Fix: support array form of tsconfig extends (TypeScript 5.0) (#1017)

TS 5.0 allows `"extends": ["./a", "./b"]`. _read_tsconfig_aliases called
`extends.startswith("@")` directly, so an array raised
`AttributeError: 'list' object has no attribute 'startswith'`. _safe_extract
caught it and skipped the WHOLE file — and since alias resolution fires on any
path-aliased import, every TS file using `@/`-style imports in a repo with
array-extends tsconfig was dropped, yielding an empty graph.

Normalize `extends` to a list (str -> [str], list kept, else []), process
entries in order (later overrides earlier, current config's paths override all
parents per TS semantics), and skip scoped npm configs (`@...`) per entry as
before.

Adds regression test test_tsconfig_array_extends_alias_resolves_existing_ts_file.

graphify/extract.py

@@ -190,9 +190,24 @@ def _read_tsconfig_aliases(tsconfig: Path, base_dir: Path, seen: set) -> dict[st
         return {}
 
     aliases: dict[str, str] = {}
+    # `extends` may be a string or, since TypeScript 5.0, an array of paths.
+    # For an array, parents are processed in order with later entries
+    # overriding earlier ones; the extending config (paths below) overrides
+    # all parents. Without the list branch, an array `extends` raised
+    # `AttributeError: 'list' object has no attribute 'startswith'`, which
+    # _safe_extract turned into a skip of the whole file.
     extends = data.get("extends")
-    if extends and not extends.startswith("@"):
-        extended_path = (base_dir / extends).resolve()
+    if isinstance(extends, str):
+        extends_list = [extends]
+    elif isinstance(extends, list):
+        extends_list = [e for e in extends if isinstance(e, str)]
+    else:
+        extends_list = []
+    for ext in extends_list:
+        # Skip scoped npm package configs (e.g. @tsconfig/svelte) — not on disk.
+        if not ext or ext.startswith("@"):
+            continue
+        extended_path = (base_dir / ext).resolve()
         if not extended_path.suffix:
             extended_path = extended_path.with_suffix(".json")
         if extended_path.exists():

tests/test_js_import_resolution.py

@@ -334,6 +334,32 @@ def test_tsconfig_alias_import_resolves_existing_ts_file(tmp_path: Path):
     assert _has_edge(result, "src/routes/page.ts", "src/lib/types/type-helpers.ts")
 
 
+def test_tsconfig_array_extends_alias_resolves_existing_ts_file(tmp_path: Path):
+    # TypeScript 5.0 allows `extends` as an array; later entries override
+    # earlier ones. The `paths` alias is inherited from the second parent.
+    # Regression: an array `extends` previously raised
+    # `AttributeError: 'list' object has no attribute 'startswith'`, which
+    # _safe_extract turned into a skip of every file using the alias.
+    _write(tmp_path / "tsconfig.base.json", json.dumps({"compilerOptions": {"strict": True}}))
+    _write(
+        tmp_path / "tsconfig.paths.json",
+        json.dumps({"compilerOptions": {"baseUrl": ".", "paths": {"$lib/*": ["src/lib/*"]}}}),
+    )
+    _write(
+        tmp_path / "tsconfig.json",
+        json.dumps({"extends": ["./tsconfig.base.json", "./tsconfig.paths.json"]}),
+    )
+    target = _write(tmp_path / "src/lib/types/type-helpers.ts", "export type Helper = string\n")
+    importer = _write(
+        tmp_path / "src/routes/page.ts",
+        "import type { Helper } from '$lib/types/type-helpers'\nconst value: Helper = 'x'\n",
+    )
+
+    result = _extract_for([target, importer], tmp_path)
+
+    assert _has_edge(result, "src/routes/page.ts", "src/lib/types/type-helpers.ts")
+
+
 def test_pnpm_workspace_package_import_resolves_package_entry(tmp_path: Path):
     _write(
         tmp_path / "pnpm-workspace.yaml",