Counting the occurrences / frequency of array elements

const arr = [2, 2, 5, 2, 2, 2, 4, 5, 5, 9];

function foo (array) {
  let a = [],
    b = [],
    arr = [...array], // clone array so we don't change the original when using .sort()
    prev;

  arr.sort();
  for (let element of arr) {
    if (element !== prev) {
      a.push(element);
      b.push(1);
    }
    else ++b[b.length - 1];
    prev = element;
  }

  return [a, b];
}

const result = foo(arr);
console.log('[' + result[0] + ']','[' + result[1] + ']')
console.log(arr)