Testing Javascript modules that use the filesystem can be a little annoying to test sometimes, especially when using awesome parallel test runners like Ava. While developing MARSS, my Markdown blogging system, I tried to find a good way of running the tests, so they could effectively have their own environment (read folder) to test in. I came accross memfs, which implements the fs module in a virtual filesystem in memory. The author also created fs-monkey, which allows you to patch the fs module with a given fs-like implementation.

With these, you are able to make the code you are testing use your own specially crafted filesystem. Woot. (Un)fortunately, the require function also uses the fs module to read files, so when you patch the fs module, you also restrict importing any more modules. Luckily, the author has unionfs which allows you to patch two file systems together - phew.

Wiring it all together it allows you to create a test file system that you can then use to test Javascript that does operations on the file system. Below is the module I created for doing such a thing and how I used it in my tests.

Test file system module

The below module sets up the virtual test file system and patches the fs module for the tests.

import { patchFs } from 'fs-monkey';
import { Volume } from 'memfs';
import { ufs } from 'unionfs';
import * as fs from 'fs';

export const vol = new Volume();

const fs2 = Object.assign({}, fs);

ufs.use(fs2).use(vol);

patchFs(ufs);

Test that uses the test file system module

The test loads above module, loads tests files into is using the memfs() .fromJson() function into a folder based on the test process id.

import { vol } from '../unionfs';
import * as process from 'process';

// Import the module to test that uses the fs, eg runs a glob
import { someFunction as thingToTest } from './someFunction';

const exampleFiles = {
  'folder/file1.txt': `This is the contents of file 1`,
  'folder/file2.txt': `This is the contents of file 2`,
  'another.txt': `You get the point`
};

const testFolder = `/test/${process.pid()}`

vol.fromJSON(exampleFiles, testFolder);

test('it returns an array of files in the directory', async (t) => {
  const results = await thingToTest(testFolder);

  t.deepEqual(Object.keys(exampleFiles), results);
});