Writing test to nodejs function that saves string as columns to CSV file

Question

I am trying to write jest test to a nodejs function which basically gets a string as an argument and writes that string as a columns to CSV file using fs's createWriteStream function. Below is the code.

import fs from 'fs';

export default (str) => {
    if (!str) return console.log('Please enter string');

    const stream = fs.createWriteStream('data.csv');

    [...str].map(i => stream.write(i + ', ')).join(',');

    stream.end();
    
    console.log('CSV created!');

    return 'CSV created!';
}

The test I wrote is not complete which is below.

import CSV from './CSV';
import fs from 'fs';

test('test csv', () => {
  expect(CSV('hello world')).toBe('CSV created!');
});

Ofcourse above test gets passed but I want it to be tested properly. I think jest.fn() is something needs to be used. I went through it docs but still don't know how to properly use it in this case. My question is how can I write jest test for above function?