Function returning undefined when callback is not used, on using callback, JS says function not defined

Question

Please take into consideration that similar questions have been asked on SO and I went through most of them.

I am making a RESTful service that needs querying the DB to get the data. I wrote the code that queries the database correctly but does returns undefined all the time. The code is here:

function returnAll(){
    ModuleDBService.find({},function(err,data){
        if(err){
            console.log('Error occured while retrieving the documents!');
        }
        return data;
    });
}

I was exporting the module using:

module.exports = {
    getAll:returnAll
};

After digging SO a lot, I discovered that I will need to use callback to get the data. I went through many examples and tried to apply a similar technique to my code, the modified code looked like this:

function getAllFromDatabase(callback){
    ModuleDBService.find({},function(err,data){
        if(err){
            console.log('Error occured while retrieving the documents!');
        }
        callback(returnAll(data));
    });
}


function returnAll(data){ return data;}

and then returning it in the similar fashion as above.

But now I am getting error that ModuleDAO.getAll is not a function (I am using var ModuleDAO = require('path to the database service').

I tried many variations of the code, went through a couple of videos on YouTube, all of them either lead to returning undefined, or return to the above stated error. If anyone could fix the code and throw light on this whole callback thing (Or could provide solid documentation to understand it), it'll be a great help.

Thanks.

EDIT: After all the extremely helpful answers, here is a summary:

Callbacks cannot return data, pass the function (the callback function) that you want your program to call with the data. In my case, it was my router returning the data.

Here is the corrected code:

function returnAll(callback) {
    ModuleDBService.find({}, function (err, data) {
        if (err) {
            console.log("Error while retrieving the document!")
            callback(null);
        }
        callback(data);
    });
}

I used this code in my router as:

mainAPIRouter.post('/api/module', function (req, res) {
    try {
        moduleDAO.getAll(function(data){
            res.status(200);
            res.json(data);
        });
    } catch (error) {
        res.status(500);
        return res.send("Invalid request");
    }
});

Thanks to all those who helped! :)

Answer 1

You are close. You don't need the returnAll() function, and you need to export getAllFromDatabase and pass a callback to it:

function getAllFromDatabase(callback){
    ModuleDBService.find({},function(err,data){
        if(err) {
            console.log('Error occured while retrieving the documents!');
        }
        callback(data);
    });
}

module.exports = {
    getAllFromDatabase: getAllFromDatabase
};

Then, when you want to use it, you need a callback function:

dataModule.getAllFromDatabase(callbackHandler);

function callbackHandler(dataFromDatabase) {
    // this function will be executed when ModuleDBService executes the callback
    console.log(dataFromDatabase);
}

A small detail: if err is Truthy, you should not execute the callback:

if(err) {
    console.log('Error occured while retrieving the documents!');
} else {
    callback(data);
}

Answer 2

You want to simply call the callback() with the data you need as an argument. You are making things much more complicated by passing another function into the callback. Try something like:

function returnAll(callback) {
  ModuleDBService.find({}, function(err, data) {
    if (err) return callback(err)
    callback(null, data);
  });
}

returnAll(function(err, data)) {
// it's customary for callbacks to take an error as their first argument
  if (err) {
    console.log('Error occured while retrieving the documents!');
  } else {
    // use data here!!
  }
}

Answer 3

As previous answers mentioned, you can use a callback. You can also use a promise if you prefer:

function returnAll(){
    return new Promise(function(resolve, reject) {
    ModuleDBService.find({},function(err,data){
        if(err){
            console.log('Error occured while retrieving the documents!');
                reject(err);
        }
          resolve(data);
    });
     });
}

You would then use something like this to access it:

returnAll()
  .then(data=> {console.log(data); })
  .catch(err=> { console.log(err); });

*Edit: since you want to use a callback, thought I'd add my $0.02 there as well. The most streamlined approach would be to just use the callback you're passing in with the ModuleDBService.find, without the ad-hoc function. But it's good to verify that callback actually is a function, and if not to make it one...makes your code more fault-tolerant.

function returnAll(cb){
    if(typeof cb!=='function') cb = function() {};
    ModuleDBService.find({},cb);
}