using mock-fs with jest to assert the result of file system operations in a sync manner

example-test.js

import mock from 'mock-fs';
import fs from 'fs';
import fileHandler from '../utils/fileHandler';

beforeEach(() => {
  // Creates an in-memory file system 
  mock({
    '/test': {
      'note.md': 'hello world!'
    }
  });
});

it('throws error if path doesn\'t exist', () => {
  mock.restore(); // => called here to check if Error is thrown 
  
  const error = () => fileHandler.saveFileSync('/test/note2.md', 'ciao world!');
  
  expect(error).toThrow();
});

it('saves a file given it\'s contents', () => {
  fileHandler.saveFileSync('/test/note2.md', 'ciao world!';
  
  expect(fs.existsSync('/test/note.md')).toBeTruthy();
  
  mock.restore(); // => called here as in a regular afterEach
});

it('throws error if path doesn\'t exist', () => {
  mock.restore(); // => called here to check if Error is thrown 
  
  const error = () => fileHandler.loadFileSync('/test/note.md');

  expect(error).toThrow();
});

it('loads a file from disk', () => {
  const actual = fileHandler.loadFileSync('/test/note.md');

  expect(actual).toBe('hello world!');
  
  mock.restore(); // => called here as in a regular afterEach
});