Pokemon Damage Calculator Kata on Codewars - http://www.codewars.com/kata/536e9a7973130a06eb000e9f

Damage_Calculator.js

function calculateDamage(yourType, opponentType, attack, defense){
  
  var type = {
    fire:0,
    water:1,
    grass:2,
    electric:3
  };

  var effectivenessTable = [[0.5,0.5,2,1],[2,0.5,0.5,0.5],[0.5,2,0.5,1],[1,2,1,0.5]];
  var effectiveness = effectivenessTable[type[yourType]][type[opponentType]];
  return Math.ceil(50 * (attack/defense) * effectiveness);

}

Damagev2.js

var effects = {
  fire : {
    fire : 0.5,
    grass : 2,
    water : 0.5,
    electric : 1
  },
  grass : {
    fire : 0.5,
    grass : 0.5,
    water : 2,
    electric : 1
  },
  water : {
    fire : 2,
    grass : 0.5,
    water : 0.5,
    electric : 0.5
  },
  electric : {
    fire : 1,
    grass : 1,
    water : 2,
    electric : 0.5
  }
};

function calculateDamage(yourType, opponentType, attack, defense){
  return Math.ceil(50 * (attack/defense) * effects[yourType][opponentType]);
}