fix: detect esm only pkg w/ main and module fields (#1494)

Author: jonchurch

shared/utils/package-analysis.ts

@@ -82,8 +82,10 @@ export function detectModuleFormat(pkg: ExtendedPackageJson): ModuleFormat {
 
   // Legacy detection without exports field
   if (hasModule && hasMain) {
-    // Has both module (ESM) and main (CJS) fields
-    return 'dual'
+    // Check for dual packages (has module field and main points to cjs)
+    const mainIsCJS = pkg.main?.endsWith('.cjs') || (pkg.main?.endsWith('.js') && !isTypeModule)
+
+    return mainIsCJS ? 'dual' : 'esm'
   }
 
   if (hasModule || isTypeModule) {

test/unit/shared/utils/package-analysis.spec.ts

@@ -26,6 +26,16 @@ describe('detectModuleFormat', () => {
     expect(detectModuleFormat({ module: 'index.mjs', main: 'index.js' })).toBe('dual')
   })
 
+  it('detects dual from type + module + main fields', () => {
+    expect(detectModuleFormat({ type: 'module', module: 'index.js', main: 'index.cjs' })).toBe(
+      'dual',
+    )
+  })
+
+  it('detects esm from type + module + main fields', () => {
+    expect(detectModuleFormat({ type: 'module', module: 'index.js', main: 'index.js' })).toBe('esm')
+  })
+
   it('detects ESM from module field without main', () => {
     expect(detectModuleFormat({ module: 'index.mjs' })).toBe('esm')
   })