fix(lambda): can't find entry file under ESM module system

packages/aws-cdk-lib/aws-lambda-nodejs/lib/function.ts

@@ -292,5 +292,8 @@ function findDefiningFile(scope: Construct): string {
     throw new ValidationError('Cannot find defining file.', scope);
   }
 
-  return sites[definingIndex].getFileName();
+  // Fixes issue #21630.
+  // ESM modules return a 'file://' prefix to the filenames, this should be removed for
+  // compatibility with the NodeJS filesystem functions.
+  return sites[definingIndex].getFileName().replace(/^file:\/\//, '');
 }

packages/aws-cdk-lib/aws-lambda-nodejs/test/function.test.ts

@@ -37,6 +37,11 @@ let stack: Stack;
 beforeEach(() => {
   stack = new Stack();
   jest.clearAllMocks();
+  // pretend the calling file is in a fake file path
+  mockCallsites.mockImplementation(() => [
+    { getFunctionName: () => 'NodejsFunction' },
+    { getFileName: () => bockPath`function.test.ts` },
+  ]);
 });
 
 // We MUST use a fake file system here.
@@ -57,12 +62,6 @@ bockfs({
 });
 const bockPath = bockfs.workingDirectory('/home/project');
 
-// pretend the calling file is in a fake file path
-mockCallsites.mockImplementation(() => [
-  { getFunctionName: () => 'NodejsFunction' },
-  { getFileName: () => bockPath`function.test.ts` },
-]);
-
 afterAll(() => {
   bockfs.restore();
 });
@@ -231,6 +230,23 @@ test('throws when entry is not js/ts', () => {
   })).toThrow(/Only JavaScript or TypeScript entry files are supported/);
 });
 
+test('NodejsFunction with .js handler in an ESM package', () => {
+  // pretend the calling file is in a fake file path
+  // In ESM, callsites are prepended with 'file://'
+  mockCallsites.mockImplementation(() => [
+    { getFunctionName: () => 'NodejsFunction' },
+    { getFileName: () => `file://${bockPath`function.test.ts`}` },
+  ]);
+
+  // WHEN
+  new NodejsFunction(stack, 'handler2');
+
+  // THEN
+  expect(Bundling.bundle).toHaveBeenCalledWith(stack, expect.objectContaining({
+    entry: expect.stringContaining('function.test.handler2.js'), // Automatically finds .ts handler file
+  }));
+});
+
 test('accepts tsx', () => {
   const entry = bockPath`handler.tsx`;