fix: refresh package type inference per compilation

loader/utils/getModuleSystem.js

@@ -1,8 +1,20 @@
 const fs = require('node:fs/promises');
 const path = require('node:path');
 
-/** @type {Map<string, string | undefined>} */
-let packageJsonTypeMap = new Map();
+/** @type {WeakMap<object, Map<string, string | undefined>>} */
+const compilationPackageJsonTypeMaps = new WeakMap();
+
+function getPackageJsonTypeMap(compilation) {
+  if (!compilation) return new Map();
+
+  let packageJsonTypeMap = compilationPackageJsonTypeMaps.get(compilation);
+  if (!packageJsonTypeMap) {
+    packageJsonTypeMap = new Map();
+    compilationPackageJsonTypeMaps.set(compilation, packageJsonTypeMap);
+  }
+
+  return packageJsonTypeMap;
+}
 
 /**
  * Infers the current active module system from loader context and options.
@@ -47,6 +59,8 @@ async function getModuleSystem(ModuleFilenameHelpers, options) {
     return 'cjs';
   }
 
+  const packageJsonTypeMap = getPackageJsonTypeMap(this._compilation);
+
   // We will assume CommonJS if we cannot determine otherwise
   let packageJsonType = '';
 

test/loader/unit/getModuleSystem.test.js

@@ -1,3 +1,5 @@
+const fs = require('fs/promises');
+const os = require('os');
 const path = require('path');
 const { ModuleFilenameHelpers } = require('webpack');
 
@@ -132,4 +134,29 @@ describe('getModuleSystem', () => {
       )
     ).resolves.toBe('cjs');
   });
+
+  it('should re-read package type for a new compilation', async () => {
+    const fixturePath = await fs.mkdtemp(path.join(os.tmpdir(), 'react-refresh-module-system-'));
+    const packageJsonPath = path.join(fixturePath, 'package.json');
+    const loaderContext = {
+      resourcePath: path.join(fixturePath, 'index.js'),
+      rootContext: fixturePath,
+      addDependency: () => {},
+      addMissingDependency: () => {},
+    };
+
+    try {
+      await fs.writeFile(packageJsonPath, JSON.stringify({ type: 'module' }));
+      await expect(
+        getModuleSystem.call({ ...loaderContext, _compilation: {} }, ModuleFilenameHelpers, {})
+      ).resolves.toBe('esm');
+
+      await fs.writeFile(packageJsonPath, JSON.stringify({ type: 'commonjs' }));
+      await expect(
+        getModuleSystem.call({ ...loaderContext, _compilation: {} }, ModuleFilenameHelpers, {})
+      ).resolves.toBe('cjs');
+    } finally {
+      await fs.rm(fixturePath, { recursive: true, force: true });
+    }
+  });
 });