initial commit

__tests__/data/package.json

@@ -1,4 +1,9 @@
 {
+  "devEngines": {
+    "packageManager": {
+      "name": "npm"
+    }
+  },
   "engines": {
     "node": "^20.0.0"
   }

__tests__/main.test.ts

@@ -240,6 +240,15 @@ describe('main tests', () => {
   });
 
   describe('cache on GHES', () => {
+    beforeEach(() => {
+      // Reset mocks and environment variables before each test
+      jest.resetModules();
+      jest.clearAllMocks();
+      jest.spyOn(core, 'saveState').mockImplementation(() => {});
+      jest.spyOn(core, 'warning').mockImplementation(() => {});
+      delete process.env['GITHUB_SERVER_URL'];
+    });
+
     it('Should throw an error, because cache is not supported', async () => {
       inputs['node-version'] = '12';
       inputs['cache'] = 'npm';
@@ -253,6 +262,9 @@ describe('main tests', () => {
       process.env['GITHUB_SERVER_URL'] = 'https://www.test.com';
       isCacheActionAvailable.mockImplementation(() => false);
 
+      // Mock package.json with no packageManager field
+      jest.spyOn(fs, 'readFileSync').mockReturnValue(JSON.stringify({}));
+
       await main.run();
 
       expect(warningSpy).toHaveBeenCalledWith(
@@ -273,11 +285,107 @@ describe('main tests', () => {
       process.env['GITHUB_SERVER_URL'] = '';
       isCacheActionAvailable.mockImplementation(() => false);
 
+      // Mock package.json with no packageManager field
+      jest.spyOn(fs, 'readFileSync').mockReturnValue(JSON.stringify({}));
+
       await main.run();
 
       expect(warningSpy).toHaveBeenCalledWith(
         'The runner was not able to contact the cache service. Caching will be skipped'
       );
     });
+
+    it('Should enable caching by default when packageManager field is defined', async () => {
+      inputs['node-version'] = '12';
+      inputs['cache'] = undefined; // User did not specify cache
+
+      inSpy.mockImplementation(name => inputs[name]);
+      const toolPath = path.normalize('/cache/node/18.0.0/x64');
+      findSpy.mockImplementation(() => toolPath);
+
+      process.env['GITHUB_SERVER_URL'] = 'https://www.test.com';
+      isCacheActionAvailable.mockImplementation(() => true);
+
+      // packageManager in package.json
+      (fs.readFileSync as jest.Mock).mockReturnValue(
+        JSON.stringify({
+          packageManager: 'yarn@3.2.0'
+        })
+      );
+
+      await main.run();
+
+      // Should enable caching (saveState called with 'yarn')
+      expect(core.saveState).toHaveBeenCalledWith(expect.anything(), 'yarn');
+    });
+
+    it('Should enable cache if devEngines.packageManager field is present', async () => {
+      inputs['node-version'] = '12';
+      inputs['cache'] = undefined; // Explicitly opt-in via package.json
+
+      inSpy.mockImplementation(name => inputs[name]);
+      findSpy.mockImplementation(() =>
+        path.normalize('/cache/node/12.16.1/x64')
+      );
+      process.env['GITHUB_SERVER_URL'] = 'https://www.test.com';
+      isCacheActionAvailable.mockImplementation(() => true);
+
+      // devEngines.packageManager in package.json
+      (fs.readFileSync as jest.Mock).mockReturnValue(
+        JSON.stringify({
+          devEngines: {
+            packageManager: {name: 'pnpm'}
+          }
+        })
+      );
+
+      await main.run();
+
+      // Should enable caching (saveState called with 'pnpm')
+      expect(core.saveState).toHaveBeenCalledWith(expect.anything(), 'pnpm');
+    });
+
+    it('Should NOT enable caching when neither cache nor packageManager is set', async () => {
+      inputs['node-version'] = '12';
+      inputs['cache'] = undefined;
+
+      inSpy.mockImplementation(name => inputs[name]);
+      findSpy.mockImplementation(() =>
+        path.normalize('/cache/node/12.16.1/x64')
+      );
+      process.env['GITHUB_SERVER_URL'] = 'https://www.test.com';
+      isCacheActionAvailable.mockImplementation(() => true);
+
+      // package.json with no cache info
+      (fs.readFileSync as jest.Mock).mockReturnValue(JSON.stringify({}));
+
+      await main.run();
+
+      expect(core.saveState).not.toHaveBeenCalled();
+    });
+
+    it('Should NOT enable caching when cache is explicitly disabled', async () => {
+      inputs['node-version'] = '12';
+      inputs['cache'] = ''; // Explicitly opt-out
+
+      inSpy.mockImplementation(name => inputs[name]);
+      findSpy.mockImplementation(() =>
+        path.normalize('/cache/node/12.16.1/x64')
+      );
+      process.env['GITHUB_SERVER_URL'] = 'https://www.test.com';
+      isCacheActionAvailable.mockImplementation(() => true);
+
+      // packageManager in package.json
+      (fs.readFileSync as jest.Mock).mockReturnValue(
+        JSON.stringify({
+          packageManager: 'yarn@3.2.0'
+        })
+      );
+
+      await main.run();
+
+      // Should NOT enable caching (saveState not called)
+      expect(core.saveState).not.toHaveBeenCalled();
+    });
   });
 });