add support for modify response

Author: guoxiangyang

README.md

@@ -377,6 +377,7 @@ proxyServer.listen(8015);
 *  **timeout**: timeout (in millis) for incoming requests
 *  **followRedirects**: true/false, Default: false - specify whether you want to follow redirects
 *  **selfHandleRequest** true/false, if set to true, none of the webOutgoing passes are called and its your responsibility ro appropriately return the response by listening and acting on the `proxyRes` event
+*  **modifyResponse**: do not pipe proxyRes to res, so you can respond to client with your own response. It still goes through all out going web passes unlike selfHandleRequest
 *  **buffer**: stream of data to send as the request body.  Maybe you have some middleware that consumes the request stream before proxying it on e.g.  If you read the body of a request into a field called 'req.rawbody' you could restream this field in the buffer option:
 
     ```
@@ -395,6 +396,7 @@ proxyServer.listen(8015);
 
     };
     ```
+
 **NOTE:**
 `options.ws` and `options.ssl` are optional.
 `options.target` and `options.forward` cannot both be missing
@@ -485,6 +487,30 @@ proxy.close();
 
 ### Miscellaneous
 
+### Modify response
+
+```
+
+    var option = {
+        target: target,
+        modifyResponse : true
+    };
+    proxy.on('proxyRes', function (proxyRes, req, res) {
+        var body = new Buffer('');
+        proxyRes.on('data', function (data) {
+            body = Buffer.concat([body, data]);
+        });
+        proxyRes.on('end', function () {
+            body = body.toString();
+            console.log("res from proxied server:", body);
+            res.end("my response to cli");
+        });
+    });
+    proxy.web(req, res, option);
+
+
+```
+
 #### ProxyTable API
 
 A proxy table API is available through this add-on [module](https://github.com/donasaur/http-proxy-rules), which lets you define a set of rules to translate matching routes to target routes that the reverse proxy will talk to.

lib/http-proxy/passes/web-incoming.js

@@ -181,8 +181,8 @@ module.exports = {
         proxyRes.on('end', function () {
           if (server) server.emit('end', req, res, proxyRes);
         });
-        // We do this separately since we are also checking for res.finished
-        if (!options.selfHandleResponse) proxyRes.pipe(res);
+        // We pipe to the response unless its expected to be handled by the user
+        if (!options.selfHandleResponse && !options.modifyResponse) proxyRes.pipe(res);
       } else {
         if (server) server.emit('end', req, res, proxyRes);
       }