Largest palindrome product in Javascript

I don't think, there is a real problem with your code. You just do not filter for the largest product, which is not necessarily your last output. Just add an additional check for the largest product, e.g. like this:

var x, y, product, max = 0;

for (x = 100; x <= 999; x++) {
    for (y = x; y <= 999; y++) {
        product = x * y;
        if (Palindromic(product)) {
          if( max < product ) { // this is new
            max = product;
            console.log(x + '*' + y + '=' + product);
          }
        }
    }
}

This returns

913*993=906609

as the largest result.