chore: migrate build toolchain from npm/tsc/ncc/Jest to Bun (#189)

Replace the previous Node.js-based build toolchain with Bun,
consolidating TypeScript compilation, bundling, and testing
into a single modern runtime.
This migration was informed by official Bun documentation
(bun.sh/docs) and current best practices for 2025.

## Build System Changes

**Previous approach:**
Two-step build process using TypeScript compiler (`tsc`)
followed by Vercel's `ncc` bundler,
producing a 1.2MB bundle with sourcemap-register.js
and licenses.txt as separate artifacts.

**New approach:**
Single-command build using `bun build` with integrated bundling,
minification, and source map generation.
Output is 50% smaller (607KB vs 1.2MB)
and requires no separate bundling step.

Build command configuration based on Bun's bundler documentation
(bun.sh/docs/bundler) specifying `--target=node` for Node.js
compatibility and `--format=cjs` for CommonJS output required
by GitHub Actions' node20 runtime.

## Test System Changes

Replaced Jest and ts-jest with Bun's native test runner.
Bun automatically rewrites `@jest/globals` imports
to use equivalent `bun:test` APIs,
allowing tests to run without modification.
This compatibility documented at bun.sh/guides/test/migrate-from-jest.

Test execution is significantly faster
(26ms vs previous Jest execution times)
due to Bun's optimized runtime.

## Linting Configuration

Migrated ESLint from legacy `.eslintrc.json` to flat config
(`eslint.config.js`) following ESLint v9 migration guide.
Flat config is the recommended format as of 2025
and provides better compatibility with ES modules.

Added special rule overrides for test files
to disable `@typescript-eslint/require-await`
and `@typescript-eslint/no-unused-vars`,
which are incompatible with Jest-style mock patterns
that Bun's test runner supports for compatibility.

## Dependencies Removed

- `jest` and `ts-jest`: Replaced by Bun's built-in test runner
- `@vercel/ncc`: Replaced by Bun's bundler
- `typescript`: Bun includes native TypeScript support
- `@types/node`: Bun includes Node.js type definitions

Total reduction: 5 development dependencies removed.

## Configuration Changes

**package.json:**
- Added `"type": "module"` for ES module support
  (required by ESLint flat config)
- Updated `main` from `lib/main.js` to `dist/index.js`
- Rewrote all npm scripts to use Bun commands
- Removed `package` script (merged into `build`)

**tsconfig.json:**
- Changed from `exclude` to explicit `include` pattern
  to ensure test files are included for ESLint type checking
- Added `dist` and `lib` to exclude list

**GitHub Actions:**
- Replaced `setup-node` with `mise-action`
  to ensure CI uses same Bun version as local development
  (version managed via mise.toml)
- Changed `npm install` to `bun install --frozen-lockfile`
- Updated all build/lint/test commands to use Bun

**action.yml:**
- Updated runtime from `node16` to `node20`
  following GitHub Actions best practices for 2025

## Code Fixes

**src/input.ts:**
Fixed `@typescript-eslint/no-unsafe-enum-comparison` errors
by explicitly casting string to enum type
before comparison (lines 65, 77).
Changed `InputOptions` export to type-only export
for ES module compatibility.

**src/main.ts:**
Fixed `@typescript-eslint/no-floating-promises` error
by marking `main()` call with `void` operator (line 67).

**__tests__/output.test.ts:**
Fixed `prefer-const` error by changing mutable `let`
to immutable `const` for test object (line 22).

## Information Sources

Migration decisions based on:
- Official Bun documentation (bun.sh/docs)
- Bun bundler guide (bun.sh/docs/bundler)
- Jest migration guide (bun.sh/guides/test/migrate-from-jest)
- ESLint v9 migration guide
(eslint.org/docs/latest/use/configure/migration-guide)
- GitHub Actions node20 best practices
- Package manager comparison research (2025 resources)

## Verification

All critical commands verified:
- `bun run build`: Produces 607KB minified bundle with source maps
- `bun run lint`: Passes with 0 errors/warnings
- `bun test`: All 20 tests pass in 26ms

## AI assistance

This change was entirely carried out by an LLM
with human instruction on the goal and verification criteria.

Co-authored-by: Claude <noreply@anthropic.com>

Author: abhinav

.changes/unreleased/Changed-20251102-135253.yaml

@@ -0,0 +1,3 @@
+kind: Changed
+body: Bump required NodeJS runtime to 24.
+time: 2025-11-02T13:52:53.53782-08:00

.eslintignore

@@ -21,19 +21,16 @@ jobs:
         with:
           ref: ${{ github.event_name == 'pull_request' && github.head_ref }}
 
-      - name: Set up Node
-        uses: actions/setup-node@v4
-        with:
-          node-version: 16.x
-          cache: npm
+      - name: Set up mise
+        uses: jdx/mise-action@v3
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Install dependencies
-        run: npm install
+        run: bun install --frozen-lockfile
 
       - name: Build dist
-        run: |
-          npm run build
-          npm run package
+        run: bun run build
 
       # Only verify on main
       - name: Verify dist status
@@ -53,10 +50,10 @@ jobs:
           commit_message: "Update dist"
 
       - name: Lint
-        run: npm run lint
+        run: bun run lint
 
       - name: Test
-        run: npm test
+        run: bun test
 
   test-install:
     runs-on: ubuntu-latest

.eslintrc.json

@@ -19,7 +19,7 @@ test.each([
         want: {'check-failed': 'false'}
     }
 ])('writeOutputs($name)', ({give, want}) => {
-    let got: Record<string, string> = {}
+    const got: Record<string, string> = {}
     const sink: output.OutputSink = {
         setOutput: (name: string, value: string) => {
             got[name] = value

.github/workflows/test.yml

@@ -72,5 +72,5 @@ outputs:
       This output is undefined in other modes.
 
 runs:
-  using: 'node16'
+  using: 'node24'
   main: 'dist/index.js'