where is log file in nodejs app with winston

I can not find in my app directory where is 'test.log' file?

below code is in server.js

var winston = require('winston'), mylogger = new (winston.Logger)({
  transports: [
    new (winston.transports.Console) (),
    new (winston.transports.File) ({filename: 'test.log'})
  ]
});

mylogger.log('Hello world');

my app directory:

/
  app/
  config/
  public/
  server.js