vercel/next.js

Parameter on `AppType` is used incorrectly

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#42846 opened on Nov 13, 2022

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Description

Verify canary release

  • I verified that the issue exists in the latest Next.js canary release

Provide environment information

sandbox@sse-sandbox-xhvisb:/sandbox$ yarn next info
yarn run v1.22.19
$ /sandbox/node_modules/.bin/next info

    Operating System:
      Platform: linux
      Arch: x64
      Version: #58~20.04.1-Ubuntu SMP Tue Jun 14 11:29:12 UTC 2022
    Binaries:
      Node: 14.19.3
      npm: 6.14.17
      Yarn: 1.22.19
      pnpm: N/A
    Relevant packages:
      next: 13.0.3-canary.4
      eslint-config-next: N/A
      react: 18.2.0
      react-dom: 18.2.0

warn  - Latest canary version not detected, detected: "13.0.3-canary.4", newest: "13.0.3".
        Please try the latest canary version (`npm install next@canary`) to confirm the issue still exists before creating a new issue.
        Read more - https://nextjs.org/docs/messages/opening-an-issue
Done in 0.84s.

Note: warning at the end seems to be because I'm using next@canary (as instructed) but canary version is older than latest official release. My issue is present in either version.

Describe the Bug

Prop types defined by the parameter on AppType are incorrectly applied to MyApp's props.pageProps instead of MyApp's props.

Expected Behavior

Expected TS types to correctly reflect runtime types.

Link to reproduction

https://codesandbox.io/s/polished-tree-xhvisb?file=/pages/_app.tsx

To Reproduce

  1. Open the link to reproduction
  2. Observe code on left (with TS errors) and the produced page on right.

There's a TS error on line 13, complaining Property 'foo' does not exist on type 'AppPropsType<any, MyInitialProps>', even though props.foo does exist at runtime.

Also, there's no TS error on line 14, even though props.pageProps.foo does not exist at runtime.

image

Contributor guide

Tech stack
typescriptnextjsreact
Domain
frontend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript basicsNext.js page structureUnderstanding of generics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Investigate the AppType type definition in Next.js source code (likely in packages/next/shared/lib/utils.ts or similar). Understand how the generic parameters are applied to App component props. The issue indicates that the second parameter (pageProps) is incorrectly typed as the App's own props. Look at how AppPropsType is defined and ensure custom props are properly propagated. Check the comments in the issue for any suggested fixes or maintainer input.