是否可以测试是否已将正确的回调函数分配给事件处理程序

Question

我有一个函数，该函数创建一个FileReader对象，并为它分配load和error处理程序。

handleFileSelect(files:ArrayLike<File>):FileReader|null{
    console.log("got file upload event: ");
    console.log(" image attachment count: ",this.currentImageAttachmentCount);
    if(this.currentImageAttachmentCount >= this.maxImageAttachmentCount)
    {
      console.log("reached max attachment size");

      this.showDialog("You can't attach more files",new DialogContext("",""));
      return null;
    }
    console.log("files selected:",files);
    console.log("total selected: ",files.length);
    for(let i=0;i<files.length;i++)
    {
      console.log("files name:",files[i].name);
      console.log("files object:",files[i])
    }

    //working with only 1 file at the moment
    let file = files[0];
    console.log("file at index 0 ",file);
    if (files && file) {
      console.log("reading file");
      let reader:FileReader = new FileReader();

      reader.onload = this.handleReaderLoaded.bind(this);
      reader.onerror = this.handleFileLoadError.bind(this);
      reader.readAsDataURL(file);
      this.currentImageAttachmentCount++;
      return reader;
    } else{
      return null;
    }

  }

我要测试handleFileSelect是否分配了正确的处理程序。我写的测试用例是

fit('should call assign correct error handler', () => {
    let newComponent = component;
    console.log("component is ",newComponent);

    let file1 = new File(["dd"], "file1",{type:'image/png'});

    let reader = newPracticeQuestionComponent.handleFileSelect([file1]);

    expect(reader).toBeTruthy();
    console.log("reader's onerror ",reader.onerror.name);
    console.log("component's onerror ",newComponent.handleFileLoadError.name);
    expect(reader.onerror.name).toEqual(newPracticeQuestionComponent.handleFileLoadError.name);

  });

我注意到浏览器控制台中的打印没有功能名称。我希望看到handleFileLoadError。

reading file reader's onerror

为什么我没有看到handleFileLoadError？如何测试是否分配了正确的错误处理程序？