Counting the occurrences / frequency of array elements

Question

In Javascript, I\'m trying to take an initial array of number values and count the elements inside it. Ideally, the result would be two new arrays, the first specifying each

Answer 1

var array = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4];

function countDuplicates(obj, num){
  obj[num] = (++obj[num] || 1);
  return obj;
}

var answer = array.reduce(countDuplicates, {});
// answer => {2:5, 4:1, 5:3, 9:1};

If you still want two arrays, then you could use answer like this...

var uniqueNums = Object.keys(answer);
// uniqueNums => ["2", "4", "5", "9"];

var countOfNums = Object.keys(answer).map(key => answer[key]);
// countOfNums => [5, 1, 3, 1];

Or if you want uniqueNums to be numbers

var uniqueNums = Object.keys(answer).map(key => +key);
// uniqueNums => [2, 4, 5, 9];