超级简单的一个实现，可能会有局限性，贵在简单易懂，使用的时候执行前loading,执行成功后loaded

/*
 * 显示loading遮罩层
 */
function loading() {
    var mask_bg = document.createElement("div");
    mask_bg.id = "mask_bg";
    mask_bg.style.position = "absolute";
    mask_bg.style.top = "0px";
    mask_bg.style.left = "0px";
    mask_bg.style.width = "100%";
    mask_bg.style.height = "100%";
    mask_bg.style.backgroundColor = "#777";
    mask_bg.style.opacity = 0.6;
    mask_bg.style.zIndex = 10001;
    document.body.appendChild(mask_bg);

    var mask_msg = document.createElement("div");
    mask_msg.style.position = "absolute";
    mask_msg.style.top = "35%";
    mask_msg.style.left = "42%";
    mask_msg.style.backgroundColor = "white";
    mask_msg.style.border = "#336699 1px solid";
    mask_msg.style.textAlign = "center";
    mask_msg.style.fontSize = "1.1em";
    mask_msg.style.fontWeight = "bold";
    mask_msg.style.padding = "0.5em 3em 0.5em 3em";
    mask_msg.style.zIndex = 10002;
    mask_msg.innerText = "正在执行,请稍后...";
    mask_bg.appendChild(mask_msg);
}
/*
 * 关闭遮罩层
 */
function loaded() {
    var mask_bg = document.getElementById("mask_bg");
    if (mask_bg != null)
        mask_bg.parentNode.removeChild(mask_bg);
}