What was actually happening: prepublish.sh (cp -r dist/src/* .) copies compiled TypeScript — including *.test.js files — into the root-level preprocessor/, parser/, and ast/

directories. Vitest found and ran these compiled test files alongside the TypeScript ones. The compiled preprocessor/preprocessor.test.js imported GlslSyntaxError from ../error.js
(root-level compiled), but buildPreprocessorParser dynamically imported preprocessor/index.js which was resolved by Vite's TypeScript-aware resolver to src/preprocessor/index.ts →
src/error.ts. Two different class instances → instanceof failed.

Two fixes applied:
1. vite.config.ts: Added include: ['src/**/*.test.ts'] so Vitest only runs the canonical TypeScript test files in src/, ignoring compiled JS copies elsewhere.
2. prepublish.sh: Added cleanup to delete *.test.js and *.test.d.ts from published directories — test files don't belong in the npm package and cause this discovery problem.

Author: AndrewRayCode

prepublish.sh

@@ -11,3 +11,8 @@ cp -r dist/src/* .
 # dist/parser/ and dist/preprocessor/, separate from the compiled TypeScript in dist/src/)
 cp dist/parser/parser.js parser/
 cp dist/preprocessor/preprocessor-parser.js preprocessor/
+
+# Remove test files from the published directories - they're not part of the package
+# and if present they confuse Vitest into running compiled JS tests with broken module paths
+find . -name "*.test.js" -not -path "./src/*" -not -path "./dist/*" -not -path "./node_modules/*" -delete
+find . -name "*.test.d.ts" -not -path "./src/*" -not -path "./dist/*" -not -path "./node_modules/*" -delete

vite.config.ts

@@ -8,5 +8,6 @@ export default defineConfig({
   },
   test: {
     globalSetup: './vitest.global-setup.ts',
+    include: ['src/**/*.test.ts'],
   },
 });