Enable a Rational class to handle math operators

You can't overload operators (read similar questions).

Moreover, a dedicated method like mult can be treated as a sign of good design (not only in Javascript), since changing the original operator behavior can confuse users (well, a rational number actually a good candidate for overloading).

You can change print to toString as user thg435 has suggested.

Going even further:

Rational.prototype = {
    mult : ... ,
    toString: ... ,
    valueOf: function() { return this.nominator / this.denominator; }
};

this will enable the a * b syntax (note: you don't operate on Rationals any more, but rather on primitives).

Javascript doesn't support operator overloading. However, it does provide "magic methods" toString and valueOf that are used to get a "primitive" value of an object in string and numeric contexts respectively. This allows for some simplifications in your code:

Rational.prototype.toString = function() {
    return this.nominator + '/' + this.denominator;
}
Rational.prototype.valueOf = function() {
    return this.nominator / this.denominator;
}

console.log("Result=" + a.mult(b)); // Result=1.5
console.log(a * b); // 1.5

Note that the last 1.5 is not a Rational object anymore, but rather the primitive value (number).