fix(fixRequestBody): support content-encoding on request body

src/handlers/fix-request-body.ts

@@ -1,5 +1,6 @@
 import type * as http from 'node:http';
 import * as querystring from 'node:querystring';
+import * as zlib from 'node:zlib';
 
 export type BodyParserLikeRequest = http.IncomingMessage & { body?: any };
 
@@ -28,8 +29,23 @@ export function fixRequestBody<TReq extends BodyParserLikeRequest = BodyParserLi
   }
 
   const writeBody = (bodyData: string) => {
-    proxyReq.setHeader('Content-Length', Buffer.byteLength(bodyData));
-    proxyReq.write(bodyData);
+    let proxyData: string | Buffer = bodyData;
+
+    const contentEncoding = String(proxyReq.getHeader('Content-Encoding')).toLowerCase();
+    switch (contentEncoding) {
+      case 'br':
+        proxyData = zlib.brotliCompressSync(proxyData);
+        break;
+      case 'deflate':
+        proxyData = zlib.deflateSync(proxyData);
+        break;
+      case 'gzip':
+        proxyData = zlib.gzipSync(proxyData);
+        break;
+    }
+
+    proxyReq.setHeader('Content-Length', Buffer.byteLength(proxyData));
+    proxyReq.write(proxyData);
   };
 
   // Use if-elseif to prevent multiple writeBody/setHeader calls:

test/unit/fix-request-body.spec.ts

@@ -1,6 +1,7 @@
 import { ClientRequest, IncomingMessage, ServerResponse } from 'node:http';
 import { Socket } from 'node:net';
 import * as querystring from 'node:querystring';
+import * as zlib from 'node:zlib';
 
 import { BodyParserLikeRequest, fixRequestBody } from '../../src/handlers/fix-request-body';
 
@@ -213,4 +214,20 @@ describe('fixRequestBody', () => {
     expect(proxyRequest.write).toHaveBeenCalledTimes(0);
     expect(proxyRequest.destroy).toHaveBeenCalledTimes(0);
   });
+
+  it('should re-encode body when the source was encoded', () => {
+    const proxyRequest = fakeProxyRequest();
+    proxyRequest.setHeader('content-type', 'application/json; charset=utf-8');
+    proxyRequest.setHeader('content-encoding', 'gzip');
+
+    jest.spyOn(proxyRequest, 'setHeader');
+    jest.spyOn(proxyRequest, 'write');
+
+    const data = { someField: 'some value' };
+    fixRequestBody(proxyRequest, createRequestWithBody(data));
+
+    const expectedBody = zlib.gzipSync(JSON.stringify(data));
+    expect(proxyRequest.setHeader).toHaveBeenCalledWith('Content-Length', expectedBody.length);
+    expect(proxyRequest.write).toHaveBeenCalledWith(expectedBody);
+  });
 });