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1遮罩层其实就是一个覆盖全界面的半透明的DIV，并处理zIndex使他浮于其他元素之上，是用户不能点击下边的元素，或者说点击没有反应。
2在遮罩层上方在弹出一个层，由于遮罩层挡住了其他所有元素，用户只能点击弹出层，制造出模式窗口的假象。
3JS代码
function openDiv(newDivID){var newMaskID = "mask"; //遮罩层idvar newMaskWidth =document.body.scrollWidth;//遮罩层宽度var newMaskHeight =document.body.scrollHeight;//遮罩层高度 //mask遮罩层 var newMask = document.createElement("div");//创建遮罩层newMask.id = newMaskID;//设置遮罩层idnewMask.style.position = "absolute";//遮罩层位置newMask.style.zIndex = "1";//遮罩层zIndexnewMask.style.width = newMaskWidth + "px";//设置遮罩层宽度newMask.style.height = newMaskHeight + "px";//设置遮罩层高度newMask.style.top = "0px";//设置遮罩层于上边距离newMask.style.left = "0px";//设置遮罩层左边距离newMask.style.background = "red";//#33393C//遮罩层背景色newMask.style.filter = "alpha(opacity=40)";//遮罩层透明度IEnewMask.style.opacity = "0.40";//遮罩层透明度FFdocument.body.appendChild(newMask);//遮罩层添加到DOM中//新弹出层var newDivWidth = 400;//新弹出层宽度var newDivHeight = 200;//新弹出层高度var newDiv = document.createElement("div");//创建新弹出层newDiv.id = newDivID;//设置新弹出层ＩＤnewDiv.style.position = "absolute";//新弹出层位置newDiv.style.zIndex = "9999";//新弹出层zIndexnewDiv.style.width = newDivWidth + "px";//新弹出层宽度newDiv.style.height = newDivHeight + "px";//新弹出层高度var newDivtop=(document.body.scrollTop + document.body.clientHeight/2 - newDivHeight/2) ;//新弹出层距离上边距离var newDivleft=(document.body.scrollLeft + document.body.clientWidth/2 - newDivWidth/2);//新弹出层距离左边距离newDiv.style.top = newDivtop+ "px";//新弹出层距离上边距离newDiv.style.left = newDivleft + "px";//新弹出层距离左边距离newDiv.style.background = "#EFEFEF";//新弹出层背景色newDiv.style.border = "1px solid #860001";///新弹出层边框样式newDiv.style.padding = "5px";//新弹出层newDiv.innerHTML = "content;content";//新弹出层内容document.body.appendChild(newDiv);//新弹出层添加到DOM中//弹出层滚动居中function newDivCenter(){newDiv.style.top = (document.body.scrollTop + document.body.clientHeight/2 - newDivHeight/2) + "px";newDiv.style.left = (document.body.scrollLeft + document.body.clientWidth/2- newDivWidth/2) + "px";}if(document.all)//处理滚动事件，使弹出层始终居中{window.attachEvent("onscroll",newDivCenter);}else{window.addEventListener('scroll',newDivCenter,false);}//关闭新图层和mask遮罩层var newA = document.createElement("span");newA.href = "#";newA.style.position = "absolute";//span位置newA.style.left=350+ "px";newA.innerHTML = "Close";newA.onclick = function()//处理关闭事件{if(document.all){window.detachEvent("onscroll",newDivCenter);}else{window.removeEventListener('scroll',newDivCenter,false);}document.body.removeChild(newMask);//移除遮罩层 document.body.removeChild(newDiv);移除弹出框return false;}newDiv.appendChild(newA);//添加关闭span
}<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD><TITLE> New Document </TITLE></HEAD><BODY><a οnclick="openDiv('newDiv');" style="cursor:pointer">TestOpenDiv</a>
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<a οnclick="openDiv('newDiv');" style="cursor:pointer">TestOpenDiv</a></BODY>
</HTML>http://blog.csdn.net/alex197963/article/details/4456174