How to mock JSON file in Jest

Question

I am using Jest to test my API and when I run my tests, my JSON file results.json gets written to due to the following line in my API app.js (which I don't want happening):

fs.writeFile('results.json', JSON.stringify(json), (err, result) => {
    if (err) console.log('error', err);
});

This is what my Jest file looks like:

const request = require('supertest');
const app = require('./app');


// Nico Tejera at https://stackoverflow.com/questions/1714786/query-string-encoding-of-a-javascript-object
function serialise(obj){
    return Object.keys(obj).map(k => `${encodeURIComponent(k)}=${encodeURIComponent(obj[k])}`).join('&');
}


describe('Test /addtask', () => {
    test('POST /addtask Successfully redirects if newDate and newTask filled in correctly', () => {
        const params = {
                    newTask: 'Example',
                    newDate: '2020-03-11'
        };
        return request(app)
            .post('/addtask')
            .send(serialise(params))
            .expect(301);
    });
});

I tried creating a mock of the JSON file and placed it outside the describe statement to prevent the actual results.json file being written to:

jest.mock('./results.json', () => ({ name: 'preset1', JSONtask: [], JSONcomplete: [] }, { name: 'preset2', JSONtask: [], JSONcomplete: [] }));

But this doesn't change anything. Does anyone have any suggestions?

I have seen other solutions to similar problems but they don't provide the answer I'm looking for.

EDIT: Although not a very good method, one solution to my problem is to wrap the fs.writeFile within the statement

if (process.env.NODE_ENV !== 'test') {
//code
};

although this would mean that fs.writeFile cannot be tested upon.

NOTE: I am still accepting answers!

Answer 1

Your issue is that the code you want to test has a hard-coded I/O operation in it, which always makes things harder to test.

What you'll want to do is to isolate the dependency on fs.writeFile, for example into something like a ResultsWriter. That dependency can then be injected and mocked for your test purposes.

I wrote an extensive example on a very similar case with NestJS yesterday under how to unit test a class extending an abstract class reading environment variables, which you can hopefully adapt to your needs.

Answer 2

jest.mock(path, factory) is for mocking JS modules, not file content.

You should instead mock fs.writeFile and check that it has been called with the expected arguments. The docs explain how to do it.