Question

I want to add a random modifier to this code, something that will either add to or subtract from the damage based off of a range of +-20% of the base value. The damage would randomize between a range of 80% of result to 120% of result. For a numeric example:

attacker.Strength(20) – defender.Defense(10) = result

20 – 10 = range(8 to 12)

var Fight = function (attacker, defender) {
    var result;
    result = (attacker.Strength - defender.Defense);
    defender.HP = defender.HP - result;

    if(defender.HP >= 1) {
         return defender.Name + " has taken " + result + " damage!";
    } else {
         return defender.Name + " has been slain";
    }
};

Answer 1

Math.random returns a number between zero and one. Map this to your range from 0.8 to 1.2 with this:

var factor = Math.random() * 0.4 + 0.8;
//                |           |     |
//              [0, 1[        |     |
//                    \      /      |
//                    [0, 0.4[     /
//                         \      /
//                        [0.8, 1,2[
var result = Math.round((attacker.Strength - defender.Defense) * factor);

Not sure whether you want to round somewhere so that your player’s health points are always natural numbers. If not, you should change your death condition to defender.HP > 0.