node.js module answer undefined [duplicate] - node.js

Question

This question already has answers here:
How do I return the response from an asynchronous call?
(41 answers)
Closed 4 years ago.
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));

Answer 1

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);
});

Answer 2

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);
});
Read more:
How do I promisify the AWS JavaScript SDK?
https://aws.amazon.com/ru/blogs/developer/support-for-promises-in-the-sdk/