Spy and callThrough native WebSocket constructor with Jasmine

Question

I'm trying to spy on native WebSocket constructor, so I wrote:

it("should spy and call through WebSocket constructor", function (done) {
  var WSspy = spyOn(window, "WebSocket").and.callThrough();

  var ws = new WebSocket("ws://some/where");

  expect(WSspy).to.toHaveBeenCalledWith("ws://some/where");
});

but it results in error:

TypeError: Failed to construct 'WebSocket': Please use the 'new' operator, this DOM object constructor cannot be called as a function.

How should I callThrough such constructor?

Answer 1

I found a workaround with callFake:

it("should spy and callFake WebSocket constructor", function (done) {
  var realWS = WebSocket;  
  var WSSpy = spyOn(window, "WebSocket").and.callFake(function(url,protocols){
    return new realWS(url,protocols);
  });        

  var ws = new WebSocket("ws://some/where");

  expect(WSSpy).toHaveBeenCalledWith("ws://some/where");
  done();
});

It works quite fine, but it overwrites the WebSocket.prototype so make sure you use it or save the reference before creating the spy, like

var realWS = WebSocket;  
var messageSpy = spyOn(WebSocket.prototype, "close").and.callThrough();      
var WSSpy = spyOn(window, "WebSocket").and.callFake(function(url,protocols){
  return new realWS(url,protocols);
});