Question 173794 in Javascript Question posted in
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Javascript – NodeJs returned array from database query is undefined

An NodeJs app we are wrting has a problem with the query data returned as an array being undefined. I cannot find an exact match for our issue.

The following test case reproduces the problem. It is something to do with the calling function not waiting for the called function to finish? I’m not a JavaScript expert, and this behaviour does not make sense to me.

themod.js

module.exports = {

   getData: () => {
      let dbc = require('./mods/db.js');
      dbc.query(`

         SELECT 1 rn, 'One' rt UNION
         SELECT 2 rn, 'Two' rt UNION
         SELECT 3 rn, 'Three' rt ;

         `,
         function (err, rows) {
            if (err) {
              console.log ( ' Error 1: ' , err );
            } else {
               arows =  module.exports.getSubData();
               console.log ( 'arows.length: ', arows.length );
            }
          })
   },

   getSubData: () => {
         let dbc = require('./mods/db.js');
         dbc.query(`

         SELECT 10 rn, 'Ten' rt UNION
         SELECT 20 rn, 'Twenty' rt UNION
         SELECT 30 rn, 'Thirty' rt ;

         `,
         function (err, rows) {
            if (err) {
              console.log ( ' Error 3: ' , err );
            } else {
               console.log ( 'arows: ', rows.length );
               return( rows );
            }
          })
   }

}

theapp.js:

let tm = require('./themod.js');

tm.getData();

When it’s run:

 node theapp.js
/path/node/node_modules/mysql/lib/protocol/Parser.js:437
      throw err; // Rethrow non-MySQL errors
      ^

TypeError: Cannot read property 'length' of undefined
    at Query.<anonymous> (/path/node/themod.js:17:54)
    at Query.<anonymous> (/path/node/node_modules/mysql/lib/Connection.js:526:10)
    at Query._callback (/path/node/node_modules/mysql/lib/Connection.js:488:16)
    at Query.Sequence.end (/path/node/node_modules/mysql/lib/protocol/sequences/Sequence.js:83:24)
    at Query._handleFinalResultPacket (/path/node/node_modules/mysql/lib/protocol/sequences/Query.js:149:8)
    at Query.EofPacket (/path/node/node_modules/mysql/lib/protocol/sequences/Query.js:133:8)
    at Protocol._parsePacket (/path/node/node_modules/mysql/lib/protocol/Protocol.js:291:23)
    at Parser._parsePacket (/path/node/node_modules/mysql/lib/protocol/Parser.js:433:10)
    at Parser.write /path/node/node_modules/mysql/lib/protocol/Parser.js:43:10)
    at Protocol.write (/path/node/node_modules/mysql/lib/protocol/Protocol.js:38:16)

As requested, mods/db.js:

var mysql = require('mysql');

var dbConn = mysql.createConnection({
   host     : 'localhost',
   user     : 'unam',
   password : 'pwrd',
   database : 'dbname'
});

dbConn.connect ( function(err) {
   if (err) {
      console.error( "DB Connect failed ", err);
   }
});

module.exports = dbConn;
Tags:

2

Answers


  1. 0

    JavaScript works with synchronous and asynchronous calls.
    Asynchronous calls are named Promises and use what we call callback
    functions to execute code once the asynchronous sequences is done.

    In your case, it seems the second argument of

    dbc.query(arg1, arg2)

    is the callback function.

    Can you provide definition of query function in ./mods/db.js please it will help to find out what is the problem

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  2. 0

    I think your problem come from the scope of your getSubData function.

    You do not return anything, you return row in the callback function but nothing at the getSubData level.
    That’s why arrow return null and then .length cannot be red.

    You can try with this

       getSubData: () => {
         let dbc = require('./mods/db.js');
         return dbc.query(`

         SELECT 10 rn, 'Ten' rt UNION
         SELECT 20 rn, 'Twenty' rt UNION
         SELECT 30 rn, 'Thirty' rt ;

         `,
         function (err, rows) {
            if (err) {
              console.log ( ' Error 3: ' , err );
            } else {
               console.log ( 'arows: ', rows.length );
               return( rows );
            }
          })
   }

    you will then get the full query and the result inside

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