How do I write FileReader test in Jasmine?

Question

I'm trying to make this test work, but I couldn't get my head around how to write a test with FileReader. This is my code

function Uploader(file) {
    this.file = file;
}

Uploader.prototype =  (function() {

    function upload_file(file, file_contents) {
        var file_data = new FormData()
        file_data.append('filename', file.name)
        file_data.append('mimetype', file.type)
        file_data.append('data', file_contents)
        file_data.append('size', file.size)

        $.ajax({
            url: "/upload/file",
            type: "POST",
            data: file_contents,            
            contentType: file.type,
            success: function(){

                // $("#thumbnail").attr("src", "/upload/thumbnail");    

            },
            error: function(){
                alert("Failed");
            },
            xhr: function() {
                myXhr = $.ajaxSettings.xhr();
                if(myXhr.upload){
                    myXhr.upload.addEventListener('progress',showProgress, false);
                } else {
                    console.log("Upload progress is not supported.");
                }
                return myXhr;
            }
        });
    }

    return {
        upload : function() {
            var self = this,
                reader = new FileReader(),
                file_content = {};

            reader.onload = function(e) {
                file_content = e.target.result.split(',')[1];

                upload_file(self.file, file_content);
            }
        }
    };
})();

And this is my test

describe("Uploader", function() {
    it("should upload a file successfully", function() {
        spyOn($, "ajax");
        var fakeFile = {};

        var uploader = new Uploader(fakeFile);
        uploader.upload();

        expect($.ajax.mostRecentCall.args[0]["url"]).toEqual("/upload/file");
    })
});

But it never gets to reader.onload.

Answer 1

The problem here is the use of reader.onload which is hard to test. You could use reader.addEventListener instead so you can spy on the global FileReader object and return a mock:

eventListener = jasmine.createSpy();
spyOn(window, "FileReader").andReturn({
 addEventListener: eventListener
})

then you can fire the onload callback by yourself:

expect(eventListener.mostRecentCall.args[0]).toEqual('load');
eventListener.mostRecentCall.args[1]({
  target:{
    result:'the result you wanna test'
  }
})