var mock = require('mock-fs');

var fs = mock.fs({
  'path/to/fake/dir': {
    'some-file.txt': 'file content here',
    'empty-dir': {/** empty directory */}
  },
  'path/to/some.png': new Buffer([8, 6, 7, 5, 3, 0, 9]),
  'some/other/path': {/** another empty directory */}
});

var rewire = require("rewire");
var moduleToTest = rewire('./path/to/module');

// inject the mock fs created above
moduleToTest.__set__('fs', fs);

// now functions in moduleToTest will use
// your mock fs instead of the real one