function handler(e) {
    e = e || window.event;

    let pageX = e.pageX;
    let pageY = e.pageY;

    // IE 8
    if (pageX === undefined) {
        pageX = e.clientX + document.body.scrollLeft + document.documentElement.scrollLeft;
        pageY = e.clientY + document.body.scrollTop + document.documentElement.scrollTop;
    }

    $mask.css({
        'top': pageY - $(window).scrollTop(),
        'left': pageX
    })
}

Unhandled promise rejection TypeError: "e is undefined"