fix: preserve res.write() return value so backpressure is not swallowed

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fix
+- `res.write()` now returns the underlying write's boolean result, restoring backpressure signalling for streaming/SSE responses [@IanMcNelly](https://github.com/IanMcNelly).
+
+
 ## [4.0.3] - 2025-04-14
 
 ### Fix
@@ -55,4 +61,4 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Removed support for objects configuration in array fields from [@ugolas](https://github.com/ugolas).
 - Fixed vulnerabilities from [@ugolas](https://github.com/ugolas).
 - Replaced coverage reporter from istanbul to nyc from [@ugolas](https://github.com/ugolas).
-- Removed support for node 6 from [@ugolas](https://github.com/ugolas).
+- Removed support for node 6 from [@ugolas](https://github.com/ugolas).

lib/express-logger.js

@@ -21,7 +21,7 @@ var audit = function (req, res, next) {
 
     res.write = function (chunk) {
         chunks.push(Buffer.from(chunk));
-        oldWrite.apply(res, arguments);
+        return oldWrite.apply(res, arguments);
     };
 
     // decorate response#json method from express
@@ -107,4 +107,4 @@ function setBodyLengthFields(options) {
     const isValid = field => field && !isNaN(field) && field > 0;
     options.request.maxBodyLength = !isValid(options.request.maxBodyLength) ? undefined : options.request.maxBodyLength;
     options.response.maxBodyLength = !isValid(options.response.maxBodyLength) ? undefined : options.response.maxBodyLength;
-}
+}

test/express-logger-test.js

@@ -201,6 +201,22 @@ describe('express-logger tests', function(){
             should(auditResponseStub.calledOnce).eql(true);
             should(res._bodyStr).eql('chunk');
         });
+        it('Should return the underlying write() return value to preserve backpressure', function(){
+            var auditMethod = expressLogger();
+            // Start request
+            auditMethod(req, res, next);
+            should(next.calledOnce).eql(true);
+
+            // Underlying write() signals its buffer is full
+            resWriteStub.returns(false);
+            should(res.write('chunk')).eql(false);
+
+            // ... and that it has drained
+            resWriteStub.returns(true);
+            should(res.write('chunk')).eql(true);
+
+            should(resWriteStub.calledTwice).eql(true);
+        });
         it('Should add body from end chunk to response', function(){
             var auditMethod = expressLogger();
             // Start request
@@ -236,4 +252,4 @@ describe('express-logger tests', function(){
             should(res).eql(false);
         })
     });
-});
+});