Journal

Progress

• TypeScript 7.0.2 pinned in the course’s own package.json and lock file, with @types/node 24.13.4 and tsx 4.23.13
• CI: tsc on the project and on each error snippet, every example and solution run with Node.js 24.21.0, the C# and Java comparisons compiled and run, all compared with their expected output on three OSes
• Lesson 1: the compiler and the tooling
• Lesson 2: structural typing
• Lesson 3: unions and narrowing
• Lesson 4: generics
• Lesson 5: type-level programming

QA

The TypeScript compiler at 7.0.2 — the Go port — with Node.js 24 and npm 11. Half of these rows are deliberate changes that break an assumption rather than defects, and the Status column says which. The fourth row is the one place where the author expected an error, did not get one, and the compiler turned out to be right.

There is no Experiments table: that fourth row is the journal’s only prediction written before its test, and a table of one row would read like a section that failed to fill.

Expected	What happens	Where	Measure	Status
tsc --noEmit with errors returns the same exit code it always has	The code changed between 6.0 and 7.0	tsc 6.0.3 against 7.0.2	6.0.3 returns 2, 7.0.2 returns 1. In 7.0.2, 1 means errors prevented the output and 2 means the output was written despite them	By design, a breaking change. A CI step testing for 2 breaks on the upgrade; test for non-zero 2026-09-15
tsc file.ts next to a tsconfig.json quietly ignores the config	Since 6.0 it is an error	tsc 6.0 and later	TS5112. --ignoreConfig restores the old behaviour; the course builds a small tsconfig per error snippet instead	By design 2026-09-15
Union members print in the order they were declared	They do not, and the order changed between versions	tsc 5.9.3 against 7.0.2	On GA’s ForceRadiant.tsx: `“warning”	“error”
A wrong out variance annotation is refused	interface Mislabeled<out T> { accept(value: T): void } compiles	tsc 7.0.2	Method parameters are bivariant, so T in that position satisfies both annotations. C# reports the same interface as CS1961. The snippet was replaced with in T on a method returning T, which tsc does catch as TS2636	By design. The author expected an error and the compiler was right — the lesson changed, not the compiler 2026-09-15
Array.isArray narrows an unknown safely	It narrows to any[], so a property access compiles with no check at all	tsc 7.0.2, the lib declaration of Array.isArray	In lesson 2’s exercise-2 solution, item.name compiled without a check, and tsc never asked for the typeof checks that make it correct	By design, and worth knowing 2026-09-15
A stable API is silent	module.stripTypeScriptTypes still prints an ExperimentalWarning, although type stripping itself is stable and silent	Node.js 24.21.0	The warning, on every call	Reproduced, not reported — the best upstream candidate in this journal 2026-09-15
Node’s error points at the line as written	The source line in the message has spaces where the types were	Node.js 24.21.0 type stripping	const guitarSource = instrumentSource; — that is how stripping preserves line and column numbers	By design 2026-09-15
dotnet run file.cs prints the compiler’s warnings on every run	It builds incrementally, so a second run with an unchanged source prints them once and never again	.NET 10 file-based programs	The first CI run failed on all three OSes on two files: CS8602 and CS8509 were in the CI’s output and not in the author’s. Fixed with dotnet clean then dotnet run --no-cache	Reproduced in CI and locally, not reported 2026-09-14
A newer compiler reports the same errors	TypeScript 7 finds more of them, much faster	tsc 5.9.3 against 7.0.2 on ga-react-components	180 errors in 14.7 s with 5.9.3; 197 in 1.9 s with 7.0.2. New codes: TS2591, because types now defaults to []; TS2871 “this expression is always nullish”; TS2550 for .at() with lib at ES2020	Reproduced. The journal records its own caveat: the dependency versions came from a non-frozen install 2026-09-15
npm runs install scripts	npm 11.19 no longer runs them by default	npm 11.19 with tsx 4.23.13	npm warn install-scripts esbuild@0.28.2 (postinstall: node install.js). tsx works anyway on the three OSes, because esbuild’s binary arrives as an optional platform package	By design, a changed default 2026-09-14
TypeScript 7.0 offers a programmatic API	It has none yet; tools that load the compiler as a library use @typescript/typescript6, a compatibility package that provides 6.0 with a tsc6 command	TypeScript 7.0	Read in the documentation, not exercised — the course did not need it	By design, a known gap in the Go port 2026-09-14
A lock file written on Windows needs rewriting on Linux	It lists all twenty platform packages as optional dependencies, so one file works on three systems	typescript 7.0.2, the Go port	@typescript/typescript-win32-x64 locally; the CI step lists typescript-linux-x64 and typescript-darwin-arm64	By design, and it is what makes the three-OS CI cheap 2026-09-14

2026-09-14 — Installing TypeScript 7.0.2

• npm view typescript version gives 7.0.2, the Go port, published on 2026-07-08. The typescript package is now small: npm installs the compiler as one package per platform, @typescript/typescript-win32-x64 on my machine, and the CI’s info step lists typescript-linux-x64 on Linux and typescript-darwin-arm64 on macOS. The lock file lists all twenty platform packages as optional dependencies, so npm ci works on the three OSes from the one lock file written on Windows.
• TypeScript 7.0 has no programmatic API yet. Tools that load the compiler as a library can use @typescript/typescript6, a compatibility package that provides TypeScript 6.0 with a tsc6 command; I didn’t need it for the course.
• The course pins its versions in code/typescript-for-csharp-java/package.json, not in the site’s: the site itself has no typescript dependency.
• npm 11.19 no longer runs install scripts by default. Installing tsx printed npm warn install-scripts esbuild@0.28.2 (postinstall: node install.js), and tsx works anyway, on the three OSes: esbuild’s binary comes from a platform package installed as an optional dependency.

2026-09-14 — The CI

• typescript-examples.yml installs Node.js 24.21.0, .NET 10 and Java 25, runs npm ci in the course folder, then check.sh with REQUIRE_COMPARE=1, so that a missing dotnet or java fails the run instead of skipping the comparisons.
• check.sh runs tsc once on the whole project, which must pass, and once per error snippet, with a generated tsconfig that extends the course’s and lists that one file; a snippet can ask for extra options with a // tsc options: comment, as lesson 2’s noUncheckedIndexedAccess does. normalize.mjs, adapted from the JavaScript course, makes paths relative and removes stack frames and process IDs.
• The first run failed on the three OSes, on two files only, and for the same reason: the C# warnings CS8602 and CS8509 were in the CI’s output and not in mine. dotnet run file.cs builds a file-based program incrementally, and a second run with an unchanged source doesn’t call the compiler, so its warnings are printed only once. check.sh now runs dotnet clean on the file before dotnet run --no-cache, and the outputs captured on Windows match on Linux and macOS. The next run passed on the three OSes in 2 minutes 41 seconds, Windows being the slowest job.

2026-09-15 — Surprises while writing lessons 1-4

tsc’s exit code changed between 6.0 and 7.0. With --noEmit and errors, tsc 6.0.3 returns 2, and 7.0.2 returns 1. In 7.0.2, 1 means that errors prevented the output, with noEmit or noEmitOnError, and 2 that the output was written despite errors, which lesson 1 shows with l01-emit. A CI step that tests $? -eq 2 would break on the upgrade; test for a non-zero code.

tsc file.ts next to a tsconfig.json is an error since 6.0. TS5112 refuses to ignore the configuration silently, as older versions did; --ignoreConfig restores the old behavior. check.sh builds a small tsconfig for each error snippet instead.

Union members don’t print in declaration order. The same comparison in GA’s ForceRadiant.tsx prints "warning" | "error" | "unknown" | "contradictory" with 5.9.3 and "contradictory" | "error" | "unknown" | "warning" with 7.0.2. The 6.0 release notes explain it: TypeScript 7 sorts types by content so that parallel checkers agree. Lesson 3’s expected outputs are those of 7.0.2 only.

A wrong out annotation compiles. I wrote interface Mislabeled<out T> { accept(value: T): void } for lesson 4’s error snippet, expecting an error, and tsc 7.0.2 accepted it: accept is a method, method parameters are bivariant, and T in that position satisfies both annotations. C# reports the same interface with CS1961. The snippet now uses the mistake that tsc does catch, in T on a method that returns T (TS2636).

Array.isArray brings any back. In the solution of lesson 2’s second exercise, the value was unknown, and Array.isArray(value) narrowed it to any[]: item.name compiled without a check. The solution’s typeof checks make it correct; tsc didn’t ask for them. Lesson 2 points it out.

Node.js prints the stripped line. When a .ts file fails at run time, the source line in Node.js’s message has spaces where the types were, as in lesson 4’s const guitarSource = instrumentSource;. That is how type stripping keeps line and column numbers.

module.stripTypeScriptTypes still prints an ExperimentalWarning in 24.21.0, although type stripping itself is stable and silent; lesson 1 shows the warning.

2026-09-15 — What tsc found in GuitarAlchemist/ga (a826864)

I cloned GuitarAlchemist/ga at a826864 into a scratch folder, installed the two front ends, and ran tsc with 5.9.3, the version that the component library’s pnpm-lock.yaml resolves, and with 7.0.2.

• Names used and never defined. Apps/ga-client/src/components/Chat/ChatInterface.tsx uses VIRTUALIZATION_THRESHOLD (lines 69 and 219) and VirtualizedMessageList (lines 254 and 255), and nothing defines or imports them: TS2304. The /ai-copilot route of App.tsx loads that component, so the page should throw a ReferenceError when it renders (to verify in a browser). Reproduction: cd Apps/ga-client && npm ci && npx tsc -b --pretty false | grep ChatInterface. This looks like a bug worth an issue.
• Type errors don’t fail the build. Both front ends build with vite build, which strips types without checking them; ga-client reports 346 errors with tsc -b (165 of them TS6133, unused variables), and ga-react-components 186. A tsc --noEmit step in CI would have caught the missing names.
• A stale lock file. In ReactComponents/ga-react-components, pnpm install --frozen-lockfile fails with ERR_PNPM_OUTDATED_LOCKFILE: 21 dependencies of package.json are missing from pnpm-lock.yaml, and the versions of @react-three/drei and @react-three/fiber differ. I installed without --frozen-lockfile, so the component library’s error counts may depend on the versions I got.
• Statuses that the type rules out. ForceRadiant.tsx, lines 721 and 725, compares a GovernanceHealthStatus with 'ok' and 'critical', which the union doesn’t contain (TS2367), and DataLoader.ts, lines 276-278, passes a SignalR message’s healthStatus: string through as unknown as GovernanceNode. Either the union is missing two statuses or the comparisons are dead code (to verify in GA’s hub). Line 1464 of the same file compares a signal severity, 'info' | 'warning' | 'emergency', with 'critical'. Lesson 3 reduces the first case.
• noImplicitAny: false next to strict: true in ga-react-components/tsconfig.app.json. Turning it back on adds only four errors, in BSPDoomExplorer.tsx and IxqlFormPanel.tsx; lesson 2 lists them.
• Unchecked data. The component library has 34 as unknown as, 42 calls to JSON.parse and 18 as any. The camera is restored from localStorage with JSON.parse(saved) as { px: number; … } (ForceRadiant.tsx, line 3558), loadQueue<T> returns JSON.parse as a T[] (CourseViewer.tsx, line 152), and ga-client’s isApiResponse<T> checks two property names before promising an ApiResponse<T>, with json as T as its fallback (musicService.ts, lines 17-49). Lessons 3 and 4 and their exercises write the checked versions.
• TypeScript 7 finds more. tsc -p tsconfig.app.json on the component library gives 180 errors in 14.7 seconds with 5.9.3 and 197 in 1.9 seconds with 7.0.2. The new errors are Node.js names in two scripts (TS2591, because types now defaults to []), TS2871 (“this expression is always nullish”) in BrainstormPanel.tsx line 40 and GitHubPollingManager.ts line 48, and TS2550 for .at() in ChatWidget.tsx line 1254, with lib set to ES2020; 5.9.3 reported neither code.
• Small things. ga-client/tsconfig.json has "sourceMaps": true at its top level, where tsc ignores it; the option is sourceMap, inside compilerOptions. src/components/PrimeRadiant/index.ts in the component library exports RemediationAction twice (TS2300, lines 101 and 129), and ThreeFretboard.tsx line 780 uses a GuitarModelStyle type that doesn’t exist. ga-client’s ShowcasePanel.test.tsx uses Node.js’s global, which the browser project’s types don’t declare.

2026-09-15 — What tsc found in this site (8ba378e)

npx -p typescript@7.0.2 tsc --noEmit at commit 8ba378e, with the site’s tsconfig.json, reports three errors, the same with 6.0.3. None breaks the site.

• astro.config.mjs, line 146: the sidebar imported from src/streeling-sidebar.json doesn’t match Starlight’s SidebarItemUserConfig, because the inferred union of its entries adds fr?: undefined to the entry that has only a Spanish translation. Lesson 3 explains the inference.
• code/javascript-for-csharp-java/errors/l04_private_outside.js: the site’s tsconfig.json includes **/*, so tsc checks the course code, including an error snippet that is wrong on purpose. The TypeScript course’s errors/*.ts will be picked up the same way; excluding code/ in the site’s tsconfig.json would keep editors and tsc on the site’s own files.
• src/content.config.ts: astro:content is declared only after astro sync.

To verify

• The ReferenceError on GA’s /ai-copilot page, in a browser.
• Whether GA’s governance hub sends ok and critical as health statuses.
• What GA’s ForceRadiant does with a saved camera that lacks a coordinate.
• new[] { 1, "two", null } in C#, quoted in lesson 2 as refused for lack of a best common type.
• The installation commands of lesson 1 on Linux and macOS outside CI.