mockfs.js

function _patchFs() {
    if (fs.hasOwnProperty('_mockedFiles')) {
        return;
    }
    fs._readFileReal = fs.readFile;
    fs._mockedFiles = {};
    fs.mockFile = function(filePath, fileContent) {
        fs._mockedFiles[filePath] = fileContent;
    };
    fs.readFile = function(filePath, cb) {
        if (fs._mockedFiles.hasOwnProperty(filePath)) {
            cb(null, Buffer.from(fs._mockedFiles[filePath]));
        } else {
            fs._readFileReal(...arguments);
        }
    };
}

function _unpatchFs() {
    if (!fs.hasOwnProperty('_mockedFiles')) {
        return;
    }
    fs.readFile = fs._readFileReal;
    delete fs._mockedFiles;
    delete fs.mockFile;
}