unit testing - Faking out FormData append with Jasmine -

i need test unit of code constructs formdata object, appends file , submits ajax request server. issue don't have file object provide formdata append method call. wanted spyon append method , call fake function of own. possible? if not, how can test code.

the following unit of code want test:

var fd = new formdata();  // listoffiles array of files , filename string var file = _.findwhere(listoffiles, { name: filename });  fd.append("file", file, filename);  $.ajax(/* options */); 

the following have unit test, doesn't job:

it("does something", function() {     var mockfiles = [{         name: "test file.pdf"     }];      // adddocs call adds files listoffiles array above     someview.adddocs(mockfiles);      var ajaxspy = spyon($, "ajax");      // doesn't work     spyon(formdata.prototype, "append").andreturn(false);      // event causes above unit of code run     app.trigger('someevent');      // more stuff... }); 

right following error:

failed execute 'append' on 'formdata': no function found matched signature provided 

which happens because spy doesn't called.

any appreciated how spy on formdata methods.

turns out wrong , following code spy correctly on formdata.append method:

spyon(formdata.prototype, "append"); 

the issue put far nested in describe blocks other tests failing because tried call formdata.append , no spy set tests.

while working through solution, realized following concept work since don't care uploading file:

spyon(window, 'formdata').andreturn({     "append": jasmine.createspy() }); 

hopefully can needs spy on formdata!