感谢：https://blog.csdn.net/yelin042/article/details/79707855

$(function () {
    var wxopenid=getcookie('wxopenid');
    var key=getcookie('key');
    if (key==''){
        var access_code=GetQueryString('code');
        if (wxopenid==""){
            if (access_code==null)
            {   
                var fromurl=location.href;
                var url='https://open.weixin.qq.com/connect/oauth2/authorize?appid=填你自已的appid哟&redirect_uri='+encodeURIComponent(fromurl)+'&response_type=code&scope=snsapi_base&state=STATE%23wechat_redirect&connect_redirect=1#wechat_redirect';
                location.href=url;
            }
            else
            {   
                $.ajax({
                    type:'get',
                    url:ApiUrl+'/index.php?act=payment&op=getopenid', 
                    async:false,
                    cache:false,
                    data:{code:access_code},
                    dataType:'json',
                    success:function(result){                 
                            if (result!=null && result.hasOwnProperty('openid') && result.openid!=""){
                               addcookie('wxopenid',result.openid,360000);                           
                               getlogininfo(result.openid);
                            } 
                            else
                            {
                              alert('微信身份识别失败 \n '+result);
                              location.href=fromurl;
                            }
                        }
                    });    
            }
        }else{
           if (key=='' && wxopenid!='')
               getlogininfo(wxopenid);  
        }
 
        function getlogininfo(wxopenid){       
            $.ajax({
               type:'get',
               url: ApiUrl + '/index.php?act=login&op=autologininfo',
               data: { wxopenid:wxopenid},
               dataType:'json',
               async:false,
               cache:false,               
               success: function (result) {                   
                       if (result.return_code=='OK'){
                           addcookie('key',result.memberinfo.key);
                           addcookie('username',result.memberinfo.username);
                       }else{
                           alert(result.return_msg);
                           location.href=WapSiteUrl+'/tmpl/member/login.html';
                       }
               }
            });
        }
    }
});

函数

// 写cookie
function addcookie(name,value,expireHours){
	var cookieString=name+"="+escape(value)+"; path=/";
	//判断是否设置过期时间
	if(expireHours>0){
		var date=new Date();
		date.setTime(date.getTime+expireHours*3600*1000);
		cookieString=cookieString+"; expire="+date.toGMTString();
	}
	document.cookie=cookieString;
}
 
//	获取cookie
function getcookie(name){
	var strcookie=document.cookie;
	var arrcookie=strcookie.split("; ");
	for(var i=0;i<arrcookie.length;i++){
	var arr=arrcookie[i].split("=");
	if(arr[0]==name)return decodeURIComponent(arr[1]); //增加对特殊字符的解析
	}
	return "";
}
 
//	删除cookie
function delCookie(name){//删除cookie
	var exp = new Date();
	exp.setTime(exp.getTime() - 1);
	var cval=getcookie(name);
	if(cval!=null) document.cookie= name + "="+cval+";
	path=/;expires="+exp.toGMTString();
}

感谢：https://blog.csdn.net/sinat_33010325/article/details/83795026

URL参数值获取GetQueryString

function GetQueryString(name) {
       var reg = new RegExp("(^|&)" + name + "=([^&]*)(&|$)");
       var r = window.location.search.substr(1).match(reg);
       if(r != null) {
             return decodeURIComponent(r[2]);
       }
       return '';
}

讲解

其中"(^|&)" + name + "=([^&]*)(&|KaTeX parse error: Expected 'EOF', got '&' at position 19: …意思就是取以name开头或者以&̲开头以&或者以结尾的字符串
window.location.search.substr(1).match(reg)的意思是找出?之后匹配reg的字符串
decodeURIComponent(r[2])的意思是对所选的字符串进行解码