Unit testing of websocket application by sinon.js

Question

I have tried to do unit testing for a web socket application using sinon.js,

One of the users on github of sinon, did this, but I am not able to understand how it does help to unit test websocket applications for validating the received data which was sent to fake server.

var dummySocket = { send : sinon.spy()};
sinon.stub(window, 'WebSocket').returns(dummySocket);
dummySocket = new WebSocket('ws://html5rocks.websocket.org/echo');
dummySocket.onopen();
dummySocket.onmessage(JSON.stringify({ hello : 'from server' }));
// You can assert whether your code sent something to the server like this:
sinon.assert.calledWith(dummySocket.send, '{"the client":"says hi"}');

My questions are

• How can I receive the same data from fake server which have been sent to server earlier.
• How can I send data to fake server by using send method of fake socket object(eg:- socket.send())?
• How can I get data from server on dummySocket.onmessage = function (msg){}

With sinon.js, I could not get the any process to create fake websocket object like for fake XMLHttpRequest and server by using respectively useFakeXMLHttpRequest() and fakeServer.create()

Is there any process to achieve this on sinon.js?

Answer 1

Normally, you would do ws = sinon.createStubInstance(WebSocket), but this isn't possible since properties on the WebSocket.prototype throw exceptions when reading them. There are two ways around this.

1. You could add a useFakeWebSocket to sinon to overwrite WebSocket. This would be similar to what useFakeXMLHttpRequest does to XMLHttpRequest.

2. Duck type out a WebSocket object by iterating over the prototype.

   beforeEach(function () {
       var ws = {};
       for (var prop in WebSocket.prototype) {
           ws[prop] = function () {}; // some properties aren't functions.
       }
   });
   

If you wanted to implement a mock echo WebSocket server so that you can test your event handlers, you could do that with this:

var ws;
beforeEach(function () {
    ws = {
        send: function (msg) {
            this.onmessage({ data: msg });
        },
        onmessage: function (e) {
            // stub
        }
    };
});
it('should echo', function () {
    var spy = sinon.spy(ws, 'onmessage');
    ws.send('this is a test');
    assertEquals(spy.args[0][0].data, 'this is a test');
});

I hope this is what you're looking for.