Nodejs + Express + mysql callback issue

Question

I 'm having some trouble understanding the behavior of callbacks . For example, the following code does not work as expected cos callbacks . I tried async library but without obtaining the desired behavior.

module.exports.list = function ( req, res ) {
    model.list( id, idx, function( boo ){
       async.forEachOf( boo, function ( b, k, e ) {
            model.sublist( b.text, function( foo ) {
               b.foo = foo;
            })
       })
       res.json({ data : boo }) //boo dont have foo porperty
    });
};

model.list = execute some big mysql query, and return rows (boo is an array).
model.sublist = same as model.list

Edit Solution:

module.exports.list = function ( req, res ) {
    model.list( id, idx, function( boo ){
        async.forEachOf( boo, function ( b, k, e ) {
            model.sublist( b.text, function( foo ) {
               b.foo = foo;
               e();
            })
        })
        res.json({ data : boo })
    });
};

Possible duplicate of Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference — Felix Kling
– Felix Kling, Commented Sep 21, 2016 at 2:14
Should move res.json(...) into 2nd callback, after model.sublist — tmquang6805
– tmquang6805, Commented Sep 21, 2016 at 2:14
The reason WHY node frameworks give you a response object that you can send messages on instead of having your function return a value (the way frameworks usually work in other languages) is exactly so that you can get data asynchronously and THEN send a response. So just send the response at the correct place, not before you receive it. — slebetman
– slebetman, Commented Sep 21, 2016 at 2:22
boo is an array, for each intem inside boo y need to execute some query based on boo item.text — 9879800
– 9879800, Commented Sep 21, 2016 at 2:32

Abhilash Km · Accepted Answer · 2016-09-21 07:00:30Z

2

should call response only in 2nd callback. read async documentation

 module.exports.list = function ( req, res ) {
    model.list( id, idx, function( boo ){
       async.forEachOf( boo, function ( b, k, e ) {
            model.sublist( b.text, function( foo ) {
               b.foo = foo;
            });
            e();
       },function(){
           res.json({ data : boo }) //boo dont have foo porperty
       });
    });
 }

answered Sep 21, 2016 at 7:00

Abhilash Km

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1 Comment

9879800 Over a year ago

e() is executed before sublist callback ends, so b.foo = undefined , how i can achieve this?

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