Cause analysis: There is a problem with the execution order of the callback function. Please add some print information to see

The line of res.write() returns the query you initially defined and does not perform the operations in req.on(). At this time, the query is still the string "what" you defined, which is not a json. Object, so there is no number attribute, so it is undefined.
Solution: You can write res.write() inside req.on(), like this:

Why don’t I see you judging the request method?
This is an example I wrote before, you can refer to it:

var http=require('http');
var url=require('url');
var fs=require('fs');
var querystring=require('querystring');
var mgd=require('./mongodb.js');

http.createServer(function(req,res){
    switch(req.method){
        case 'POST':
            update(req,res);
            break;
        case 'GET':
            get(req,res);
            break;
        default:break;
    }
}).listen(8080);

function update(req,res){
    var pathname=url.parse(req.url).pathname;
    var postData='';
    /*接收评论*/
    if(pathname=='/postComment'){
        req.addListener('data',function(data){
            postData+=data;
        });
        req.addListener('end',function(){
            var json=querystring.parse(postData);
            mgd(function(c){
                c.insert('comment',json,function(){
                    var json={}
                    json.code=1;
                    res.writeHead('Content-Type:application/json;charset=UTF-8');
                    res.write(JSON.stringify(json));
                    res.end();
                });
            });

        })
    }
}

function get(req,res){
    var pathname=url.parse(req.url).pathname;
    
    /*主页*/
    if(pathname==='/'){
        fs.readFile('test.html',function(err,file){
            res.end(file);
        })
    }

    /*获取评论列表*/
    if(pathname=='/comment'){
        mgd(function(c){
            c.find('comment',{},function(data){
                var json={};
                if(data.length!=0){
                    json.code=1;
                    json.data=data;
                }else{
                    json.code=0;
                    json.data=null;
                }
                json=JSON.stringify(json)
                res.writeHead('Content-Type:application/json;charset=UTF-8');
                res.write(json);
                res.end();
            })
            
        })
    }

}

mongodb.js, you can check it out on my github. It’s actually just a simple comment demo