javascript获取url中的参数值

function QueryString(fieldName)
{
var urlString = document.location.search;
if(urlString !=null)
{
var typeQu = fieldName+"=";
var urlEnd = urlString.indexOf(typeQu);
if(urlEnd !=-1)
{
var paramsUrl = urlString.substring(urlEnd+typeQu.length);
var isEnd =  paramsUrl.indexOf('&');
if(isEnd !=-1)
{
return paramsUrl.substring(0, isEnd);
}
else
{
return paramsUrl;
}
}
else
{
return null;
}
}
else
{
return null;
}
}