How 1,762 files get made

The pipeline

The steps, in order

1. prebuild. scripts/copy-wasm-assets.mjs copies the 46 TypeScript declaration files listed in lib/tsLibFiles.json into public/wasm/typescript-lib/. The editor's type checker needs them. Then scripts/build-react-sandbox.mjs uses esbuild to bundle lib/reactSandbox/runtime.ts into a minified ES2020 script, public/wasm/react-sandbox.js, which is 205 KB. The React exercises run inside it.
2. Compile. Turbopack bundles the app. In the last production build's trace this took 2.2 seconds.
3. Type check. tsc runs inside next build and took 2.8 seconds. When the type check runs on its own on a fresh checkout, npx next typegen has to run first, because the global PageProps and LayoutProps helper types have to exist on disk before a separate tsc --noEmit can check app/.
4. Static generation. Every generateStaticParams is called, and every page, route handler and metadata file is rendered to disk. This took 21.5 seconds.

The whole next-build span in .next/trace-build was 29.6 seconds. About seventy percent of it is the step that grows with the content.

Where the routes come from

24 files under app/ export generateStaticParams. The 20 topic [chapter] routes call topicChapterParams(topicId) in components/reader/topicPages.tsx, which returns every chapter in the topic, including outlines. An outline still gets a page, a roadmap card with its syllabus section and a "Meanwhile" link to a related written topic, but its metadata sets index: false. Crawlers can follow its links, but it stays out of search results. /problems/[slug] and /problems/[slug]/cases both map over exercises(), and /level/[topic] maps over topics that have levels.

Nothing reads the content directly except lib/content.ts, which is also where derived fields come from. The reading time shown on every chapter is computed there: anything inside a <script>, <pre>, <style> or <svg> element is dropped, the remaining tags are stripped, the words are counted and divided by 180, and the result is rounded with a minimum of two minutes.

Recording test cases for other languages

The most unusual build step is lib/polyglot/record.ts. Every exercise ships a JavaScript reference solution and JavaScript tests. The playground can also grade Python, Ruby, PHP, Lua, C and C++. Rather than hand-write tests in six more languages, the build records what the JavaScript does:

• It finds the solution's top-level function and its parameter names with a regular expression. Anything else is declined with a reason.
• It keeps only tests made entirely of direct assert.equal, strictEqual, deepEqual or deepStrictEqual calls on that function, plus const set-up lines.
• It runs the solution and each test inside vm.runInNewContext with a two-second timeout. The function is wrapped so that every call logs a deep copy of its arguments and its return value.
• Recorded values have to be plain: numbers, strings, booleans and nested arrays. From those it infers a signature, int, float, bool, string or list of one of them, and it gives up if an argument changes type between calls.

app/problems/[slug]/cases/route.ts is force-static, so this runs 538 times during the build and never in production. Of those, 222 exercises came out gradable in other languages, with 1,439 tests and 2,048 recorded cases, and 395 tests were skipped as not replayable. The other 316 carry a reason the page shows instead:

The cost is size. Most case files are a one-line refusal, and the median is 97 bytes. But recording keeps every call, including the recursive and repeated calls a test makes, so the largest file, ex-maximal-square, is 1,002,059 bytes. Together they come to 4.2 MB. A reader only downloads one after switching that problem to another language.

Indexes, sitemap and preview images

The 21 search indexes are force-static route handlers too. The search chapter covers what goes in them. app/sitemap.ts reads the same interface and lists 1388 URLs. Outlines are left out. A topic's pages carry the meta.updated month from its notes file as lastModified; everything with no such date, including every problem page, carries one shared constant that moves only when that content actually changes, so deploys stop telling crawlers that everything just did. app/robots.ts disallows /api/ and points at the sitemap.

Nine opengraph-image.tsx files draw 1200 by 630 PNG cards with next/og's ImageResponse, through ogCard in lib/og.tsx. The topic cards count their written chapters and exercises at build time, so the chips on a shared link cannot go stale. Each PNG is between 37 and 58 KB, and all of them are prerendered. lib/metadata.ts gives every other page one of those nine images by matching its path to the nearest section, so a shared link always carries a picture, not bare text.

What next.config.ts does, and does not, do

It sets headers and redirects, turns off the automatic trailing-slash redirect so that proxy.ts can do it in one hop, and nothing else: no images block, no experimental flags, no webpack changes. The CSP inside it is assembled from two helpers. One is wasmOrigins() from lib/wasmAssets.ts. The other is sentryOrigin(), which adds nothing unless a DSN is set.

lib/wasmAssets.ts is where every runtime version is pinned: Pyodide 314.0.6, sql.js 1.14.2, wasmoon 1.16.0 for Lua, Ruby WASI 2.10.1, php-wasm 0.1.0, clang 22.0.0-git20542-10 and the WASI shim 0.4.2. By default they are all loaded from cdn.jsdelivr.net. The prebuild script can copy Pyodide and sql.js into public/wasm/ with --all, and three environment variables can point the loaders there, but the default build does neither. tests/wasm-assets.test.ts fails if the pinned Pyodide or sql.js version stops matching the installed package, or if a base URL loses its trailing slash.

What the build leaves behind

The prerendered HTML alone comes to 63.3 MB. .next/server/app, with the RSC payloads and route bodies, is 198 MB on disk.