Incorrect Output, need synchronous for loop - node.js

Question

exports.calculateGstBaseOnInput = function(req, res) {
console.log("welcome");
for (var item of req.body.so_items) {
req.productid = item.productid;
req.qty = item.qty;
getItemDetail(req, res).then(function(result) {
return getCartItems(req, res);
}).then(function(result) {
return calculateGST(req, res);
})
}
}
getItemDetail = function(req, res) {
return new Promise(function(resolve, reject) {
console.log("inside getItemDetail");
var SQL = "mysql query";
mysqlConnect.query(SQL, function(err, result, fields) {
if (err) {
console.log("inside err");
res.json({ status: 'Failure', statusMessage: 'item does not exist' });
} else if (result.length < 0) {
console.log("inside length 0");
res.json({ status: 'Failure', statusMessage: 'item does not exist' });
} else {
req.itemdetail = result;
console.log("price inside getitemdetail= ", req.itemdetail[0].price);
//callback();
}
});
resolve('done');
});
}
getCartItems = function(req, res) {
return new Promise(function(resolve, reject) {
console.log("inside getCartItems");
var SQL = "mysql query";
mysqlConnect.query(SQL, function(err, result, fields) {
if (err) {
res.json({ status: 'Failure', statusMessage: 'item does not exist' });
} else if (result.length < 0) {
res.json({ status: 'Failure', statusMessage: 'item does not exist' });
} else {
req.cartItems = result;
//callback();
}
});
resolve('done');
});
}
calculateGST = function(req, res) {
return new Promise(function(resolve, reject) {
console.log("inside calculateGST");
if (req.userDetails[0].is_gst_included) {
//total = req.qty * req.itemdetail[0].price;
// console.log("price = ",req.itemdetail[0].price);
//callback();
} else {
//total = req.qty * req.itemdetail[0].price;
//console.log("price = ",req.itemdetail[0].price);
//total = req.qty * req.itemdetail[0].price;
}
resolve('done');
});
}
Actual output :
inside getItemDetail
inside getItemDetail
inside getCartItems
inside getCartItems
inside calculateGST
inside calculateGST
Expected Output(output i wanted):
inside getItemDetail
inside getCartItems
inside calculateGST
inside getItemDetail
inside getCartItems
inside calculateGST
how do i achieve this without setting any time.

Answer 1

You are resolveing immediately instead of resolving within the callback provided to mysqlConnect.query().
Consider an abbreviated version of your getItemDetail function:
function getItemDetail(req, res) {
return new Promise(function(resolve, reject) {
var SQL = "mysql query";
mysqlConnect.query(SQL, function(err, result, fields) {
// Stuff
});
resolve('done');
});
}
It's logic is:
Create a new promise
Begin a query
resolve()
Whatever called getItemDetail does what it should because getItemDetail resolved
query finishes sometime later
Instead, you should likely be doing something like this, where resolve is within the query callback:
function getItemDetail(req, res) {
return new Promise(function(resolve, reject) {
var SQL = "mysql query";
mysqlConnect.query(SQL, function(err, result, fields) {
// Stuff
resolve('done');
});
});
}
The logic here is:
Create a new promise
Begin a query
getItemDetail caller shouldn't do anything yet because getItemDetail isn't yet resolved
query finishes at some point and the callback is triggered which calls resolve
getItemDetail caller will now proceed since it has been told that the function resolved
You need to follow this pattern in any function where you need a query to actually finish before the caller should move on.
You should also consider leveraging async/await. Consider this abbreviated example using your code as a base:
const mysqlConnect = {
query(sql, cb) {
setTimeout(() => {
cb(null, ["foo"], ["bar"]);
});
}
};
(async function() {
const t = await calculateGstBaseOnInput({
body: {
so_items: ["a", "b", "c"]
}
});
}());
async function calculateGstBaseOnInput(req, res) {
for (var item of req.body.so_items) {
const itemDetail = await getItemDetail(req, res);
const cartItems = await getCartItems(req, res);
const gst = await calculateGST(req, res);
}
}
function getItemDetail(req, res) {
console.log("getItemDetail");
return new Promise(function(resolve, reject) {
var SQL = "mysql query";
mysqlConnect.query(SQL, function(err, result, fields) {
// Stuff
resolve('done');
});
});
}
function getCartItems(req, res) {
return new Promise(function(resolve, reject) {
console.log("---getCartItems");
var SQL = "mysql query";
mysqlConnect.query(SQL, function(err, result, fields) {
// Stuff
resolve('done');
});
});
}
function calculateGST(req, res) {
return new Promise(function(resolve, reject) {
console.log("------calculateGST");
// Stuff
resolve('done');
});
}
This outputs:
getItemDetail
---getCartItems
------calculateGST
getItemDetail
---getCartItems
------calculateGST
getItemDetail
---getCartItems
------calculateGST