maximumKey = array => {
  let max = array[0].key;
  const length = array.length;

  for (let i = 1; i < length; i++) {
    if (array[i].key > max) {
      max = array[i].key;
    }
  }

  return max;
}
Finds the maximum key from an array of objects

Asymptotic Complexity: O(array.length)

@param Array
@return Integer