JS element is visible ?

element-is-visible.js

    /* Element is visible ?
    *******************************************************************************/
    $.fn.isVisible = function(partial) {

        if( typeof $(this).offset() === 'undefined' ) return false;

        var $t = $(this),
            $w = $(window),
            viewTop = $w.scrollTop(),
            viewBottom = viewTop + $w.height() - 100,
            _top = $t.offset().top,
            _bottom = _top + $t.height(),
            compareTop = partial === true ? _bottom : _top,
            compareBottom = partial === true ? _top : _bottom;

        return ((compareBottom <= viewBottom) && (compareTop >= viewTop));
    };