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');
    });
}

实际输出：

inside getItemDetail
inside getItemDetail
inside getCartItems
inside getCartItems
inside calculateGST
inside calculateGST

预期输出（我想要的输出）：

inside getItemDetail
 inside getCartItems
    inside calculateGST
    inside getItemDetail
    inside getCartItems
    inside calculateGST

如何在不设置任何时间的情况下实现这一目标。

Answer 1

您resolve立即{而不是在提供给mysqlConnect.query()的回调中解析。

考虑getItemDetail函数的缩写版本：

function getItemDetail(req, res) {
    return new Promise(function(resolve, reject) {
        var SQL = "mysql query";
        mysqlConnect.query(SQL, function(err, result, fields) {
            // Stuff
        });
        resolve('done');
    });
}

这是逻辑：

创建新承诺
开始query
resolve()
所谓的getItemDetail做了什么，因为getItemDetail解决了
query稍后完成

相反，您可能应该这样做，其中resolve位于query回调中：

function getItemDetail(req, res) {
    return new Promise(function(resolve, reject) {
        var SQL = "mysql query";
        mysqlConnect.query(SQL, function(err, result, fields) {
            // Stuff
            resolve('done');
        });
    });
}

这里的逻辑是：

创建新承诺
开始query
getItemDetail来电者不应该做任何事情，因为getItemDetail尚未解决
query在某个时刻结束并触发回调调用resolve
getItemDetail调用者现在将继续，因为已经告知函数已解决

在调用者继续前需要查询实际完成的任何函数中，您需要遵循此模式。

您还应考虑利用async/await。以您的代码为基础考虑这个缩写示例：

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');
    });
}

输出：

getItemDetail
---getCartItems
------calculateGST
getItemDetail
---getCartItems
------calculateGST
getItemDetail
---getCartItems
------calculateGST

输出不正确，需要同步循环

1 个答案: