fix(express): map multer errors by code instead of message (#17769)

* fix(express): map multer errors by code instead of message

multer 2.4.0 changed the LIMIT_UNEXPECTED_FILE message from "Unexpected field" to
"Unexpected file field". transformException compared error.message against the old
text, so a file sent under an unexpected field name became a 500 instead of a 400.

Map multer errors by their code, which multer documents as the stable identifier,
and keep matching on the message for errors without one, such as busboy's. The
LIMIT_UNEXPECTED_FILE constant is updated to the new message.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* Update packages/platform-express/multer/multer/multer.utils.ts

Co-authored-by: Micael Levi L. Cavalcante <micalevisk@gmail.com>

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-authored-by: Micael Levi L. Cavalcante <micalevisk@gmail.com>

Author: 3 people

packages/platform-express/multer/multer/multer.constants.ts

@@ -6,7 +6,7 @@ export const multerExceptions = {
   LIMIT_FIELD_KEY: 'Field name too long',
   LIMIT_FIELD_VALUE: 'Field value too long',
   LIMIT_FIELD_COUNT: 'Too many fields',
-  LIMIT_UNEXPECTED_FILE: 'Unexpected field',
+  LIMIT_UNEXPECTED_FILE: 'Unexpected file field',
   MISSING_FIELD_NAME: 'Field name missing',
   LIMIT_FIELD_NESTING: 'Field name nesting too deep',
 };

packages/platform-express/multer/multer/multer.utils.ts

@@ -8,12 +8,17 @@ import { multerExceptions, busboyExceptions } from './multer.constants.js';
 // Multer may add in a 'field' property to the error
 // https://github.com/expressjs/multer/blob/aa42bea6ac7d0cb8fcb279b15a7278cda805dc63/lib/multer-error.js#L19
 export function transformException(
-  error: (Error & { field?: string }) | undefined,
+  error: (Error & { field?: string; code?: string }) | undefined,
 ) {
   if (!error || error instanceof HttpException) {
     return error;
   }
-  switch (error.message) {
+  // Multer identifies its errors by `code`, while the messages may change
+  // between releases (e.g. "Unexpected field" became "Unexpected file field")
+  const exception = isMulterExceptionCode(error.code)
+    ? multerExceptions[error.code]
+    : error.message;
+  switch (exception) {
     case multerExceptions.LIMIT_FILE_SIZE:
       return new PayloadTooLargeException(error.message);
     case multerExceptions.LIMIT_FILE_COUNT:
@@ -37,3 +42,9 @@ export function transformException(
   }
   return error;
 }
+
+function isMulterExceptionCode(
+  code: unknown,
+): code is keyof typeof multerExceptions {
+  return typeof code === 'string' && Object.hasOwn(multerExceptions, code);
+}

packages/platform-express/test/multer/multer/multer.utils.spec.ts

@@ -3,6 +3,7 @@ import {
   HttpException,
   PayloadTooLargeException,
 } from '@nestjs/common';
+import multer from 'multer';
 import {
   busboyExceptions,
   multerExceptions,
@@ -71,6 +72,36 @@ describe('transformException', () => {
         );
       });
     });
+    describe('and is an error thrown by multer', () => {
+      it('should return "BadRequestException" for LIMIT_UNEXPECTED_FILE', () => {
+        const err = new multer.MulterError('LIMIT_UNEXPECTED_FILE', 'photo');
+        const result = transformException(err);
+        expect(result).toBeInstanceOf(BadRequestException);
+        expect(result!.message).toBe(`${err.message} - photo`);
+      });
+      it('should return "PayloadTooLargeException" for LIMIT_FILE_SIZE', () => {
+        const err = new multer.MulterError('LIMIT_FILE_SIZE', 'avatar');
+        expect(transformException(err)).toBeInstanceOf(
+          PayloadTooLargeException,
+        );
+      });
+      it('should map by code even when the message differs', () => {
+        const err = {
+          code: 'LIMIT_UNEXPECTED_FILE',
+          message: 'Some other wording',
+          field: 'photo',
+        };
+        const result = transformException(err as any);
+        expect(result).toBeInstanceOf(BadRequestException);
+        expect(result!.message).toBe('Some other wording - photo');
+      });
+      it('should behave as identity for a code that is not a multer code', () => {
+        const err = Object.assign(new Error('no such file'), {
+          code: 'ENOENT',
+        });
+        expect(transformException(err)).toBe(err);
+      });
+    });
     describe(`and has a 'field' property`, () => {
       it('should return the field propery appended to the error message', () => {
         const err = {