EU21

var div_sum = function(num)
{
    var s = 0;
    for (var i=1; i <= (num / 2) + 1; i++)
    {
        if (num % i == 0)
        {
            s += i;
        }
    }

    return s;
};

var sum = function(seq)
{
    var res = 0;

    for (var item of seq)
    {
        res += item;
    }

    return res;
};

var getAmicSet = function(upperEnd)
{
    var amic_nums = new Set();

    for (var i=1; i < upperEnd; i++)
    {
        if (!amic_nums.has(i))
        {
            var d_i = div_sum(i);
            var d_d_i = div_sum(d_i);
            if (d_d_i == i && d_i != i)
            {
                amic_nums.add(i);
                amic_nums.add(d_i);
            }
        }
    }

    return sum(amic_nums.keys());

};

var solveEU21 = function()
{
    var start = performance.now();
    var solution = getAmicSet(10000);
    var elapsed = performance.now() - start;

    return [solution, elapsed];
};

var res = solveEU21();

document.writeln("Solution is: " + res[0] + ". Took " + res[1] + ". milliseconds.");
document.writeln("<br />");