js 打开新页面在屏幕中间的实现方法

<a href="javascript:void(0)" onclick="window.open('https://www.jb51.net', 'newwindow', 'height=500, width=900, top='+Math.round((window.screen.height)/2<span style="font-family: Arial, Helvetica, sans-serif;">-250</span><span style="font-family: Arial, Helvetica, sans-serif;">)+',left='+Math.round((window.screen.width)/2</span><span style="font-family: Arial, Helvetica, sans-serif;">-450</span><span style="font-family: Arial, Helvetica, sans-serif;">)+',toolbar=no, menubar=no, scrollbars=no, resizable=no,location=no, status=no')">同意交换</a></span> 

</pre><pre code_snippet_id="1613432" snippet_file_name="blog_20160317_3_5070514" name="code" class="html"> 

<pre name="code" class="html"><script> 
function openwin(){ 
var width=Math.round((window.screen.width)/2<span style="font-family: Arial, Helvetica, sans-serif;">-400</span>); 
var height=Math.round((window.screen.height)/2<span style="font-family: Arial, Helvetica, sans-serif;">-200</span><span style="font-family: Arial, Helvetica, sans-serif;">);</span> 
window.open('https://www.jb51.net', 'newwindow', 'height=200, width=400, top='+height+',left='+width+',toolbar=no, menubar=no, scrollbars=no, resizable=no,location=no, status=no') 
} 
</script> 
<a href="javascript:void(0)" onclick="openwin();">同意交换</a> 

<div id="cnblogs_post_body"><pre class="best-text mb-10">===================各项参数============