Function in node.js

Question

In my node application i am using function.But i am getting the result of that function:

My code:

var connection = mysql.createConnection({
  host     : 'localhost',  
  user     : 'xxxx',
  password : 'xxxxx',
  database : 'xxxxxxx',
  debug : true,
})
connection.connect(function(err) {
    if ( !err ) {
        console.log("Connected to MySQL");
    } else if ( err ) {
        console.log(err);
    }
});

 if(level5 == undefined)
{
   var result1=querylevel5();
   console.log("vvvvvvvvv="+result1)
   res.writeHead(200, { 'Content-Type': 'application/json'});
   res.end(JSON.stringify(result1,null,"\n"));
}

My function:

    function querylevel5()
    {
      var result;    
      connection.query("select * from levels ", function(err, row1, fields) {
        result= row1;
        /*res.writeHead(200, { 'Content-Type': 'application/json'});
        res.end(JSON.stringify(row1,null,"\n"));*/
      });
      return result;
    }

I want to get the row1 result ,in my calling function..But its printing "undefined"..I am new to this node.js..So please help me to solve this..Thanks in advance..

Answer 1

Most likely the problem is that the function querylevel5 returns immediately before result . You have to do like most any other node.js framework does, like for example the connection.query function, namely use callbacks:

if(level5 == undefined) {
    querylevel5(function(result) {
        console.log("vvvvvvvvv="+result)
        res.writeHead(200, { 'Content-Type': 'application/json'});
        res.end(JSON.stringify(result,null,"\n"));
    });
}

And

function querylevel5(callback) {
    connection.query("select * from levels ", function(err, row1, fields) {
        callback(row1);
    });
}

Answer 2

问题是结果是在回调中传递的（即异步），但是您同步调用该方法并打印结果 - 因此在您尝试打印时实际上并未返回结果。