## Summary
- Handle the `extends` array form (TypeScript 5.0+) when resolving
tsconfig/jsconfig path aliases and `baseUrl` for `next.config.ts` loading.
- Normalize `extends` (string | string[] | undefined) into an ordered list at
both loader sites; iterate parents so later entries override earlier ones.
- Add array-extends regression tests (single parent, multi-parent precedence,
child override).
## Root Cause
Both tsconfig loaders only handled the string form of `extends`
(`typeof parsed.extends === "string"`). When `extends` was an array
(`["./a.json", "./b.json"]`), it was silently dropped, so inherited
`compilerOptions.paths`/`baseUrl` were lost and path-aliased imports in
`next.config.ts` failed to resolve. Next.js normalizes via
`Array.isArray(config.extends) ? config.extends : [config.extends]` and merges
parents in order; this change mirrors that behavior at both vinext sites
(packages/vinext/src/config/tsconfig-paths.ts and packages/vinext/src/index.ts).
## References
- Fixes#1974
- Next.js parity: packages/next/src/build/next-config-ts/transpile-config.ts (loadTsConfigFile)
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Next.js's own loader (packages/next/src/build/next-config-ts/transpile-config.ts)
reads compilerOptions.paths + baseUrl from tsconfig.json and passes them to SWC
so next.config.ts can import via tsconfig aliases like:
import { foo } from '@/foo'
vinext's loadNextConfig() calls Vite's runnerImport(), but did not configure any
resolve.alias entries, so the same imports fail with:
[vinext] Failed to load next.config.ts:
Cannot find module '@/foo' imported from '<dir>/next.config.ts'
This is a parity gap surfaced by the Next.js deploy test suite.
Fix: extract a small tsconfig-paths loader (config/tsconfig-paths.ts) that
reads compilerOptions.paths (following extends, honoring baseUrl) and returns
absolute Vite resolve.alias entries. loadNextConfig now passes those aliases
into runnerImport's inlineConfig.
Affected suites (from run 25870737355):
- e2e/app-dir/next-config-ts/import-alias-paths-only/
- e2e/app-dir/next-config-ts/import-alias-paths-with-baseurl/
- e2e/app-dir/next-config-ts/import-js-extensions-cjs/
- e2e/app-dir/next-config-ts/import-js-extensions-esm/
- e2e/app-dir/next-config-ts-native-ts/dynamic-import-esm/
- e2e/app-dir/next-config-ts-native-mts/import-from-node-modules/
Verification:
- 9 new unit tests for loadTsconfigPathAliasesForRoot
- 4 new unit tests covering loadNextConfig + tsconfig paths (ported from
Next.js's import-alias-paths-only / import-alias-paths-with-baseurl
fixtures), including extends + jsconfig fallback
- Reproduced against .nextjs-ref/test/e2e/app-dir/next-config-ts/
import-alias-paths-only/ — config now loads with `foo === "foo"`
- vp check (fmt + lint + types) clean
- bash -n scripts/*.sh clean
Refs: cloudflare/vinext#actions/runs/25870737355
Xplod13