How to write a reverse function on an array that has integers

reverse function

Do you know that the sort method does not work so well on integers in an array since it is used for strings! 
That is something I found out today so then how to use a sort method on an array that has integers? Simply add a 
custom function to a sort method as an argument like below!
var arr = [1,100,4,30,5,80]
var compare = function(num1,num2){
    if (num1 < num2){
        return -1;
    }
    else if (num1 > num2){
        return 1;
    }
    else{
        return o;
    }   
}
then run the following method using the compare variable as an argument in the sort method like below! 
document.write(arr.sort(compare))
Then how would you modify this code so that the array goes from the highest # to the lowest?