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Intercept WebSocket messages

With ajax requests it can be done with this code:
let oldXHROpen = window.XMLHttpRequest.prototype.open;
window.lastXhr = '';
window.XMLHttpRequest.prototype.open = function(method, url, async, user, password) {
this.addEventListener('load', function() {
window.lastXhr = this.responseText;
});
return oldXHROpen.apply(this, arguments);
};
lastXhr variable will hold the last response.
But how can this be achieved for websockets too?

you would need to make this wrapper as soon as possible
#brunoff you're correct in that you can always use your functions before a server's by puppet window logic, or you could just hijack the data from the MessageEvent itself:
function listen(fn){
fn = fn || console.log;
let property = Object.getOwnPropertyDescriptor(MessageEvent.prototype, "data");
const data = property.get;
// wrapper that replaces getter
function lookAtMessage() {
let socket = this.currentTarget instanceof WebSocket;
if (!socket) {
return data.call(this);
}
let msg = data.call(this);
Object.defineProperty(this, "data", { value: msg } ); //anti-loop
fn({ data: msg, socket:this.currentTarget, event:this });
return msg;
}
property.get = lookAtMessage;
Object.defineProperty(MessageEvent.prototype, "data", property);
}
listen( ({data}) => console.log(data))
You can try putting in the code and running it in the console on this page and then running their WebSocket example.

To intercept the messages, you will have to spy on the onmessage = fn and addEventListener("message", fn) calls.
To be able to modify the onmessage we have to override the global WebSocket in the first place. The below is intercepting the incoming messages, but in a similar way you can spy on the send method to intercept the outgoing messages (the ones sent by the client to the server).
I tested this on a page using Firebase and it works nicely, but you have to initialize it before the other scripts making sure that the websocket library (it can be socket.io, ws, etc) is using the overridden WebSocket constructor.
Spy the Incoming Messages and modify the data
Eventually you can override the data before calling the real message listener – this becomes handy if you do not have control over the page functionality and want to inject your own data in the message listener.
const OriginalWebsocket = window.WebSocket
const ProxiedWebSocket = function() {
console.log("Intercepting web socket creation")
const ws = new OriginalWebsocket(...arguments)
const originalAddEventListener = ws.addEventListener
const proxiedAddEventListener = function() {
if (arguments[0] === "message") {
const cb = arguments[1]
arguments[1] = function() {
// Here you can get the actual data from the incoming messages
// Here you can even change the data before calling the real message listener
Object.defineProperty(e, "data", { value: 'your injected data' })
console.log("intercepted", arguments[0].data)
return cb.apply(this, arguments)
}
}
return originalAddEventListener.apply(this, arguments)
}
ws.addEventListener = proxiedAddEventListener
Object.defineProperty(ws, "onmessage", {
set(func) {
return proxiedAddEventListener.apply(this, [
"message",
func,
false
]);
}
});
return ws;
};
window.WebSocket = ProxiedWebSocket;
If you do not need to modify the data, you can follow the second part of the answer.
Spy the Incoming messages without modifying the data
If you want to listen for messages only, without overriding the data, things are simpler:
const OriginalWebsocket = window.WebSocket
const ProxiedWebSocket = function() {
const ws = new OriginalWebsocket(...arguments)
ws.addEventListener("message", function (e) {
// Only intercept
console.log(e.data)
})
return ws;
};
window.WebSocket = ProxiedWebSocket;
Spy the Outgoing Messages
In a very similar way, you can proxy the send method which is used to send data to the server.
const OriginalWebsocket = window.WebSocket
const ProxiedWebSocket = function() {
const ws = new OriginalWebsocket(...arguments)
const originalSend = ws.send
const proxiedSend = function() {
console.log("Intercepted outgoing ws message", arguments)
// Eventually change the sent data
// arguments[0] = ...
// arguments[1] = ...
return originalSend.apply(this, arguments)
}
ws.send = proxiedSend
return ws;
};
window.WebSocket = ProxiedWebSocket;
Feel free to ask any questions if anything is unclear.

In a solution similar to yours, where the window.XMLHttpRequest was replaced with a wrapped version that feeds window.lastXhr, we replace window.WebSockets with a wrapped version that feeds window.WebSocketMessages with all messages and timestamps received from all websockets created after this script.
window.watchedWebSockets = [];
window.WebSocketMessages = [];
function WebSocketAttachWatcher(websocket) {
websocket.addEventListener("message", (event)=>window.WebSocketMessages.push([event.data,Date.now()]));
window.watchedWebSockets.push(websocket);
}
// here we replace WebSocket with a wrapped one, that attach listeners on
window.WebSocketUnchanged = window.WebSocket;
window.WebSocket = function(...args) {
const websocket = new window.WebSocketUnchanged(...args);
WebSocketAttachWatcher(websocket);
return websocket;
}
Differently from your XMLRequest case, the websocket may already exist. If you need garanties that all websockets would be catched then you would need to make this wrapper as soon as possible. If you just can't, there's an not so good trick to capture already existing websockets once they send a message:
// here we detect existing websockets on send event... not so trustable
window.WebSocketSendUnchanged = window.WebSocketUnchanged.prototype.send;
window.WebSocket.prototype.send = function(...args) {
console.log("firstsend");
if (!(this in window.watchedWebSockets))
WebSocketAttachWatcher(this);
this.send = window.WebSocketSendUnchanged; // avoid passing here again on next send
window.WebSocketSendUnchanged.call(this, ...args);
}
It is not so trustable since if they don't send but receive they will stay unnoticed.

Intro
The question/bounty/op is specifically asking for a reputable source.
Instead of rolling a custom solution, my proposal is that a known proven library should be used - that has been used, audited, forked, and in general used by the community and that is hosted on github.
The second option is to roll your own (though not recommended) and there are many exccelent answers on how to do it involving the addEventListener
wshook
Wshook is a library (hosted on github) that allows to easily intercept and modify WebSocket requests and message events. It has been starred and forked multiple times.
Disclaimer: I don't have any relationship with the specific project.strong text
Example:
wsHook.before = function(data, url, wsObject) {
console.log("Sending message to " + url + " : " + data);
}
// Make sure your program calls `wsClient.onmessage` event handler somewhere.
wsHook.after = function(messageEvent, url, wsObject) {
console.log("Received message from " + url + " : " + messageEvent.data);
return messageEvent;
}
From the documentation, you will find:
wsHook.before - function(data, url, wsObject):
Invoked just before
calling the actual WebSocket's send() method.
This method must return data which can be modified as well.
wsHook.after - function(event, url, wsObject):
Invoked just after
receiving the MessageEvent from the WebSocket server and before
calling the WebSocket's onmessage Event Handler.
Websocket addEventListener
The WebSocket object supports .addEventListener().
Please see: Multiple Handlers for Websocket Javascript

if you are using nodejs then you can use socket.io
yarn add socket.io
after installation, you can use the middleware of socket.io
io.use(async (socket, next) => {
try {
const user = await fetchUser(socket);
socket.user = user;
} catch (e) {
next(new Error("unknown user"));
}
});

How do I call a Python function from Node.js?

I'm working on making a Homebridge plugin for a project. Homebridge is a Node.js server which I have running on a Raspberry Pi which emulates an Apple HomeKit Bridge.
Using this link, I was able to execute Python code from the following Node.js code:
var Service, Characteristic;
var spawn = require('child_process').spawn;
var py = spawn('python', ['/home/pi/Desktop/RFbulb/nRF24L01PLUS.py']);
var data = [10,10,10];
var dataString = '';
var RFstatus = true;
module.exports = function(homebridge) {
Service = homebridge.hap.Service;
Characteristic = homebridge.hap.Characteristic;
homebridge.registerAccessory("homebridge-RFbulb", "RFbulb", RFbulbAccessory);
}
function RFbulbAccessory(log, config) {
this.log = log;
this.config = config;
this.name = config["name"];
this.address = config["address"];
this.service = new Service.Lightbulb(this.name);
this.service
.getCharacteristic(Characteristic.On)
.on('get', this.getOn.bind(this))
.on('set', this.setOn.bind(this));
}
RFbulbAccessory.prototype.setOn = function(on, callback) { // This is the function throwing the error
var state = on ? "on": "off";
if (state == "on") {
data = [1,parseInt(this.address, 10),100];
dataString = '';
py.stdout.on('data', function(data) {
dataString += data.toString();
});
py.stdout.on('end', function() {
console.log(dataString);
});
py.stdin.write(JSON.stringify(data));
py.stdin.end();
RFstatus = true;
}
callback(null);
}
RFbulbAccessory.prototype.getServices = function() {
return [this.service];
}
Interestingly enough, when I activate the setOn function the first time (for example, to turn the device on) it works fine, but when I activate the setOn function a second time (to turn the device off) I get the following errors and the server exits:
events.js:141
throw er; // Unhandled 'error' event
^
Error: write after end
at writeAfterEnd (_stream_writable.js:166:12)
at Socket.Writable.write (_stream_writable.js:211:5)
at Socket.write (net.js:642:40)
at RFbulbAccessory.setOn (/usr/lib/node_modules/homebridge-RFbulb/index.js:47:12)
at emitThree (events.js:97:13)
at emit (events.js:175:7)
at Characteristic.setValue (/usr/lib/node_modules/homebridge/node_modules/hap-nodejs/lib/Characteristic.js:155:10)
at Bridge.<anonymous> (/usr/lib/node_modules/homebridge/node_modules/hap-nodejs/lib/Accessory.js:710:22)
at Array.forEach (native)
at Bridge.Accessory._handleSetCharacteristics (/usr/lib/node_modules/homebridge/node_modules/hap-nodejs/lib/Accessory.js:655:8)
What could be causing this error? Especially since the function appears to work fine for a single use.

You're getting that error because you're closing the input stream:
py.stdin.end();
After a stream has been closed, you can no longer write to it like you are here:
py.stdin.write(JSON.stringify(data));
If the Python program you're running accepts multiple commands over STDIN then simply remove the py.stdin.end() line.
However, it's likely that your Python program runs once then completes. If that's the case, you will need to respawn the process every time you want the program to run.
if (state === "on") {
py = spawn('python', ['/home/pi/Desktop/RFbulb/nRF24L01PLUS.py']);
...
}

NodeJS, WebSockets, upgrade head

I'm playing with NodeJS and WebSockets, there is the upgrade event with it's head parameter, as I understood from here that is basically data that directly trails the headers, but for my use case it's always empty so I don't really know what it means, I'd be glad if someone could provide a simple use case where the data parameter within the upgrade event isn't empty.

Looking at the source that emits that upgrade event in the node repo, you'll see the following implementation:
function socketOnData(d) {
assert(!socket._paused);
debug('SERVER socketOnData %d', d.length);
var ret = parser.execute(d);
onParserExecuteCommon(ret, d);
}
function onParserExecute(ret, d) {
debug('SERVER socketOnParserExecute %d', ret);
onParserExecuteCommon(ret, undefined);
}
function onParserExecuteCommon(ret, d) {
if (ret instanceof Error) {
debug('parse error');
socket.destroy(ret);
} else if (parser.incoming && parser.incoming.upgrade) {
// Upgrade or CONNECT
var bytesParsed = ret;
var req = parser.incoming;
debug('SERVER upgrade or connect', req.method);
if (!d)
d = parser.getCurrentBuffer();
socket.removeListener('data', socketOnData);
socket.removeListener('end', socketOnEnd);
socket.removeListener('close', serverSocketCloseListener);
unconsume(parser, socket);
parser.finish();
freeParser(parser, req, null);
parser = null;
var eventName = req.method === 'CONNECT' ? 'connect' : 'upgrade';
if (EventEmitter.listenerCount(self, eventName) > 0) {
debug('SERVER have listener for %s', eventName);
var bodyHead = d.slice(bytesParsed, d.length);
// TODO(isaacs): Need a way to reset a stream to fresh state
// IE, not flowing, and not explicitly paused.
socket._readableState.flowing = null;
self.emit(eventName, req, socket, bodyHead);
...
The passed parameter, bodyHead, reflects the data from the passed parameter d on the socketOnData function, which is the default socket data event handler. On the other hand, it could be undefined if the onParserExecuteCommon was called from onParserExecute. I'd have to look through the source more to understand which cases would apply depending on how your server is implemented. Maybe you could enable the debug logs to see which methods are being called.

Node.js Recursive Loop fail

Below is the Node.js Script. It downloads the images contained in a div. The loop works fine for 9.86% that is upto id = 36. When id > 36 it exits the loop. I am using the node 0.12 version. The loop needs to run 365 times before its completion. I am usign the method of recursive callback.
Code:
//Required modules
var fs = require('fs'),
cheerio = require('cheerio'),
request = require('request');
//Default Variables
var baseURI = 'http://www.website/';
var year = 2013;
var id = 1;
var savePath = process.argv[2];
//Download Function
var download = function(uri, filename, callback){
request({ uri: uri }, function(err, res, body){
var $ = cheerio.load(body);
var imgDiv = $('#img-wallpaper').children()['0'];
if(err)
console.err(err);
if(typeof imgDiv !== 'undefined') {
request(imgDiv.attribs.src).pipe(fs.createWriteStream(filename)).on('close', callback);}
});
};
//Main Function
console.log("Downloading . . .");
// Loop function to create a recursive effect
(function loop(){
download(baseURI+year+'/'+id+'/wallpaper/', savePath+id+'.jpg',
function(){
console.log(((id/365)*100).toFixed(2)+'% completed');
if(id == 330)
year = "2014";
if(((id/365)*100) != 100){
id=id+1;
loop();}
});
})(1)

Do I understand correctly that if you set the starting value for the id more than 35 (36?) the script is not downloaded any images?
Test the script on the fixed uri and on the fixed image by changing only the variables. Script is expected to work out? If this is the case:
Or not called callback for body request
Or a false condition typeof imgDiv !== 'undefined'
Or not called callback for request image
Thus, when an error in one of these points the script stops working. It is necessary to change the severity of conditions.

As #stdob said, The error was caused due to
Or a false condition typeof imgDiv !== 'undefined'
Though the answer is not the right way to overcome the error, it is more of an hack type. It ignores the error and continues the script!
if(typeof imgDiv !== 'undefined') {
request(imgDiv.attribs.src).pipe(fs.createWriteStream(filename)).on('close', callback);
}
else{
id++;
request(uri).pipe(fs.createWriteStream(filename)).on('close', callback);
}

node js fs module says object does not have method appendFile

I'm building a node js app(for learning) where i'm logging each operation into a file called log.txt
The logger module has the following code :
var fs = require('fs');
function write(data,filename)
{
var entry = 'Time: '+new Date();
if(filename !=null || filename != undefined) entry = entry+'\n\tFile: '+filename;
if(data !=null || data != undefined) entry = entry+'\n\tMessage: '+data;
entry = entry+'\n';
fs.appendFile('./log.txt',entry,function(err){
if(err){console.log('Log NOT Appended with data:\n\t'+entry);}
else{console.log('Log Appended with data:\n\t'+entry);}
});
}
exports.write = write;
now in my app.js i'm requiring it as :
var logger = require('./logger');
var fs = require('fs');
function ReadFile()
{
var data = fs.readFileSync('./config.txt');
if(data==null)
logger.write("Config data not found");
else
logger.write(data,"app.js");
}
ReadFile();
This throws me back an error saying :
Object #<Object> has no method 'appendFile'
However this worked fine earlier on a different pc, I noticed this when tried to run my
app on my system.

This was a problem with the version, i was using v0.6.12 which did not have that method, i upgraded to v0.10.0 which solved it, thanks to michaelt for pointing it out.