Testing functions with properties using Jest and Sinon - node.js

Question

am trying to write tests to test streams on my app, dealing with fs library with its createWriteStream function, so the stub i created is as follows:
writeStreamStub = sinon.stub()
onceStreamEventStub = sinon.stub()
endStreamStub = sinon.stub()
onStreamStub = sinon.stub()
createStreamStub = sinon.stub(fs, 'createWriteStream').returns({
write: writeStreamStub,
once: onceStreamEventStub,
end: endStreamStub,
on: onStreamStub
})
So now I can test for whether the functions are called and the returned functions are also called. But I am using the --coverage flag and the code of the callbacks of the returned functions is not covered, the write method is called inside a process.nextTick and I have no idea how to go about this. Is it possible to cover the whole code and the code inside the callbacks, and if so, how do I go about it. Thanks in advance.
N.B. The variables are globaly declared

Answer 1

If there's no cogent reason to use both sinon and jest, I'd recommend just using one library. If you decide to go with jest, here's a simple example. Assume you have a class like
const fs = require('fs');
module.exports = class FileWriter {
constructor() {
this.writer = fs.createWriteStream('./testfile.txt');
}
writeFile() {
process.nextTick(() => {
this.writeContent('hello world');
});
}
writeContent(content) {
this.writer.write(content);
this.writer.end();
}
};
and in your unit-test you want to mock the behaviour of all the used fs-functions (createWriteStream, writer, end in this case) and just check if they are called with the correct arguments. You could do this with something like this:
const fs = require('fs');
const FileWriter = require('./FileWriter');
// use this to have mocks for all of fs' functions (you could use jest.fn() instead as well)
jest.mock('fs');
describe('FileWriter', () => {
it('should write file with correct args', async () => {
const writeStub = jest.fn().mockReturnValue(true);
const endStub = jest.fn().mockReturnValue(true);
const writeStreamStub = fs.createWriteStream.mockReturnValue({
write: writeStub,
end: endStub,
});
const fileWriter = new FileWriter();
fileWriter.writeFile();
await waitForNextTick();
expect(writeStreamStub).toBeCalledWith('./testfile.txt');
expect(writeStub).toBeCalledWith('hello world');
expect(endStub).toHaveBeenCalled();
});
});
function waitForNextTick() {
return new Promise(resolve => process.nextTick(resolve));
}