below.

EatSleepGamesOP•5w ago

I found a workaround that makes this setup work correctly. For some reason, importing the interface types directly inside the server API file is necessary — even if they aren't used elsewhere in the code. Once I do that, useFetch on the frontend gets proper type inference automatically, without needing to define or pass types manually in the client code. Here’s the working code:

import { useApiClient } from '~~/server/lib/apiClient';
import type { ApiResponse } from '~~/server/types/apiResponse';
import type { Meta } from '~~/server/types/meta';

export default defineEventHandler(async (event) => {
  const api = useApiClient(event);

  const { data, meta } = await api<ApiResponse<Tags[]>>('bo/tags');

  return {
    tags: data as Tags[],
    meta: meta as Meta,
  };
});

import { useApiClient } from '~~/server/lib/apiClient';
import type { ApiResponse } from '~~/server/types/apiResponse';
import type { Meta } from '~~/server/types/meta';

export default defineEventHandler(async (event) => {
  const api = useApiClient(event);

  const { data, meta } = await api<ApiResponse<Tags[]>>('bo/tags');

  return {
    tags: data as Tags[],
    meta: meta as Meta,
  };
});

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Losing types when returning data from server API handler

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