feat(logger): add cause field to formatted error

packages/logger/src/formatter/LogFormatter.ts

@@ -1,6 +1,23 @@
 import { LogFormatterInterface } from '.';
 import { LogAttributes, UnformattedAttributes } from '../types';
 
+/**
+ * Typeguard to monkey patch Error to add a cause property.
+ *
+ * This is needed because the `cause` property was added in Node 16.x.
+ * Since we want to be able to format errors in Node 14.x, we need to
+ * add this property ourselves. We can remove this once we drop support
+ * for Node 14.x.
+ *
+ * @see 1361
+ * @see https://nodejs.org/api/errors.html#errors_error_cause
+ */
+const isErrorWithCause = (
+  error: Error
+): error is Error & { cause: unknown } => {
+  return 'cause' in error;
+};
+
 /**
  * This class defines and implements common methods for the formatting of log attributes.
  *
@@ -31,6 +48,11 @@ abstract class LogFormatter implements LogFormatterInterface {
       location: this.getCodeLocation(error.stack),
       message: error.message,
       stack: error.stack,
+      cause: isErrorWithCause(error)
+        ? error.cause instanceof Error
+          ? this.formatError(error.cause)
+          : error.cause
+        : undefined,
     };
   }
 

packages/logger/tests/unit/formatter/PowertoolLogFormatter.test.ts

@@ -307,6 +307,71 @@ describe('Class: PowertoolLogFormatter', () => {
 
       expect(shouldThrow).toThrowError(expect.any(URIError));
     });
+
+    test('when an error with cause of type Error is formatted, the cause key is included and formatted', () => {
+      // Prepare
+      const formatter = new PowertoolLogFormatter();
+      class ErrorWithCause extends Error {
+        public cause?: Error;
+        public constructor(message: string, options?: { cause: Error }) {
+          super(message);
+          this.cause = options?.cause;
+        }
+      }
+
+      // Act
+      const formattedURIError = formatter.formatError(
+        new ErrorWithCause('foo', { cause: new Error('bar') })
+      );
+
+      // Assess
+      expect(formattedURIError).toEqual({
+        location: expect.stringMatching(/PowertoolLogFormatter.test.ts:[0-9]+/),
+        message: 'foo',
+        name: 'Error',
+        stack: expect.stringMatching(
+          /PowertoolLogFormatter.test.ts:[0-9]+:[0-9]+/
+        ),
+        cause: {
+          location: expect.stringMatching(
+            /PowertoolLogFormatter.test.ts:[0-9]+/
+          ),
+          message: 'bar',
+          name: 'Error',
+          stack: expect.stringMatching(
+            /PowertoolLogFormatter.test.ts:[0-9]+:[0-9]+/
+          ),
+        },
+      });
+    });
+
+    test('when an error with cause of type other than Error is formatted, the cause key is included as-is', () => {
+      // Prepare
+      const formatter = new PowertoolLogFormatter();
+      class ErrorWithCause extends Error {
+        public cause?: unknown;
+        public constructor(message: string, options?: { cause: unknown }) {
+          super(message);
+          this.cause = options?.cause;
+        }
+      }
+
+      // Act
+      const formattedURIError = formatter.formatError(
+        new ErrorWithCause('foo', { cause: 'bar' })
+      );
+
+      // Assess
+      expect(formattedURIError).toEqual({
+        location: expect.stringMatching(/PowertoolLogFormatter.test.ts:[0-9]+/),
+        message: 'foo',
+        name: 'Error',
+        stack: expect.stringMatching(
+          /PowertoolLogFormatter.test.ts:[0-9]+:[0-9]+/
+        ),
+        cause: 'bar',
+      });
+    });
   });
 
   describe('Method: formatTimestamp', () => {