Can I write scripts for hubot in Javascript?

Yes, you can write your hubot scripts in pure JavaScript. Following is a simple hubot script written in pure JavaScript and put under the /scripts/ directory of my customized hubot:

// Description:
//   holiday detector script
//
// Dependencies:
//   None
//
// Configuration:
//   None
//
// Commands:
//   hubot is it weekend ?  - returns whether is it weekend or not
//   hubot is it holiday ?  - returns whether is it holiday or not

module.exports = function(robot) {
    robot.respond(/is it (weekend|holiday)\s?\?/i, function(msg){
        var today = new Date();

        msg.reply(today.getDay() === 0 || today.getDay() === 6 ? "YES" : "NO");
    });
}