React Testing Library - Simulating uploading file with FileReader - reactjs

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)