This is the code that I have in the node which prepares data before it is sent to the Nextion serial protocol.

preparing_data_for_nextion.js

var str = msg.payload;
var buf = [];

for (var i=0, l = str.length; i < l; i++) {
    var ascii = str.charCodeAt(i);
    buf.push(ascii);
}
buf.push(255);
buf.push(255);
buf.push(255);

msg.payload = new Buffer(buf);

return msg;