TypeScript: expand ${configDir} and compute rootDir the way tsc does

[0xfauzi]

Fixes #13, and removes the guess at rootDir that #13's first commit left in place.

Two commits. The first expands ${configDir}. The second replaces the guessed source root with the one the project's own tsc would use. Both rules were measured with the compiler before they were written, and each has a test whose docstring states the acceptance first.

Commit 1: expand ${configDir} in inherited tsconfig paths

${configDir} stands for the folder of the top-level tsconfig.json, the one the project owns, so a shared base config can point at the project that extends it. Before, the text was kept as written, so @sindresorhus/tsconfig left ky's outDir at node_modules/@sindresorhus/tsconfig/${configDir}/distribution, a folder that does not exist.

The rule, from tsc --showConfig on TypeScript 7.0.2 with the variable at three levels of an extends chain:

value written	in	tsc resolved it to
${configDir}/source (rootDir)	npm package	./source under the top-level folder
${configDir}/typings (typeRoots)	nested config	./typings under the top-level folder
./local-typings (typeRoots)	nested config	./config/nested/local-typings, the declaring folder
${configDir}/src/* (paths)	npm package	the absolute top-level src/*
lib/* (paths)	npm package	lib/*, unchanged, relative to baseUrl
cache/${configDir}/x (tsBuildInfoFile)	nested config	left literal, resolved from the declaring folder

So: in every file of the chain, a baseUrl, paths target, rootDir or outDir value that starts with the variable becomes the top-level folder; anywhere else it stays literal; a plain relative value still means the declaring file's folder. The same holds for files, include and exclude, which commit 2 reads.

Commit 2: compute rootDir the way the project's tsc does

With rootDir unset, the reader used to guess src/ or the repository root. That is a guess, and on a TypeScript 5 project with include: ["source"] it maps out/source/cli.js to a file that tsc would never have produced. The new test shows that case failing on the old code.

What tsc does, measured by where it emits with rootDir unset, on ten layouts:

layout	5.9.3	6.0.3	7.0.2
all sources under source/	root is source/	root is the tsconfig folder, and error TS5011 asks for an explicit rootDir	root is the tsconfig folder
source/ and scripts/	the common folder, the repository	the tsconfig folder	the tsconfig folder
include: ["source"] with scripts/ present	source/	the tsconfig folder, TS5011	the tsconfig folder
files: ["source/index.ts"]	source/
exclude: ["scripts"]	source/
a .d.ts outside source/	source/ (declarations do not count)
a test file under test/	the repository (tests count)	the tsconfig folder	the tsconfig folder
composite: true	the tsconfig folder	the tsconfig folder	the tsconfig folder
include: ["${configDir}/source"] in a base config	source/

Blank cells were not run; the 6 and 7 columns did not vary with the input set in any run.

The rule the reader now follows, in root_candidates:

1. An explicit rootDir is the answer.
2. composite means the tsconfig folder, on every version.
3. Otherwise the project's TypeScript major decides. 5 takes the longest common folder of the non-declaration files that files, include and exclude select, test files included. 6 and later take the tsconfig folder.
4. The major is read from node_modules/typescript/package.json, then from the typescript range in package.json (^5.9.3 names 5). When neither exists, both roots are tried, and a compiled target maps to a module only when exactly one root names one. Two matches leave the target unmapped, because which root tsc used is not knowable, and a wrong answer costs more than none.

The input-file selection is checked against tsc

files, include and exclude follow tsconfig glob rules: **/ crosses folders, * and ? stay inside one, a folder entry means everything under it, and outDir is always left out. To make sure this matches the compiler rather than my reading of the docs, I ran tsc --listFilesOnly (5.9.3) on five of the pinned repositories and compared the non-declaration source files it lists with input_files:

repository	tsc	input_files	only in tsc	only in input_files
ky	31	31	0	0
taxonomy	123	123	0	0
zod	467	467	0	0
nest	1073	1073	0	0
excalidraw	637	637	0	0

hono and date-fns are solution-style configs (files: [] or include: [] with references), so both tsc and input_files see no inputs there; systemap does not follow references, and on those two the major (6 and 7) picks the tsconfig folder anyway.

Effect on the pinned repositories

bench/typescript_adapter.py gives the same counts as the run recorded on #12 for all seven, every column. ky is the one whose path changed: its include: ["source"] and TypeScript ^5.9.3 give root source/, and with the src//source/ fallback disabled its seven package entries still map, through outDir and rootDir alone. Before this commit they only mapped through the fallback.

The extra walk input_files makes costs 0.05 s on zod and 0.21 s on nest, measured in the container.

Verification

• uv run pytest -q: 400 passed, including the two new tests.
• uv run pre-commit run with each commit staged against its parent: every hook passes, including the complexity and file-length ratchets.
• uv run mypy src/systemap: clean.
• uv run systemap refresh, extract --check, check, judgement --strict, render --check: pass; docs/map/ is updated.

Branch

This branch sits on #12's current head 084122e, because typescript_config.py only exists there. Until #12 merges, this PR shows #12's six commits plus two. After #12 merges, I will rebase so only these two remain.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CzLdCRXKdAP2swLysEfacu

[github-actions]

What this change does to the map

83aabb818069b62c3cce16dea34dcb3d4a0ebda7 to 91e63d5d26a09fcb856d217aca5ad1056bbb16b4: 8 modules changed, 5 added, 0 removed, 0 moved; 2 of 16 cards named.

Changed, nothing to do (5)

• added: systemap.language, claimed by FactsExtractor
• added: systemap.language_config, claimed by Config
• added: systemap.typescript, claimed by FactsExtractor
• added: systemap.typescript_config, claimed by Config
• added: systemap.typescript_surface, claimed by FactsExtractor

Next to the change

FactsExtractor holds most of what changed, and a flow joins it to 9 of the 16 cards, so naming them would say nothing.

The committed map at 91e63d5; the change map itself is systemap figure --mode change --base 83aabb8 --out change.svg.
Nothing to decide: the map already covers this change. systemap refresh brings the facts up to date.