perf(translations): load every date-fns locale from one file on the server

[AlessioGr]

Written by AI (Claude Code). The measurements were also run by AI.

What

importDateFNSLocale loads one of 40 date-fns locales, each through its own import(). That's right for browsers, which only download the language they use. But bundlers compile every import() target, so server components and SSR also compiled all 40 locale files and their loaders.

Servers don't download anything, so they now load all locales from one bundled file, dist/dateFNS/all.js. The translations package imports its loader through #importDateFNSLocale, a package-private import defined in the imports field of package.json: browsers get the per-locale version, and everything else gets dist/importDateFNSLocale.server.js. The build checks that the server version only reads locales that all.js exports.

imports works like exports, but for paths a package only uses internally. Node, Next.js, Vite and esbuild all support it, and publishConfig points it at dist when the package is published, the same way it does for exports. Inside the monorepo, both point at the source file, as before.

Results

Fresh website template (Next.js 16.4.0, Turbopack), cold next dev at 4 threads, with locally packed packages: main, and main plus only this PR. Next.js 16.4 no longer logs compile times, so the /admin response time is measured without a browser and covers compiling and rendering the page. Times are medians of 6 interleaved runs, which vary by about ±0.2 s.

Cold next dev, 4 threads	main	This PR
Server component modules compiled for /admin	1,108	1,029 (−79)
SSR modules compiled for /admin	468	389 (−79)
Browser modules compiled for /admin	603	603
All modules compiled for /admin	2,250	2,092
/admin response time, opened first	4.73 s	4.71 s
/admin response time, after the front-end page	2.71 s	2.59 s

On its own, the response time change is within run-to-run noise.

🤖 Generated with Claude Code

[github-actions]

📦 esbuild Bundle Analysis for payload

This analysis was generated by esbuild-bundle-analyzer. 🤖
This PR introduced no changes to the esbuild bundle! 🙌