Return Variable in Node Module Call Back

Question

I'm using this NPM package to get the CSV output of table from an access database.

I have my function structured as follows:

function createDB(mdbfile){
  var rawDB = mdb(mdbfile);
  var db = {}
  rawDB.tables(function(err, table){
    table.forEach(function(item){
      var rawTable = null;
      rawDB.toCSV(item, function(err, csv){
        rawTable = csv; //Doesn't work
      });
      console.log(rawTable); //Always null :(
      
      var newTable = Do stuff with CSV;
      db.item = newTable;
    });
  });

  return db
}

I'm sure this is some scoping issue but I'm new to node and don't know how to search for this properly. From what I can tell the module is synchronous but the scope doesn't work.

Any help would be appreciated! Unfortunately a client I have MUST use access DB file and I'm just trying to make this work.

akz · Accepted Answer · 2017-04-30 18:59:20Z

1

The scoping looks fine, but the toCsv function does not look synchronous. You can double check the source code: https://github.com/maxogden/node-mdb/blob/master/index.js. So, you would try to print out rawTable before the toCsv callback is called.

I would also add proper error handling within both callbacks to see whether you are getting any errors.

EDIT:

Since your createDB function makes asynchronous calls, it should take a callback. The code below is an example that could possibly work for your case.

// look into the async module, specifically forEach
const asyncForEach = require('async/forEach');

function createDB(mdbfile, next ){
  var rawDB = mdb(mdbfile);
  var db = {}
  rawDB.tables(function(err, table){

    if( err ) return next(err);     

    asyncForEach(
        table,
        function(item, cb){
            rawDB.toCSV(item, ( err, csv )=>{
                if( err ) return cb( err );
                // this processing can be done here or in the callback to asynForEach
                var newTable = Do stuff with CSV;
                db.item = newTable;
                cb(null, csv);
            };
        },
        function(err, csvs){
            // csvs can be processed and added to db here as well
            if ( err ) return next( err );
            return next( null, db );
        } );
   } );
}

You would then have to provide a callback to createDB when you call it:

createDB( mdbFile , ( err, db )=>{
    if( err ) return err;
    // use db e.g. res.send( db )
}

Or you could look into how to accomplish this with promises.

edited Apr 30, 2017 at 18:59

answered Apr 28, 2017 at 4:25

akz

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2 Comments

Segfaulter Over a year ago

Thanks! It looks like there are no errors. But is there a good way to go back to synchronous after this asynchronous call? Eventually my express router needs to be able to access the variable.

Segfaulter Over a year ago

Thanks! I have been looking to promises for this but I'm not sure how to convert the modules built in functions.

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