gistfile1.txt

var compress = function (buf, cb) {
  console.log('before ' + buf);

  zlib.gzip(buf, function (err, result) {
    buf = result;
    console.log('in buf ' + buf);
    console.log('in string ' + buf.toString());
    cb(buf)
  });
};

>>>

before Hello World!
after Hello World!

in buf <BINARY GOBBLEDEGOOK>
in string <BINARY GOBBLEDEGOOK>