Sinon: Mocking websockets/ws - node.js

Question

How can I mock up websockets/ws using Sinon? I'm trying to test that my application behaves as expected when using WebSockets, without necessarily needing to connect each time (eg: testing event handlers, etc).
Coming from a C# background, I'd just mock out the whole interface using a library like Moq, and then verify that my application had made the expected calls.
However, when trying to do this with Sinon, I'm running into errors.
An example of a test:
const WebSocket = require('ws');
const sinon = require('sinon');
const webSocket = sinon.mock(WebSocket);
webSocket.expects('on').withArgs(sinon.match.any, sinon.match.any);
const subject = new MyClass(logger, webSocket);
This class is then calling:
this._webSocket.on("open", () => {
this.onWebSocketOpen();
});
But when I try and run my tests, I get this error:
TypeError: Attempted to wrap undefined property on as function
What's the correct way to mock out an object like this using Sinon?
Thanks.

Answer 1

If your just trying to test if the given sockets 'on' method was called when passed in, this is how you would do it:
my-class/index.js
class MyClass {
constructor(socket) {
this._socket = socket;
this._socket.on('open', () => {
//whatever...
});
};
};
module.exports = MyClass;
my-class/test/test.js
const chai = require('chai');
const expect = chai.expect;
const sinon = require('sinon');
const sinon_chai = require('sinon-chai');
const MyClass = require('../index.js');
const sb = sinon.sandbox.create();
chai.use(sinon_chai);
describe('MyClass', () => {
describe('.constructor(socket)', () => {
it('should call the .prototype.on method of the given socket\n \t' +
'passing \'open\' as first param and some function as second param', () => {
var socket = { on: (a,b) => {} };
var stub = sb.stub(socket, 'on').returns('whatever');
var inst = new MyClass(socket);
expect(stub.firstCall.args[0]).to.equal('open');
expect(typeof stub.firstCall.args[1] === 'function').to.equal(true);
});
});
});