api - How to convert an asynchronous function into synchronous function using promise in Node.js? -

i have changed code async suggested still not able access value , getting error of cannot read property '0' of undefined.the task_id not accessible third function not created:

 app.post('/api/add_task',function(req,res,error){   var sql = "insert `campus_ambassador`.`tasks` ( `task_type`, `task_name`, `details`, `social_media_handles`, `media`, `created_date`, `updated_date`) values ('offline', 'golfed', 'ssss', '#soccer', 'sdfsd', '2012-12-26 00:00:00', '2012-12-26 00:00:00');"   var sql1 = "select task_id tasks task_name='golfed'";   var sql2 = "insert `campus_ambassador`.`task_level` (`task_id`, `level_id`, `updated_on`, `created_on`) values ("+"'"+res1[0].task_id+"'"+", '3', '2012-12-26 00:00:00', '2012-12-26 00:00:00')";   var res1=''    async.waterfall([   function(callback){         db.select(sql,function(err){           console.log("made task");           callback(null);            })        },        function(callback){   db.select(sql1,function(err,data){                 res1 = data;                 console.log(res1);                 console.log(res1[0].task_id);                 console.log("got task id of created task")                 res.end();                  callback(null,data);                        })           },     function(callback){               db.select(sql2,function(err){               console.log(res1[0].task_id)             console.log("updated task level table")             callback(null);           })        }   ],     function(err,results){         console.log(json.stringify(results));     }) }) 

this depend on few things. version of node using , if lib db support promise.

turning callback promise

if db doesnt support promise default have yourself, this:

var dbselectpromise = function(db, sql) {     return new promise(function(resolve, reject) {         db.select(sql, function(err, data) {             if (err) {                 reject(err)             } else {                 resolve(data)             }         })     }) } 

using promise

once have promise (either made it, or native db) can use this

dbselectpromise(db, sql) .then(function(fristresult) {     return dbselectpromise(db, sql1) }) .then(function(secondresult) {     return dbselectpromise(db, sql2) }) .then(function(thirdresult) {     // , on... }) .catch(function(err) {     // here can handle errors of them }) 

using async/await

if node version 7.6 or higher can better. frist change promise function:

var dbselectpromise(db, sql) = async function {     return new promise(function(resolve, reject) {         db.select(sql, function(err, data) {             if (err) {                 reject(err)             } else {                 resolve(data)             }         })     }) } 

then can use sync code:

var fristresult = await dbselectpromise(db, sql) var secontresult = await dbselectpromise(db, sql1) var thirdresult = await dbselectpromise(db, sql2) 

edit save result accross multiples "then" can save result in outer scope, so:

var frist, second, third dbselectpromise(db, sql) .then(function(fristresult) {     frist = fristresult     return dbselectpromise(db, sql1) }) .then(function(secondresult) {     second = secondresult     return dbselectpromise(db, sql2) }) .then(function(thirdresult) {     third = thirdresult     // , on... }) .catch(function(err) {     // here can handle errors of them }) 

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