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TypeScript reports errors when union type passed as Body to RouteGeneric since Fastify 4 #4063

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svsool opened this issue Jun 21, 2022 · 3 comments · Fixed by #4076
Closed
2 tasks done

TypeScript reports errors when union type passed as Body to RouteGeneric since Fastify 4 #4063

svsool opened this issue Jun 21, 2022 · 3 comments · Fixed by #4076

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@svsool
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svsool commented Jun 21, 2022 •
edited

Prerequisites

  • I have written a descriptive issue title
  • I have searched existing issues to ensure the bug has not already been reported

Fastify version

4.1.0

Plugin version

No response

Node.js version

18.x

Operating system

macOS

Operating system version (i.e. 20.04, 11.3, 10)

12.3

Description

Hi, I came across the following problem after updating to 4.x.

Body property of request gets resolved to unknown instead of { filters?: { ids: string[] } } | null. Use-case: we have some routes that optionally accept body using ajv's anyOf keyword. It used to work in Fastify 3.

Fastify 4:
image

vs.

Fastify 3:
image

Steps to Reproduce

This snippet can be used to reproduce the problem:

import { fastify } from 'fastify';

const app = fastify();

app.route<{
    Body: { filters?: { ids: string[] } } | null;
}>({
    method: 'DELETE',
    url: '/entities',
    schema: {}, // omitted for clarity
    async handler(request, reply) {
        const filters = request.body?.filters;
    },
});

Expected Behavior

body type gets resolved to { filters?: { ids: string[] } } | null instead of unknown.
@svsool svsool changed the title TypeScript reports errors with union type passed as Body to RouteGeneric since Fastify 4 TypeScript reports errors when union type passed as Body to RouteGeneric since Fastify 4 Jun 21, 2022
@A5rocks
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A5rocks commented Jun 21, 2022 •
edited

I ran into this and investigated a bit (didn't realize someone else had made an issue). Here's what I found:

Firstly, body is governed by ResolveFastifyRequestType<TypeProvider, SchemaCompiler, RouteGeneric>["body"]. In my case, TypeProvider is FastifyTypeProviderDefault, SchemaCompiler is FastifySchema, RouteGeneric is { Body: z.infer<typeof body>, [snip] } where body is a zod.union([..., ..., ...]).

This then gets moved into ResolveRequestBody.

fastify/types/type-provider.d.ts

Lines 34 to 35 in 4361770

type ResolveRequestBody<TypeProvider extends FastifyTypeProvider, SchemaCompiler extends FastifySchema, RouteGeneric extends RouteGenericInterface> =
UndefinedToUnknown<keyof RouteGeneric['Body'] extends never ? CallTypeProvider<TypeProvider, SchemaCompiler['body']> : RouteGeneric['Body']>

See, keyof <union> can be never. This means it goes to CallTypeProvider<TypeProvider, SchemaCompiler['body']> which returns unknown (as we have not extended FastifyTypeProviderDefault).

What is there to do? Well, checking StackOverflow, I find that something like type KeysOf<T> = T extends T ? keyof T : never; can be used for unions. The only problem is that I'm not quite sure why this conditional exists in the first place for ResolveRequestBody.

tl;dr

Why is ResolveRequestBody conditional on keyof <body type>?

If that is intentional, then type KeysOf<T> = T extends T ? keyof T : never; would work; keyof <body type> -> KeysOf<<body type>>.

@mcollina
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Thanks for reporting! Would you like to work on a PR with a fix? I'm not sure if your proposed solution is ok, however it mostly depends if it breaks any other tests.

@A5rocks A5rocks mentioned this issue Jun 23, 2022
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@A5rocks
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A5rocks commented Jun 23, 2022 •
edited

I believe I fix this in #4076

image

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Use a union-aware keyof operator
3 participants
@mcollina @svsool @A5rocks