text: example = function (param1, opt_param, callback) {
    if (callback === undefined) {
      // only two paramaters were passed, so the callback is actually in `opt_param`
      callback = opt_param;
  
      //give `opt_param` a default value
      opt_param = "and a default parameter";
    }
    callback(param1, opt_param);
  }
  
  example("This is a necessary parameter", console.log);
  example("This is a necessary parameter", "and unnecessary parameter", console.log);
  