变量不保留值

Question

我试图将一个变量设置为从查询获得的结果，但该变量没有保留该值。var queryString = "SELECT price FROM menu_items WHERE id = " +     req.query.items + ";";mysql.pool.query(queryString, function(err, rows, fields){    if (err){        next(err);        return;    }    itemPrice = rows[0].price;    console.log("item price is2: " + itemPrice);});console.log("item price is: " + itemPrice);我希望它打印"item price is2: 10.99"然后"item price is 10.99"但它打印项目价格是undefined然后item price is2: 10.99

Answer 1

var queryString = "SELECT price FROM menu_items WHERE id = " + 

    req.query.items + ";";

mysql.pool.query(queryString, function(err, rows, fields){

    if (err){

        next(err);

        return;

    }

    itemPrice = rows[0].price;

    console.log("item price is2: " + itemPrice);

    console.log("item price is: " + itemPrice);

});

任何网络调用都是异步运行的，因此一旦进行了 db 调用，下一行就开始执行。

使用异步等待

const {promisify} = require('util');

(async ()=> {

    var queryString = "SELECT price FROM menu_items WHERE id = " + 

        req.query.items + ";";

        const query = promisify( mysql.pool.query).bind(mysql.pool);

        const result = await query(queryString)

            if (!result) {

                next(result);

                return;

            }

            console.log(result)

            itemPrice = result[0].price;

            console.log("item price is2: " + itemPrice);

            console.log("item price is: " + itemPrice);

})()

Answer 2

you can do it using following method

export const getPriceValue = async () => {

var queryString = "SELECT price FROM menu_items WHERE id = " + 

    req.query.items + ";";

mysql.pool.query(queryString, function(err, rows, fields){

    if (err){       

        return 0;

    }

    if(row) {

      return(rows[0].price);

    }

});

}

let priceValue = await getPriceValue();

console.log('price::', priceValue);

Answer 3

var queryString = "SELECT price FROM menu_items WHERE id = " + 

    req.query.items + ";";

mysql.pool.query(queryString, function(err, rows, fields){

    if (err){

        next(err);

        return;

    }

    itemPrice = rows[0].price;

    console.log("item price is2: " + itemPrice);

    console.log("item price is: " + itemPrice);

});