Merge pull request #7714 from backstage/freben/stringify-error

Add `stringifyError`

packages/errors/api-report.md

@@ -18,7 +18,7 @@ export class ConflictError extends CustomErrorBase {}
 export class CustomErrorBase extends Error {
   constructor(message?: string, cause?: Error | unknown);
   // (undocumented)
-  readonly cause?: Error;
+  readonly cause?: Error | undefined;
 }
 
 // @public
@@ -98,4 +98,7 @@ export function serializeError(
     includeStack?: boolean;
   },
 ): SerializedError;
+
+// @public
+export function stringifyError(error: unknown): string;
 ```

packages/errors/src/errors/CustomErrorBase.ts

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+import { stringifyError } from '../serialization';
 import { isError } from './assertion';
 
 /**
@@ -32,25 +33,12 @@ import { isError } from './assertion';
  * ```
  */
 export class CustomErrorBase extends Error {
-  readonly cause?: Error;
+  readonly cause?: Error | undefined;
 
   constructor(message?: string, cause?: Error | unknown) {
-    let assignedCause: Error | undefined = undefined;
-
     let fullMessage = message;
     if (cause !== undefined) {
-      let causeStr;
-      if (isError(cause)) {
-        assignedCause = cause;
-        causeStr = String(cause);
-
-        // Prefer the cause.toString, but if it's not implemented we use a nicer fallback
-        if (causeStr === '[object Object]') {
-          causeStr = `${cause.name}: ${cause.message}`;
-        }
-      } else {
-        causeStr = `unknown error '${cause}'`;
-      }
+      const causeStr = stringifyError(cause);
       if (fullMessage) {
         fullMessage += `; caused by ${causeStr}`;
       } else {
@@ -63,6 +51,6 @@ export class CustomErrorBase extends Error {
     Error.captureStackTrace?.(this, this.constructor);
 
     this.name = this.constructor.name;
-    this.cause = assignedCause;
+    this.cause = isError(cause) ? cause : undefined;
   }
 }

packages/errors/src/serialization/error.test.ts

@@ -14,7 +14,8 @@
  * limitations under the License.
  */
 
-import { deserializeError, serializeError } from './error';
+import { NotModifiedError } from '../errors';
+import { deserializeError, serializeError, stringifyError } from './error';
 
 class CustomError extends Error {
   readonly customField: any;
@@ -61,4 +62,30 @@ describe('serialization', () => {
     expect(withoutStack1.stack).not.toBeDefined();
     expect(withoutStack2.stack).not.toBeDefined();
   });
+
+  it('stringifies all supported forms', () => {
+    expect(stringifyError({})).toEqual("unknown error '[object Object]'");
+    expect(
+      stringifyError({
+        toString() {
+          return 'str';
+        },
+      }),
+    ).toEqual("unknown error 'str'");
+    expect(
+      stringifyError({
+        name: 'not used',
+        message: 'not used',
+        toString() {
+          return 'str';
+        },
+      }),
+    ).toEqual('str');
+    expect(stringifyError({ name: 'N', message: 'm1' })).toEqual('N: m1');
+    expect(stringifyError(new NotModifiedError('m2'))).toEqual(
+      'NotModifiedError: m2',
+    );
+    expect(stringifyError(new Error('m3'))).toEqual('Error: m3');
+    expect(stringifyError(new TypeError('m4'))).toEqual('TypeError: m4');
+  });
 });

packages/errors/src/serialization/error.ts

@@ -19,6 +19,7 @@ import {
   deserializeError as deserializeErrorInternal,
   serializeError as serializeErrorInternal,
 } from 'serialize-error';
+import { isError } from '../errors';
 
 /**
  * The serialized form of an Error.
@@ -78,3 +79,19 @@ export function deserializeError<T extends Error = Error>(
   }
   return result;
 }
+
+/**
+ * Stringifies an error, including its name and message where available.
+ *
+ * @param error - The error.
+ * @public
+ */
+export function stringifyError(error: unknown): string {
+  if (isError(error)) {
+    // Prefer error.toString, but if it's not implemented we use a nicer fallback
+    const str = String(error);
+    return str !== '[object Object]' ? str : `${error.name}: ${error.message}`;
+  }
+
+  return `unknown error '${error}'`;
+}

packages/errors/src/serialization/index.ts

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-export { deserializeError, serializeError } from './error';
+export { deserializeError, serializeError, stringifyError } from './error';
 export type { SerializedError } from './error';
 export { parseErrorResponse } from './response';
 export type { ErrorResponse } from './response';