filesystem

Mocking Example: Interacting with the filesystem

For example in clean-directory.js we have the following function:

export default function cleanDirectory() {
    const files = fs.readdirSync(DIR_TO_CLEAN).map((f) => path.join(DIR_TO_CLEAN, f));

    files.forEach((file) => {
        fs.unlinkSync(file);
    });

    return files.length;
}

The challenge

We want mock out the contents of the filesystem per test.

Solution

See the tests

Alternatives

1. Dependency injection. We could have fs or readdirSync and unlink be default arguments to the cleanDirectory function:

   export default function cleanDirectory(_readdirSync = readdirSync, _unlink = unlink) {

   Given the availability of battle-tested modules to mock out the file system, I think it's a nicer development experience to use one of those (e.g. memfs, mock-fs) - rather writing our own readdirSync and unlink functions each time and littering our function interface.