I am having issues with exporting in node

Question

I am trying to make an api endpoint for data coming from dynamoDB. I believe that I have everything connected but when I run postman to check the api (api/db) it doesn't recognize the functions from the db.js in the db.js (for routes). I have run a test on api/test and am getting the information back. Here is the code from both files:

1.  This scans the database and I'm trying to export it to another file.

var AWS = require('aws-sdk');

var params = {
    TableName : "iotbuttonsn",
    //KeyConditionExpression: "serialNumber =:serialNumber",

    //ExpressionAttributeValues: {
    //    ":serialNumber":"*"
//},

ScanIndexForward: false,
Limit: 3,
Select: 'ALL_ATTRIBUTES'
};

AWS.config.update({
  region: "us-east-1",
  endpoint: "https://dynamodb.us-east-1.amazonaws.com"
});

var docClient = new AWS.DynamoDB.DocumentClient();

var getDatabase = (function(){

    return {

    scanDB: function(){
        docClient.scan(params, onScan);
        var onScan = function(err, data){
            if (err) {
                console.log(err.message);
            } else {
                console.log('scan success');
                len = data.Items.length;
                for (n=0; n<len; n++) {
                    clickTypes[n] = data.Items[n].payload.clickType;
                    serialNums[n] = data.Items[n].serialNumber;
                }
            }
        };
    },
    clickTypes: [],
    serialNums: []
  };    
})();

module.exports = getDatabase;

2.  This is where I'm trying to input but db.scanDB() isn't working:

var router = require('express').Router();
var db = require('../routes/db.js');

router.get('/', function(req, res){
    db.scanDB();
    buttons = 
    [
        iot_buttonOne = {
            serialNum: db.serialNum[0],
            clickType: db.clickTypes[0]
    },
        iot_buttonTwo = {
            serialNum: db.serialNum[1],
            clickType: db.clickTypes[1]
        }

        ]
    .then(
        function scanSuccess(data){
            res.json(data);
        },

        function scanError(err){
            res.send(500, err.message);
        }

    );

});

module.exports = router;

Answer 1

Change your db.scan() function to properly return an asynchronous result:

// db.js
module.exports = {
    scanDB: function(cb){

        docClient.scan(params, function(err, data) {
            var clickTypes = [], serialNums = [];
            if (err) {
                console.log(err.message);
                cb(err);
            } else {
                console.log('scan success');
                len = data.Items.length;
                for (n=0; n<len; n++) {
                    clickTypes[n] = data.Items[n].payload.clickType;
                    serialNums[n] = data.Items[n].serialNumber;
                }
                cb(null, {clickTypes, serialNums});
            }
        });
    }
};    

Then, when you use it:

var db = require('../routes/db.js');
db.scanDB(function(err, data) {
    if (!err) {
        // data.clickTypes
        // data.serialNums
    } else {
        // process error
    }
});

It really does not good to put the scanDB result on the DB object the way you were doing because there was no way for the caller to know when the asynchronous operation was done. So, since you have to provide some notification for the caller when the async operation is done (either via callback or promise), you may as well just pass the results there too.

Also, the .then() handler in your router.get(...) handler does not belong there. I don't know why it's there at all as there are no promises involved in the code you show. Perhaps a cut/paste error when creating the question?

Note, I removed the IIFE from your getDatabase() definition since there was no benefit to it other than a little more complicated code.