how to return array in Node.js from module

Question

getting undefined all the time "main.js":

var dbAccess = require('../dao/dbAccess');
dbaInstance = new dbAccess();
var wordPool = dbaInstance.getWordPool();
console.log (wordPool);

and "dbAccess.js" contains:

var DatabaseAccess = function() {}

DatabaseAccess.prototype.getWordPool = function () {
    RoundWord.find({},'words decoys', function(err, wordPoolFromDB) {
        if (err) throw err;
        //console.log(wordPoolFromDB); -working ok
        return (wordPoolFromDB);
    });
}

module.exports = DatabaseAccess;

why is it not working?

"console.log (wordPool);" shows "undefined" instead of array — V.Pud
– V.Pud, Commented May 29, 2017 at 9:41
Possible duplicate of How do I return the response from an asynchronous call? — ponury-kostek
– ponury-kostek, Commented May 29, 2017 at 13:53

arboreal84 · Accepted Answer · 2017-05-29 09:42:16Z

2

DatabaseAccess.prototype.getWordPool is not returning any result.

Since you are using an asynchronous function, you need do one of these things:

a) Take a callback as parameter and invoke the callback with a result

DatabaseAccess.prototype.getWordPool = function (cb) {
    RoundWord.find({}, 'words decoys', function(err, results) {
        if (err) {
           return cb(err, null);
        }

        cb(null, results);
    });
}

The callback convention is: cb(error, results...)

b) Use promises

DatabaseAccess.prototype.getWordPool = function () {
    return RoundWord.find({}, 'words decoys', function (err, results) {
        if (err) {
            throw err; // however you might want to sanitize it
        }

        return results;
    });
}

To consume this result you will need to do it as a promise

databaseAccess.getWordPool()
.catch(function (err) {
    // process the error here
})
.then(function (results) {
    // something with results
});

edited May 29, 2017 at 9:42

answered May 29, 2017 at 9:14

arboreal84

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3 Comments

arboreal84 Over a year ago

What library are you using? mongoose?

V.Pud Over a year ago

yeah, it's mongoose odm

arboreal84 Over a year ago

i added it to my answer.

srfrnk · Accepted Answer · 2017-05-29 09:22:02Z

1

It will work if you change to this:

var dbAccess = require('../dao/dbAccess');
dbaInstance = new dbAccess();
dbaInstance.getWordPool(function(wordPool){console.log (wordPool);});

And:

var DatabaseAccess = function() {}

DatabaseAccess.prototype.getWordPool = function (cb) {
    RoundWord.find({},'words decoys', function(err, wordPoolFromDB) {
        if (err) throw err;
        //console.log(wordPoolFromDB); -working ok
        cb(wordPoolFromDB);
    });
}

module.exports = DatabaseAccess;

answered May 29, 2017 at 9:22

srfrnk

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3 Comments

Prashant Tapase Over a year ago

can we write var DatabaseAccess = function (cb) { RoundWord.find({},'words decoys', function(err, wordPoolFromDB) { with above code? What is roll of prototype in above code?

V.Pud Over a year ago

thanks for reply, but it isn't solution. how can i operate "wordPool" in main.js? variable is still undefined

srfrnk Over a year ago

If you use asynchronous functions you have to consume the results in an asynchronous fashion. You can't mix synchronous and asynchronous code like that. You can operate on workPool inside the callback or use a blocking synchronous function to get the results but not both. sorry.

gotomanners · Accepted Answer · 2017-05-29 09:57:10Z

1

If the function is Asynchronous you need to pass a callback to find to get the result:

DatabaseAccess.prototype.getWordPool = function (callback) {
    RoundWord.find({},'words decoys', function(err, wordPoolFromDB) {
        if (err) throw err;
        callback(err, wordPoolFromDB);
    });
}

and call it as follows in main:

dbaInstance.getWordPool(function (err, wordPool) {
    console.log (wordPool); 
   // wordPool is only available inside this scope,
   //unless assigned to another external variable
});

// cannot access wordPool here

edited May 29, 2017 at 9:57

answered May 29, 2017 at 9:14

gotomanners

7,9461 gold badge27 silver badges39 bronze badges

8 Comments

Prashant Tapase Over a year ago

is it work in sync? Because there is no callback in function. will it work for err condition?

V.Pud Over a year ago

thanks for reply, but that additional "return" returns something strange:)

gotomanners Over a year ago

@PrashantTapase Well i'd need to know what Roundwood.find does or returns. I assumed it synchronous because of how the code was written. like Array.prototype.find

gotomanners Over a year ago

@V.Pud what is this "strange" something?

V.Pud Over a year ago

ok, thanks man, its tottaly new area for exploration for me, have to read about it

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