30 riadkov kodu to nie je ani zďaleka, ale nejak takto:

function myFunction(value) {
    var num = Number(value); 
    var num1 = num;
    if (num < 0) num1 = (num * -1);
     
    var str = num1.toFixed(2);
    while (str.length < 5) str = "0" + str;
    if (num < 0) str = "-" + str;
    document.getElementById("demo").innerHTML = str ;
}