Function.prototype.curry = function() {
  if (arguments.length < 1) {
    return this; //nothing to curry. return function
  }

  var self = this;
  var args = toArray(arguments);
  return function() {
     return self.apply(this, args.concat(toArray(arguments)));
  }
}

function toArray(args) {
  return Array.prototype.slice.call(args);
}

var converter = function(factor, symbol, input){
  return input * factor + symbol;
}

var mileToKm = converter.curry(1.62, 'km');
mileToKm(3); // 4.82km

var kgToLb = converter.curry(2.2, 'lb');
kgToLb(3); // 6.6lb