fix(response-interceptor): reduce responseInterceptor buffer churn

CHANGELOG.md

@@ -6,6 +6,7 @@
 - chore(package.json): update to httpxy@0.5.3
 - fix(ipv6): preserve credentials when normalizing bracketed IPv6 target string
 - fix(ipv6): unspecified IPv6 target hostname (::)"
+- fix(response-interceptor): reduce responseInterceptor buffer churn
 
 ## [v4.0.0](https://github.com/chimurai/http-proxy-middleware/releases/tag/v4.0.0)
 

src/handlers/response-interceptor.ts

@@ -47,15 +47,24 @@ export function responseInterceptor<
   ): Promise<void> {
     debug('intercept proxy response');
     const originalProxyRes = proxyRes;
-    let buffer = Buffer.from('', 'utf8');
+    const chunks: Buffer[] = [];
+    let bufferLength = 0;
 
     // decompress proxy response
     const _proxyRes = decompress(proxyRes, proxyRes.headers['content-encoding']);
 
-    // concat data stream
-    _proxyRes.on('data', (chunk) => (buffer = Buffer.concat([buffer, chunk])));
+    // collect data chunks and concatenate once on end to avoid repeated full-buffer copies
+    _proxyRes.on('data', (chunk) => {
+      const chunkBuffer = Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk);
+      chunks.push(chunkBuffer);
+      bufferLength += chunkBuffer.length; // precalculate Buffer length for slightly better performance on Buffer.concat()
+    });
 
     _proxyRes.on('end', async () => {
+      const buffer = Buffer.concat(chunks, bufferLength);
+      chunks.length = 0; // clear chunks array
+      bufferLength = 0;
+
       // copy original headers
       copyHeaders(proxyRes, res);
 
@@ -64,15 +73,17 @@ export function responseInterceptor<
       const interceptedBuffer = Buffer.from(await interceptor(buffer, originalProxyRes, req, res));
 
       // set correct content-length (with double byte character support)
-      debug('set content-length: %s', Buffer.byteLength(interceptedBuffer, 'utf8'));
-      res.setHeader('content-length', Buffer.byteLength(interceptedBuffer, 'utf8'));
+      debug('set content-length: %s', Buffer.byteLength(interceptedBuffer));
+      res.setHeader('content-length', Buffer.byteLength(interceptedBuffer));
 
       debug('write intercepted response');
       res.write(interceptedBuffer);
       res.end();
     });
 
     _proxyRes.on('error', (error) => {
+      chunks.length = 0; // clear chunks array
+      bufferLength = 0;
       res.end(`Error fetching proxied request: ${error.message}`);
     });
   };

test/unit/response-interceptor.spec.ts

@@ -23,6 +23,48 @@ describe('responseInterceptor', () => {
     expect(res.end).toHaveBeenCalledWith();
   });
 
+  it('should combine proxy response chunks before calling interceptor', async () => {
+    const proxyRes = createMockRequest();
+    const req = createMockRequest();
+    const res = createMockResponse();
+
+    responseInterceptor(async (buffer) => {
+      expect(buffer).toEqual(Buffer.from('HPM'));
+      return buffer;
+    })(proxyRes, req, res);
+
+    proxyRes.emit('data', Buffer.from('H'));
+    proxyRes.emit('data', Buffer.from('P'));
+    proxyRes.emit('data', Buffer.from('M'));
+    proxyRes.emit('end');
+    await waitInterceptorHandler();
+
+    expect(res.setHeader).toHaveBeenCalledWith('content-length', 'HPM'.length);
+    expect(res.write).toHaveBeenCalledWith(Buffer.from('HPM'));
+    expect(res.end).toHaveBeenCalledWith();
+  });
+
+  it('should combine string proxy response chunks before calling interceptor', async () => {
+    const proxyRes = createMockRequest();
+    const req = createMockRequest();
+    const res = createMockResponse();
+
+    responseInterceptor(async (buffer) => {
+      expect(buffer).toEqual(Buffer.from('HPM'));
+      return buffer;
+    })(proxyRes, req, res);
+
+    proxyRes.emit('data', 'H');
+    proxyRes.emit('data', 'P');
+    proxyRes.emit('data', 'M');
+    proxyRes.emit('end');
+    await waitInterceptorHandler();
+
+    expect(res.setHeader).toHaveBeenCalledWith('content-length', 'HPM'.length);
+    expect(res.write).toHaveBeenCalledWith(Buffer.from('HPM'));
+    expect(res.end).toHaveBeenCalledWith();
+  });
+
   it('should end with error when receive a proxy error event', async () => {
     const proxyRes = createMockRequest();
     const req = createMockRequest();