How to check the scroll bar status is already at top or at end?

HTML:

<div id="mydiv" style="overflow: auto; height: 500px"></div>

SCRIPT:

$(document).ready(function()
{
    $("#mydiv").scroll(function()
    {
        var div = $(this);
        if (div[0].scrollHeight - div.scrollTop() == div.height())
        {
            alert("Reached the bottom!");
        }
        else if(div.scrollTop() == 0)
        {
            alert("Reached the top!");
        }
    });
});

check

if($(window).scrollTop() == 0 || $(window).scrollTop() == $(document).height()- $(window).height()) {
   // do something
}