item-sort.js

function itemsSort(items) {
    // Write your code here
    const countObj = items.reduce((rlt, val) => {
        if (val in rlt) {
            rlt[val]++
        } else {
            rlt[val] = 1
        }
        return rlt;
    }, {})
    
    return items.sort((num1, num2) => {
        if (countObj[num1] !== countObj[num2]) return countObj[num1] - countObj[num2]
        return num1 - num2
    })
}

With the following input,

items = [4, 5, 6, 5, 4, 3]

expected output:

[3, 6, 4,  4, 5, 5]