const nums = [1, 2, 3, 2, 4, 1];

function duplicateNumber(){
    const count = {};
    const duplicateNum = []
    
    for (let n of nums) {
        count[n] = (count[n] || 0) + 1;
    }
    
    for(const key in count){
        if(count[key] > 1){
            duplicateNum.push(Number(key));
        }
    }
    return duplicateNum;
}

const result = duplicateNumber(nums);
console.log(result,'---result')