在节点js中执行下一条语句之前等待回调完成

Question

New to NodeJs. I am having an issue with synchronizing method execution. heres is what I mean, I have a controller which is responsible for: a. Calling a method (in separate JS file) to open database connection and retrieve data. b. It then sends the data to the front end. From my debugging efforts it would appear I am sending the data before the method that opens the connection and gets the data completes. I know this because when the last line of salesController.js executes it gives error 'Cannot read property 'recordsets' of undefined'. In following code snippet dbAccess.js has method that opens the db and gets data. salesController.js then sends that data. Need help with the highlighted portion in salesController.js. Note if it helps I am using sql server as backend.

dbAccess.js

const dbConfig = require('../../nodeApp/dbConfig')

module.exports = {
    makeDBAccess: function(sqlString, req, res) {
        var sql = require("mssql/msnodesqlv8");
        // connect to your database
        sql.connect(dbConfig, function (err) {


            if (err) console.log(err);

            // create Request object
            var request = new sql.Request();

            // query to the database and get the records
            request.query(sqlString, function (err, recordset) {

                if (err) console.log(err)

                console.log(recordset.recordsets[0]);
                console.log(recordset.recordsets[0].length)

                return recordset;
            });
        });
    }
}

salesController.js

const express = require('express');
var router = express.Router();
var dbAccess = require('../utility/dbAccess.js')
var utility = require('../utility/readfile.js')


router.get('/totalSales2011', function (req, res) {
    var sqlString = utility.readFileFunc("queries/sales2011.sql");
    var recordSet = dbAccess.makeDBAccess(sqlString,req,res); **//Get data from database**
    res.send(recordSet.recordsets[0]); **//Wait for line above to complete before doing this.**
});

module.exports = router;

Answer 1

cause in javascript functions returns undefined if you didn't add a return statement, the makeDBAccess function doesn't contain a return so it returns undefined .

As for what to do about this, you need to use callback or promises since connecting to your database and making a database query are both non-blocking asynchronous operations.

dbAccess.js

const dbConfig = require('../../nodeApp/dbConfig')

module.exports = {
    makeDBAccess: function(sqlString, callback) {
        var sql = require("mssql/msnodesqlv8");
        // connect to your database
        sql.connect(dbConfig, function (err) {


            if (err) return callback(err);

            // create Request object
            var request = new sql.Request();

            // query to the database and get the records
            request.query(sqlString, function (err, recordset) {

                if (err) return callback(err)


                return callback(null, recordset);
            });
        });
    }
}

salesController.js

const express = require('express');
var router = express.Router();
var dbAccess = require('../utility/dbAccess.js')
var utility = require('../utility/readfile.js')


router.get('/totalSales2011', function (req, res) {
    var sqlString = utility.readFileFunc("queries/sales2011.sql");
    var recordSet = dbAccess.makeDBAccess(sqlString, (err, recordSet) => {
          if (err) console.error(err);
          else res.send(recordSet.recordsets[0]);
    });

});

module.exports = router;