ava-mockfs-rewiremock.js

import test from 'ava';
import sinon from 'sinon';
// ^ uses a real FS

// nothing before would be mocked
import rewiremock from 'rewiremock/node';

import mockfs from 'mock-fs';
import mountfs from 'mountfs';
import fs from 'fs';

rewiremock('fs').mockThrough();

mockfs.patchInPlace();

// ^ mountfs will "patch" not real FS, but a mock :P

fs.mount('/some/test/directory/', mockfs.fs({
  '/some/test/directory': {
    'some-file.txt': 'mock contents'
  }
});

This worked for me in a nodejs module after a small few adjustments:

import test from 'ava';

// Nothing before is mocked
import rewiremock from 'rewiremock/node.js';

import mockfs from 'mock-fs';
import mountfs from 'mountfs';
import fs from 'fs';

rewiremock('fs').mockThrough();

mountfs.patchInPlace();

fs.mount('/topdir/', mockfs({
    '/topdir': {
        'foobar': {
            'baz/barney': {},
            'foo.js': 'console.log("foobar")',
        },
        'foobaz/bar.json': '{}',
    },
}));