node.js module answer undefined

Question

Can someone give me a hand? I tried to figure it out but i ran out of ideas.

-------------dynamo.js....

module.exports.readUser = function (user_id) {
    AWS.config = new AWS.Config();
    AWS.config.update({region:  "eu-west-1"});

    var docClient = new AWS.DynamoDB.DocumentClient();
    var table = "user";
    var user_id = user_id;
    var params = {
        TableName: table,
        Key:{
            "user_id": user_id
        }
    };
    docClient.get(params, function(err, data) {
        if (err) {
            return err;
        } else {
            //console.log(data); <-- data is filled 
            return data;
        }
    });

var dynamo = require("./dynamo.js");
console.log(dynamo.readUser(4711998));

Can you explain the problem with more detail? Include console output if possible. — Adam, Sep 28 '18 at 20:24
there isn't so much ubuntu@ip-172-31-21-157:~$ node test.js ec2 undefined — Klaus Schmitt, Sep 28 '18 at 20:44
data is filled with a json but console.log(dynamo.readUser(4711998)); gives only a undefined — Klaus Schmitt, Sep 28 '18 at 20:51

giggo1604 · Answer 1 · 2018-09-28T20:57:19.190

The data you log out inside the function is only the returned value for the anonymous function given to docClient.get as the second argument. The exported readUser function has no return statement and therefore it is undefined.

Because of the async nature of javascript you have to access the result via a callback like so:

module.exports.readUser = function (user_id, cb) {
    ...
    docClient.get(params, function(err, data) {
        if (err) {
            return err;
        } else {
            return cb(data);
        }
    });
}

var dynamo = require("./dynamo.js");
dynamo.readUser(4711998, function (user) {
    console.log(user);
});

Lemix · Answer 2 · 2018-09-28T21:25:17.393

docClient.get is an asynchronous function. You need promisify this function or use callback.

Use callback:

module.exports.readUser = function (user_id, cb) {
    AWS.config = new AWS.Config();
    AWS.config.update({ region: "eu-west-1" });

    var docClient = new AWS.DynamoDB.DocumentClient();
    var table = "user";
    var user_id = user_id;
    var params = {
        TableName: table,
        Key: {
            "user_id": user_id
        }
    };
    docClient.get(params, cb);
}

var dynamo = require("./dynamo.js");

dynamo.readUser(4711998, function (err, user) {
    if(err)
        console.error(err);
    else
        console.log(user);
});

Use promise:

module.exports.readUser = function (user_id) {
    AWS.config = new AWS.Config();
    AWS.config.update({ region: "eu-west-1" });

    var docClient = new AWS.DynamoDB.DocumentClient();
    var table = "user";
    var user_id = user_id;
    var params = {
        TableName: table,
        Key: {
            "user_id": user_id
        }
    };
    return new Promise(function(resolve, reject) {
        docClient.get(params, function(err, data) {
            if (err) {
                reject(err);
            } else {
                resolve(data);
            }
        });
    });
}

var dynamo = require("./dynamo.js");

dynamo.readUser(4711998).then(function (user) {
    console.log(user);
}).catch(function(err) {
    console.error(err);
});

node.js module answer undefined

2 Answers2