Check for file

fs.js

fs.stat('log.txt', function(err, stat) {
    if(err == null) {
        console.log('File exists');
    } else if(err.code == 'ENOENT') {
        fs.writeFile('log.txt', 'Some log
');
    } else {
        console.log('Some other error: ', err.code);
    }
});