const array = [3, 'a', 'a', 'a', 2, 3, 'a', 3, 'a', 2, 4, 9, 3];

const getMostFrequentValue = array => {
  const countAllValues = array.reduce((acc, curr) => {
    if (curr in acc) {
      return { ...acc, [curr]: acc[curr] + 1 }
    }

    return { ...acc, [curr]: 1 }
  }, {})

  const countedValues = Object.values(countAllValues);
  return Math.max(...countedValues);
}