fix(shims): add runtime default export to next/app

[james-elicx]

Summary

Adds a runtime default export to packages/vinext/src/shims/app.ts (now .tsx) so Pages Router fixtures that follow the canonical pages/_app.js pattern can build under vinext.

The problem

Pages Router code that does:

import App from "next/app";
export default class MyApp extends App {
  static async getInitialProps(appContext) {
    const appProps = await App.getInitialProps(appContext);
    return { ...appProps, extraProp: "Hi There" };
  }
  // ...
}

currently fails to build under vinext with:

[MISSING_EXPORT] "default" is not exported by "node_modules/.../vinext/dist/shims/app.js"

The existing shim only re-exported a type as default (export type { AppProps as default }), with no runtime value.

This is the single highest-leverage shim fix in the Next.js deploy suite: 51 build failures all trace back to this missing default export.

What I ported

From packages/next/src/pages/_app.tsx (Next.js canary):

• App extends React.Component<P & AppProps<CP>, S> and renders <Component {...pageProps} />.
• App.getInitialProps(appContext) returns { pageProps } — invokes the wrapped page's getInitialProps if defined, otherwise returns { pageProps: {} }.
• App.origGetInitialProps is preserved alongside getInitialProps (Next.js userland code occasionally introspects this).
• AppProps, AppContext, and AppInitialProps types are exported.

The original Next.js implementation delegates to loadGetInitialProps in shared/lib/utils.ts. For the canonical _app pattern, the only relevant behaviour is the pageProps extraction, which is replicated inline; the dev-only validation (getInitialProps as an instance method, response-sent check, empty-object warning) is intentionally omitted — see "Intentional omissions" below.

Tests

tests/shims.test.ts gains a next/app shim describe with four cases:

1. The default export is a React class component (has prototype.render, instances are React.Component instances).
2. App.render() produces <Component {...pageProps} /> (asserted via renderToStaticMarkup).
3. App.getInitialProps is a function, returns { pageProps: <result> } when Component.getInitialProps is defined, and origGetInitialProps is preserved.
4. App.getInitialProps returns { pageProps: {} } when Component has no getInitialProps.

Run with: vp test run tests/shims.test.ts -t "next/app shim" — all 4 pass, full file (870 tests) green.

Intentional omissions

These bits of Next.js's _app.tsx were intentionally left out because they are not exercised by Pages Router test fixtures and would expand the shim surface beyond what the failing deploy-suite builds need:

• loadGetInitialProps dev-mode validation — Next.js throws if getInitialProps is defined as an instance method, warns on empty objects, and checks isResSent(res). Userland code calling App.getInitialProps(appContext) doesn't depend on these. If the deploy suite later catches a fixture that relies on them, they're easy to port over.
• AppType, AppPropsType, AppTreeType as re-exports — the typescript test suite imports AppType from next/app, but no JS fixture does. Adding these is a follow-up if the typing parity gap matters.
• Detailed NextPageContext / Router typing — ctx and router are typed as any here, matching Next.js's permissive _app.tsx generics (<P = any>).

Verification

• vp test run tests/shims.test.ts — 870/870 pass
• vp check — formatting, lint, type checks clean

Related

This is the first of a series of focused shim fixes against the Next.js deploy suite. Other shim-level parity gaps are separate PRs.

[james-elicx]

/bigbonk review

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/vinext@1204

commit: 5773166

[github-actions]

Example	Preview	Production	Original
app-router-cloudflare	preview	production
pages-router-cloudflare	preview	production
app-router-playground	preview	production	original
realworld-api-rest	preview	production
nextra-docs-template	preview	production
benchmarks	preview	production
hackernews	preview	production	original

[ask-bonk]

Looks good. The shim faithfully ports the essential behavior from Next.js _app.tsx — class shape, origGetInitialProps/getInitialProps statics, and the render() delegation all match upstream.

Tests are thorough: class component shape assertion, renderToStaticMarkup verification of render output, getInitialProps delegation with and without Component.getInitialProps.

The intentional omissions (dev-mode validation from loadGetInitialProps, AppType/AppPropsType re-exports) are well-documented and reasonable for the current scope.

Minor gap (not blocking): next-shims.d.ts lines 506-512 still declare the old minimal AppProps-only shape without the default class export, AppContext, or AppInitialProps. Since the shim map in index.ts resolves to the source .tsx file directly, this does not affect builds — but external .d.ts consumers would not see the new types. Fine as a follow-up.

One nit inline on null-coercion behavior.

[ask-bonk]

nit: Next.js's loadGetInitialProps throws here when the result is null/undefined ("should resolve to an object. But found \"${props}\" instead."), while this silently coerces to {}. The PR documents this as intentional and it's fine for the current scope — well-behaved getInitialProps implementations won't return nullish. Just flagging for awareness: if a future deploy-suite failure shows a page rendering with empty props when it should have errored, this is the divergence point.

[ask-bonk]

Review posted successfully. Here's a summary of what I reviewed and the outcome:

Verdict: APPROVED

The PR adds a runtime default export to the next/app shim, replacing a type-only default export that caused 51 build failures across the deploy suite. The implementation faithfully ports the core behavior from Next.js's _app.tsx:

• App class extends React.Component with render() returning <Component {...pageProps} />
• Static getInitialProps and origGetInitialProps delegate to the wrapped page's getInitialProps
• Tests cover class shape, render output, getInitialProps delegation, and the no-getInitialProps fallback

One nit flagged (inline comment on app.tsx:73): Next.js's loadGetInitialProps throws when getInitialProps returns null/undefined, while this shim silently coerces to {}. Documented as intentional, fine for current scope.

One minor gap noted (not blocking): next-shims.d.ts still has the old minimal type declarations and doesn't reflect the new default class export or AppContext/AppInitialProps types. Since the shim map resolves to the source .tsx directly, this doesn't affect builds — but worth a follow-up for external .d.ts consumers.

github run

[james-elicx]

/bigbonk review

[ask-bonk]

Clean, well-scoped fix. The shim faithfully replicates the essential runtime behavior from Next.js's _app.tsx — class shape, getInitialProps/origGetInitialProps statics, and render() delegation all match upstream. The type declarations in next-shims.d.ts were updated to reflect the new default class export, AppContext, and AppInitialProps. Tests are solid.

Ship it.

[ask-bonk]

Review posted: APPROVED

The PR is clean and well-scoped. Here's my assessment:

What it does: Replaces the type-only default export in packages/vinext/src/shims/app.ts (now .tsx) with a real runtime App class component, unblocking 51 deploy-suite build failures that all trace to [MISSING_EXPORT] "default" is not exported by .../shims/app.js.

What I verified:

• The App class correctly extends React.Component with render() returning <Component {...pageProps} />
• Static getInitialProps and origGetInitialProps faithfully port the core behavior from Next.js's _app.tsx
• The null-coercion divergence (line 87-89) is well-documented and reasonable for current scope
• next-shims.d.ts was updated to reflect the new default class export plus AppContext/AppInitialProps types
• Four focused test cases cover class shape, render output, getInitialProps delegation, and the no-getInitialProps fallback
• CI Check job passed; deploys and E2E tests all green

github run