js更改url字符串的某个参数

url = changeURLArg(url,'bed_id',json.bedId)
function changeURLArg(url,arg,arg_val){
    var pattern=arg+'=([^&]*)';
    var replaceText=arg+'='+arg_val; 
    if(url.match(pattern)){
        var tmp='/('+ arg+'=)([^&]*)/gi';
        tmp=url.replace(eval(tmp),replaceText);
        return tmp;
    }else{ 
        if(url.match('[\?]')){ 
            return url+'&'+replaceText; 
        }else{ 
            return url+'?'+replaceText; 
        } 
    }
}