money.js

var Money = (function(){
  function Money(qty) {
    this.whole = 0;
    this.cents = 0;
    this.add(qty);
  }
  Money.prototype.calc = function() {
    while (this.cents > 100) {
      this.cents -= 100;
      this.whole += 1;
    }
    return this;
  };
  Money.prototype.add = function(qty) {
    var parts = qty.split('.');
    this.whole += Number(parts[0]);
    if(typeof parts[1] != 'undefined'){
      this.cents += Number(parts[1]);
    }
    return this.calc();
  };
  Money.prototype.toString = function() {
    return this.whole +'.'+ this.cents;
  };
  return Money;
}());

var m = new Money('90.75').add('50.43');

console.log(m); //=> {whole: 141, cents: 18}
console.log(m.toString()); //=> "141.18"