使用js在图片鼠标右击提示没有下载权限提醒

document.oncontextmenu = function (e) {
    var target = (typeof e != "undefined") ? e.target : event.srcElement
    if (target.tagName == "IMG" || (target.tagName == 'A' && target.firstChild.tagName == 'IMG')) {
    alert("Downloading of images is disabled at the request of the copyright holders.");
    return false;
    }
}