SOURCE

function fun(num){
    let num1 = num / 10;
    let num2 = num % 10;
    if(num1 < 1){
        return num;
    }else{
        num1 = Math.floor(num1);
        return `${num2}${fun(num1)}`;
    }
}
var a = fun(1234)
console.log(a);
console.log(typeof(a))