Migrate codebase from JavaScript to TypeScript

.github/dependabot.yml

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+version: 2
+updates:
+- package-ecosystem: npm
+  directory: "/"
+  schedule:
+    interval: daily
+  open-pull-requests-limit: 10

.github/workflows/tests.yml

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+name: Tests
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Setup Node
+        uses: actions/setup-node@v3
+        with:
+          node-version-file: ".nvmrc"
+          cache: "npm"
+
+      - name: Install
+        run: npm ci
+
+      - name: Build
+        run: npm run build
+
+      - name: Test (coverage)
+        run: npm run test:coverage
+
+      - name: Test (mutation)
+        run: npm run test:mutation

.gitignore

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-# Logs
-logs
-*.log
-npm-debug.log*
-yarn-debug.log*
-yarn-error.log*
-lerna-debug.log*
-
-# Diagnostic reports (https://nodejs.org/api/report.html)
-report.[0-9]*.[0-9]*.[0-9]*.[0-9]*.json
-
-# Runtime data
-pids
-*.pid
-*.seed
-*.pid.lock
-
-# Directory for instrumented libs generated by jscoverage/JSCover
-lib-cov
-
-# Coverage directory used by tools like istanbul
+node_modules
+build
 coverage
-*.lcov
-
-# nyc test coverage
-.nyc_output
-
-# Grunt intermediate storage (https://gruntjs.com/creating-plugins#storing-task-files)
-.grunt
-
-# Bower dependency directory (https://bower.io/)
-bower_components
-
-# node-waf configuration
-.lock-wscript
-
-# Compiled binary addons (https://nodejs.org/api/addons.html)
-build/Release
-
-# Dependency directories
-node_modules/
-jspm_packages/
-
-# TypeScript v1 declaration files
-typings/
-
-# TypeScript cache
-*.tsbuildinfo
-
-# Optional npm cache directory
-.npm
-
-# Optional eslint cache
-.eslintcache
-
-# Optional REPL history
-.node_repl_history
-
-# Output of 'npm pack'
-*.tgz
-
-# Yarn Integrity file
-.yarn-integrity
-
-# dotenv environment variables file
-.env
-.env.test
-
-# parcel-bundler cache (https://parceljs.org/)
-.cache
-
-# next.js build output
-.next
-
-# nuxt.js build output
-.nuxt
-
-# gatsby files
-.cache/
-public
-
-# vuepress build output
-.vuepress/dist
-
-# Serverless directories
-.serverless/
-
-# FuseBox cache
-.fusebox/
-
-# DynamoDB Local files
-.dynamodb/
+.kelonio.state.json
+*.stryker-tmp
+reports

.husky/.gitignore

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+_

.husky/pre-push

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+#!/bin/sh
+. "$(dirname "$0")/_/husky.sh"
+
+npm run pre-push

.nvmrc

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+18

.prepush.config

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+main_branch=master
+merge_main_branch=true
+run_tests=true

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Commands
+
+```bash
+npm install          # install dependencies
+npm run build        # compile TypeScript to build/
+npm run watch        # compile in watch mode
+npm test             # run tests
+npm run test:coverage  # run tests with coverage report
+npm run test:mutation  # run Stryker mutation tests (~3 min)
+```
+
+To run a single test file:
+```bash
+npx jest src/__tests__/generate-isin.spec.ts
+```
+
+To run a test by name:
+```bash
+npx jest -t "should generate the correct check digit"
+```
+
+## Architecture
+
+This is a small npm library (`@thomaschaplin/isin-generator`) that generates valid 12-character ISINs. It depends on `@thomaschaplin/cusip-generator` to supply the 9-character CUSIP component of each ISIN.
+
+**ISIN structure:** `[2-char country code] + [9-char CUSIP] + [1 check digit]`
+
+### Source layout
+
+- `src/generate-isin.ts` — core logic. `generateIsin(countryCode?: string)` picks a random country code (or uses the one provided), calls `generateCusip()` fresh each invocation, then computes a Luhn-variant check digit via `calculateValues` → `calculateCheckDigit`.
+- `src/fixtures/country-codes.ts` — exported `string[]` of ISO 3166-1 alpha-2 country codes used for random selection.
+- `src/index.ts` — re-exports `generateIsin` as the public API.
+- `build/` — compiled output (gitignored); this is what gets published to npm per the `files` field in `package.json`.
+
+### Check digit algorithm
+
+The `generateIsinCheckDigit` function is called with a 12-character string (`code + cusip + '0'`). `calculateValues` iterates indices 0–10 (skipping the placeholder check digit at index 11), converts letters to their numeric equivalent via `charCode - 55`, and splits two-digit results. `calculateCheckDigit` then applies a Luhn-style doubling of even-indexed values to produce the final digit.
+
+### Testing approach
+
+Tests mock `@thomaschaplin/cusip-generator` with a fixed CUSIP (`"037833100"`) to enable deterministic assertions on exact ISIN output (e.g. `"US0378331005"`). When adding tests for the check digit logic, use this pattern — asserting exact output kills far more mutations than length-only checks.
+
+### Pre-push hook
+
+Husky runs `prepush.sh` before every push, which reads `.prepush.config` and interactively prompts to merge `master` and/or run tests. In non-interactive environments (e.g. CI) the `/dev/tty` read will fail gracefully and the push continues.

LICENSE

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 MIT License
 
-Copyright (c) 2019 Thomas Chaplin
+Copyright (c) 2023 Thomas Chaplin
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal