feat(errorHandler): log client errors

.changeset/honest-moose-eat.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-common': patch
+---
+
+Add optional `logClientErrors` to errorHandler to log 4XX errors

packages/backend-common/src/middleware/errorHandler.test.ts

@@ -134,4 +134,20 @@ describe('errorHandler', () => {
 
     expect(mockLogger.error).not.toHaveBeenCalled();
   });
+
+  it('log 400 errors when logClientErrors is true', async () => {
+    const app = express();
+
+    const mockLogger = { child: jest.fn(), error: jest.fn() };
+    mockLogger.child.mockImplementation(() => mockLogger as any);
+
+    app.use('/NotFound', () => {
+      throw new errors.NotFoundError();
+    });
+    app.use(errorHandler({ logger: mockLogger as any, logClientErrors: true }));
+
+    await request(app).get('/NotFound');
+
+    expect(mockLogger.error).toHaveBeenCalled();
+  });
 });

packages/backend-common/src/middleware/errorHandler.ts

@@ -28,11 +28,18 @@ export type ErrorHandlerOptions = {
   showStackTraces?: boolean;
 
   /**
-   * Logger instance to log any 5xx errors.
+   * Logger instance to log errors.
    *
    * If not specified, the root logger will be used.
    */
   logger?: Logger;
+
+  /**
+   * Whether any error < 4XX should be logged or not.
+   *
+   * If not specified, by default log any 5xx errors.
+   */
+  logClientErrors?: boolean;
 };
 
 /**
@@ -59,7 +66,6 @@ export function errorHandler(
     type: 'errorHandler',
   });
 
-  /* eslint-disable @typescript-eslint/no-unused-vars */
   return (
     error: Error,
     _request: Request,
@@ -76,7 +82,7 @@ export function errorHandler(
     const status = getStatusCode(error);
     const message = showStackTraces ? error.stack : error.message;
 
-    if (logger && status >= 500) {
+    if (options.logClientErrors || status >= 500) {
       logger.error(error);
     }