Express gzip compression issue

I`m writing application with node and express. I want go compress my static files before sending them to the client with compression package. Here is my code:

var compression = require('compression');

var index = require('./routes/index');
var login = require('./routes/login');
var logout = require('./routes/logout');
var register = require('./routes/register');
var userRoute = require('./routes/user');
var app = express();

//files compression
app.use(compression({ 
  threshold: 0,
  filter: function () { return true; },
  level: 0
}));

I tried to use it without options but got the same result. I`m expecting to see Content-Encoding: gzip in response headers, but no luck.