Migrate codebase from JavaScript to TypeScript

[thomaschaplin]

Summary

This PR rewrites the entire codebase from JavaScript to TypeScript, adds a comprehensive test suite, fixes several bugs found during the migration, and sets up CI/CD tooling.

Changes

TypeScript migration

• Replaced index.js (CommonJS) with typed ES modules under src/
• Core logic rewritten in src/generate-isin.ts with proper type annotations
• Country codes moved from country-codes.json into a typed fixture at src/fixtures/country-codes.ts
• src/index.ts added as the package entry point
• tsconfig.json added to configure compilation to build/

New features

• generateIsin() now accepts an optional countryCode parameter, allowing callers to pin the country (e.g. generateIsin("US")). Country code is treated as case-insensitive.

Bug fixes found during migration

• CUSIP was generated once at module load — every ISIN was sharing the same CUSIP. Fixed by moving generateCusip() inside generateIsin() so each call produces a fresh value.
• Orphaned dead statement — a countryCodes[Math.random() * ...] expression was computed but its result discarded. Removed.
• Error never thrown — new Error(...) was created but not thrown. Fixed to throw new Error(...).
• Wrong type annotations — calculateValues was typed as returning string[] but pushed numbers; calculateCheckDigit accepted string[] and used redundant parseInt calls. Corrected to number[].

Testing

• Jest test suite added (src/__tests__/generate-isin.spec.ts) using a mocked CUSIP for deterministic assertions
• Tests assert exact known ISIN values (e.g. US0378331005, GB0378331002) to verify correct check digit calculation end-to-end
• Additional tests cover case-insensitive country codes, per-call CUSIP generation, and fixture content
• Performance benchmark via kelonio (~0.03ms per call)
• Stryker mutation testing config added — generate-isin.ts achieves 87.8% mutation score

CI/CD & tooling

• GitHub Actions workflow (.github/workflows/tests.yml) runs build, coverage, and mutation tests on push/PR to master
• Dependabot configured for daily npm dependency updates
• Husky pre-push hook with interactive prompts to merge latest master and run tests before pushing
• release.sh and prepush.sh scripts added
• .nvmrc added to pin Node version
• examples/ directory added with a standalone TypeScript usage example

Test plan

• npm test — all 9 tests pass
• npm run test:coverage — full coverage run passes
• npm run test:mutation — 87.8% mutation score on core logic
• npm run build — TypeScript compiles cleanly to build/

🤖 Generated with Claude Code