Q:

Mutator for values of specyfic properties in object

I want to write mutators for obj.b and obj.c.  My example works fine but i don't like many ifs (maybe switch will be better) but I'm trying to write something like hashmap (key will be the same as key and value will be anonymous function).  Then I will check if obj.key exists in hashMap as property, then run function and mutate value.  Is it possible with my example? 
let obj = {'a': 'aa', 'b': 'bb', 'c': 'cc'};
for(const property in obj) {
  obj[property] = mutator(property, obj[property]);
}

function mutator(key, value) {
  if (key === 'b') {
    return value + 'kk';
  } else if (key === 'c') {
    return value + 'mm';
  }
  return value;
}

console.log(obj);

A:

Something like this? 
function mutator(key, value) {
    const mutations = {
    'b': value + 'kk',
    'c': value + 'mm',
  };

  return (key in mutations) ?  mutations[key] : value;
}

