Easier way to use memfs in tests

[sapegin]

I’m trying to use memfs in tests. Use case is like this:

1. Mock fs with memfs:

   // __mocks__/fs.js
   process.chdir('/');
   module.exports = require('memfs').fs;

2. Load the initial state from JSON using fromJSON method.

3. Run methods I’m testing.

4. Inspect the resulting JSON from toJSON.

The problem is that every time I call fromJSON it modifies the volume, keeping all existing files from previous tests.

I see two possible solutions:

1. Recreate a memfs volume before each test. Seems a bit too complex.
2. rm -rf / after each tests. But it seems scary — what if I forget to call jest.mock('fs')? ;-|

As a perfect solution I see fromJSON that can remove all existing files before loading JSON. Could be a new parameter or a new method.

What do you think?

[streamich]

module.exports = require('memfs').fs;

You can also do module.exports = require('memfs');

Recreate a memfs volume before each test. Seems a bit too complex.

I would go with this one, just create new Volume for each test.

rm -rf / after each tests.

We could add rmrf method, like vol.rmrfSync('/');. Or just a method like .reset(), .truncate(), .clear().

As a perfect solution I see fromJSON that can remove all existing files before loading JSON.

The .fromJSON() method could be extended as follows, note reset: true:

vol.fromJSON(json, {
    cwd: '/', 
    reset: true,
    softlinks: { /* ... */ },
    hardlinks: { /* ... */ },
    fds: { /* ... open file state ... */ }
});

[sapegin]

I would go with this one, just create new Volume for each test.

That’s inconvenient because I would need to export a method from this mock module that would recreate a volume.

Or just a method like .reset(), .truncate(), .clear().

I think that’s the most clear solution. I assume there is an easy way to implement that? I’d try ;-)

[streamich]

I assume there is an easy way to implement that? I’d try ;-)

Hmm, I'm thinking just resetting these properties should do the job:

https://github.com/streamich/memfs/blob/master/src/volume.ts#L509-L539

Like:

this.ino = 0;
this.inodes = {};
this.releasedInos = [];
// etc.

And also create a new root node:

 this.root = new Link(this, null, '');
 this.root.setNode(this.createNode(true))

[streamich]

BTW, could something like that do the job?

// __mocks__/fs.js
const {createFsFromVolume, Volume} = require('memfs');

process.chdir('/');

const beforeEach = () => {
    module.exports = createFsFromVolume(new Volume);
    module.exports.__beforeEach = beforeEach;
};

exports.__beforeEach = beforeEach;

And then:

beforeEach(() => {
    require('fs').__beforeEach();
});

[sapegin]

I’m not sure there’s beforeEach inside mock files. And anyway I’ve submitter a PR with reset already ;-)

I’ve tried something similar and it didn’t work — I assume all tests still use a link to the initial volume and just overwriting module.exports isn’t enough.

[sapegin]

I’m trying .reset() and test cases look really nice:

jest.mock('fs');

const { vol } = require('memfs');
const { copyFiles } = require('../fs');

afterEach(vol.reset.bind(vol));

describe('copyFiles()', () => {
	it('should copy a file', () => {
		vol.fromJSON({ 'tmpl/a': 'pizza' });

		copyFiles('tmpl', 'a');

		expect(vol.toJSON()).toMatchSnapshot();
	});

	it.skip('should copy multiple files', () => {
		vol.fromJSON({ 'tmpl/a': 'pizza', 'tmpl/b': 'coffee' });

		copyFiles('tmpl', ['a', 'b']);

		expect(vol.toJSON()).toMatchSnapshot();
	});

	it.skip('should not overwrite a file if  overwrite=false', () => {
		const json = { 'tmpl/a': 'pizza', a: 'pizza' };
		vol.fromJSON(json);

		copyFiles('tmpl', 'a', { overwrite: false });

		expect(vol.toJSON()).toEqual(json);
	});
});

[streamich]

I like the .toJSON() with .toMatchSnapshot().