Node.js on AWS Lambda and DynamoDB

Question

I'm trying to create a Lambda function to generate a .js script (in order to use with Chart.JS).

This script sends a query to a table in DynamoDB and outputs the results in .js file (which is stored in an S3 bucket).

I try for many hours to make it functional, but I'm stuck with classical problems on Node.js: order on callback functions and variables scope.

Here is the code I used:

var AWS = require('aws-sdk');
AWS.config.update({region: 'eu-west-1'});
var s3 = new AWS.S3();

var tweetValue ;
var neutralValue ;

var destBucket = "twitterappfront1";
var ddb = new AWS.DynamoDB.DocumentClient({apiVersion: '2012-08-10'});

function sentimentVal(inputparams) {
    //    function resultrequest() 
    ddb.get(inputparams, function(err, data) {
    if (err) {
        console.log("Error", err);
    } else {
        console.log("Success", data.Item);

        //Catch tweets number in DynamoTB table and store un descriptor 
        var numtweets = (JSON.parse(JSON.stringify(AWS.DynamoDB.Converter.marshall(data.Item)))).tweets ;
        var tweetsObject = Object.getOwnPropertyDescriptor(numtweets, 'N') ;
        tweetValue = tweetsObject.value ;

        console.log ("test stringify = ", numtweets) ;
        console.log (tweetsObject.value) ;
        console.log ("Value = ", tweetValue) ;
        return tweetValue ;
    }
    });
}

exports.handler = (event) => {
    // Read options from the event.

    var paramsNeutral = {
        TableName: 'twitterSentiment',
        Key: { 'sentiment':'NEUTRAL' }
    };

    // Call sentimentVal function with paramsNeutral, and setNeutralValue callback function
    // 
    sentimentVal(paramsNeutral, setNeutralValue);

    function setNeutralValue (error, tweetValue) {
        if (error) console.error('ERROR !', error) ;
        else console.log ('callback tweetValue = ', tweetValue) ;
    }
};

My problem is that it seems the callback function is never used: I have no console output "ERROR" or "Callback tweetValue ="

And I don't understand how to catch the value from the sentvimentVal function. I tried a "return", but I don't know if it is the right way.

Can you please help me ?

Thank you

Radu Diță · Accepted Answer · 2020-04-10 08:10:13Z

3

You are not waiting for the update to DynamoDB to finish.

Update it to return a promise and use async/await

async function sentimentVal(inputparams) {
    try {
     //    function resultrequest() 
     const data = await ddb.get(inputparams).promise()

     console.log("Success", data.Item);

     //Catch tweets number in DynamoTB table and store un descriptor 
     var numtweets = (JSON.parse(JSON.stringify(AWS.DynamoDB.Converter.marshall(data.Item)))).tweets ;
     var tweetsObject = Object.getOwnPropertyDescriptor(numtweets, 'N') ;
     tweetValue = tweetsObject.value ;

     console.log ("test stringify = ", numtweets) ;
     console.log (tweetsObject.value) ;
     console.log ("Value = ", tweetValue) ;
     return tweetValue ;
   } catch (err) {
      console.log("Error", err);
      throw err
   }
}

And await for it in handler

exports.handler = (event) => {
    // Read options from the event.

    var paramsNeutral = {
        TableName: 'twitterSentiment',
        Key: { 'sentiment':'NEUTRAL' }
    };

    // Call sentimentVal function with paramsNeutral, and setNeutralValue callback function
    // 
    const tweet = await sentimentVal(paramsNeutral, setNeutralValue);

    function setNeutralValue (error, tweetValue) {
        if (error) console.error('ERROR !', error) ;
        else console.log ('callback tweetValue = ', tweetValue) ;
    }
};

I'm not sure what setNeutralValue is supposed to do.

answered Apr 10, 2020 at 8:10

Radu Diță

14.3k2 gold badges33 silver badges37 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Chris4484 Over a year ago

Hello, thank you for your help ! I tried to use your corrections but I have an error on " const tweet = await sentimentVal(paramsNeutral, setNeutralValue);" line It shows an error "Parsing error: Unexpected token sentimentVal". Do you have an idea on what may cause this ? Thank you !

Chris4484 Over a year ago

I succedeed adding async in exports.handler = async (event) => {

Collectives™ on Stack Overflow

Node.js on AWS Lambda and DynamoDB

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related