Add a warning for when double application is possible

Author: fatfisz

.gitignore

@@ -1,5 +1,5 @@
 .DS_Store
-node_modules/
+/node_modules/
 npm-debug.log
 lib/
 coverage/

src/getRealPath.js

@@ -78,6 +78,26 @@ const resolvers = [
   getRealPathFromAliasConfig,
 ];
 
+function getResolvedPath(sourcePath, currentFile, opts) {
+  let resolvedPath = null;
+
+  resolvers.some((resolver) => {
+    resolvedPath = resolver(sourcePath, currentFile, opts);
+    return resolvedPath !== null;
+  });
+
+  return resolvedPath;
+}
+
+function checkIfDoubleApplicationPossible(sourcePath, resolvedPath, currentFile, opts) {
+  if (resolvedPath !== null) {
+    const resolvedAgainPath = getResolvedPath(resolvedPath, currentFile, opts);
+    if (resolvedAgainPath !== null && resolvedAgainPath !== resolvedPath) {
+      warn(`Resolving "${sourcePath}" may give different results if done multiple times. Remove cycles from the configuration or alias to absolute paths.`);
+    }
+  }
+}
+
 export default function getRealPath(sourcePath, { file, opts }) {
   if (sourcePath[0] === '.') {
     return sourcePath;
@@ -86,12 +106,11 @@ export default function getRealPath(sourcePath, { file, opts }) {
   // File param is a relative path from the environment current working directory
   // (not from cwd param)
   const currentFile = path.resolve(file.opts.filename);
-  let resolvedPath = null;
+  const resolvedPath = getResolvedPath(sourcePath, currentFile, opts);
 
-  resolvers.some((resolver) => {
-    resolvedPath = resolver(sourcePath, currentFile, opts);
-    return resolvedPath !== null;
-  });
+  if (process.env.NODE_ENV !== 'production') {
+    checkIfDoubleApplicationPossible(sourcePath, resolvedPath, currentFile, opts);
+  }
 
   return resolvedPath;
 }

test/index.test.js

@@ -509,6 +509,61 @@ describe('module-resolver', () => {
         });
       });
     });
+
+    describe('multiple alias application warning', () => {
+      const mockWarn = jest.fn();
+      jest.mock('../src/log', () => ({
+        warn: mockWarn,
+      }));
+      jest.resetModules();
+      const pluginWithMock = require.requireActual('../src').default;
+      const fileName = path.resolve('test/testproject/src/app.js');
+
+      const cycleAliasTransformerOpts = {
+        babelrc: false,
+        plugins: [
+          [pluginWithMock, {
+            alias: {
+              first: 'second',
+              second: 'first',
+            },
+          }],
+        ],
+        filename: fileName,
+      };
+
+      beforeEach(() => {
+        mockWarn.mockClear();
+        process.env.NODE_ENV = 'development';
+      });
+
+      it('should print a warning for a package that may be resolved multiple times', () => {
+        testWithImport(
+          'first',
+          'second',
+          cycleAliasTransformerOpts,
+        );
+
+        expect(mockWarn.mock.calls.length).toBe(1);
+        expect(mockWarn).toBeCalledWith('Resolving "first" may give different results if done multiple times. Remove cycles from the configuration or alias to absolute paths.');
+      });
+
+      describe('production environment', () => {
+        beforeEach(() => {
+          process.env.NODE_ENV = 'production';
+        });
+
+        it('should not print a warning for a package that may be resolved multiple times', () => {
+          testWithImport(
+            'first',
+            'second',
+            cycleAliasTransformerOpts,
+          );
+
+          expect(mockWarn.mock.calls.length).toBe(0);
+        });
+      });
+    });
   });
 
   describe('with custom cwd', () => {