React Testing Library - Simulating uploading file with FileReader

Question

I want to test a React component where certificate file is uploaded and the result of file can be shown in a text area. I am still learning writing unit test and here facing difficulty to mock FileReader, readAsText and onloadend. I have read few examples but they do not work as expected. Any guidance would be helpful

Thanks

Cert Component

export default function Cert() {

 const fileInputRef = useRef();

  function handleEvent(obj, event) {
    var event = new Event(event, { target: obj, bubbles: true });
    return obj ? obj.dispatchEvent(event) : false;
  }

 let handleChangeFile = (file) => {
    let fileData = new FileReader();
    fileData.readAsText(file);
    fileData.onloadend = () => {
      let el = document.getElementById('cert');
      el.value = event.target.result;
      handleEvent(el, 'input');
      if (fileInputRef.current) {
        fileInputRef.current.value = '';
      }
    };
    fileData.onerror = function () {
      console.log(fileData.error);
    };
  };

  return (
<CheckboxWithOptions name="int" label='app'>
 <Field
          component={ExpandingTextareaField}
          name="certificate"
          label='Upload Certificate *'
          isRequired
          validate={required}
          id="cert"
        />
        <div>
          <Button
            label='Upload file'
            type="button"
            appearance="brand"
            size="large"
            onClick={() => {
              fileInputRef.current.click();
            }}
            data-typeId="test"
          />
          <input
            style={{
              opacity: 0,
              position: 'fixed',
              top: 0,
              left: 0,
              width: 0,
            }}
            type="file"
            accept=".cert"
            ref={fileInputRef}
            onChange={(e) => handleChangeFile(e.target.files[0])}
            data-testid="testinput"
          />
        </div>
}

Cert.test.js

it('upload file to show in textarea ', async () => {
        const file = new File(['dummy'], 'test.cert', { type: 'cert' })
        render(
            <div>
                <label htmlFor="file-uploader">Upload file:</label>
                <input id="file-uploader" type="file" onChange={(e) => handleChangeFile('e')} />
            </div>,
        )
        const { getByLabelText } = render(<TestComponent />);

        await waitForElementToBeRemoved(() => screen.getByText(/loading/i));
        expect(getByLabelText("CheckBox").value).toBe("false");
        fireEvent.click(screen.getByLabelText("CheckBox"))
        const input = screen.getByLabelText(/Upload file/i)
        fireEvent.click(input)
        userEvent.upload(input, file)
        await waitFor(() => expect(handleChangeFile).toBeCalledTimes(1)); // working fine till here
   jest.spyOn(global, "FileReader")
            .mockImplementation(function () {
                readAsText = jest.fn();
            });
        fireEvent.change(input, {
            target: {
                files: [file]
            }
        });
        expect(FileReader).toHaveBeenCalled(); // not working
// test the FileReader and textarea
})

Answer 1

Had the same issue with checking some side effect that should be invoked on FileReader.onload, so I just ended up setting a short pause after triggering the event (I'm using enzyme):

const pauseFor = milliseconds => new Promise(resolve => setTimeout(resolve, milliseconds));
...
wrapper.find('.upload-box').simulate('drop', someMockedDropEvent);
// set pause was the only way to make reader.onload to fire
await pauseFor(100);
expect(something).toEqual(something)