Getting "undefined" as a return value from a function in DynamoDB

Question

Getting "undefined" as a return value from a function in DynamoDB

I've been working on a project involving AWS DynamoDB where I need to query a table for email addresses and determine the number of duplicate emails. However, when I execute the function below, it returns undefined which has left me puzzled.

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

function checkEmail(email) {
    var params = {
        TableName: "Users",
        IndexName: "email",
        KeyConditionExpression: "email=:email",
        ExpressionAttributeValues: {
            ":email": email
        }
    }
    var emails = 0;
    docClient.query(params, function (err, data) {
        if (err) {
            return -1;
        }
        else {
            emails = data.ScannedCount;
        }
    });

    return emails;
}

I suspect that the issue lies in the function completing before the table is fully queried. I'm considering using async/await but unsure of how to implement this. Any advice or guidance would be greatly appreciated.

javascript async-await amazon-dynamodb

Answer 1

Answer №1

There is a challenge with timing in this scenario: checkEmail completes before docClient.query executes its callback, as docClient.query operates asynchronously while checkEmail does not.

The solution is to modify checkEmail to also function asynchronously by utilizing Promises and the async / await syntax. As recommended by Richard Dunn, AWS offers such promises for this situation, making it easy to implement:

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

async function checkEmail(email) {
    const params = {
        TableName: "Users",
        IndexName: "email",
        KeyConditionExpression: "email=:email",
        ExpressionAttributeValues: {
            ":email": email,
        },
    };

    try {
        const data = await docClient.query(params).promise();
        return data.ScannedCount;
    } catch (error) {
        console.error(error);
        return -1;
    }
}

With the asynchronous nature of the function, you must now not only call it but also wait for it to complete its task — and only then retrieve the result:

checkEmail(email).then((emails) => {
    // `emails` is a number
});

Answer 2

There is a challenge with timing in this scenario: checkEmail completes before docClient.query executes its callback, as docClient.query operates asynchronously while checkEmail does not.

The solution is to modify checkEmail to also function asynchronously by utilizing Promises and the async / await syntax. As recommended by Richard Dunn, AWS offers such promises for this situation, making it easy to implement:

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

async function checkEmail(email) {
    const params = {
        TableName: "Users",
        IndexName: "email",
        KeyConditionExpression: "email=:email",
        ExpressionAttributeValues: {
            ":email": email,
        },
    };

    try {
        const data = await docClient.query(params).promise();
        return data.ScannedCount;
    } catch (error) {
        console.error(error);
        return -1;
    }
}

With the asynchronous nature of the function, you must now not only call it but also wait for it to complete its task — and only then retrieve the result:

checkEmail(email).then((emails) => {
    // `emails` is a number
});

Answer 3

Answer №2

Once the function is complete, the callback function will be executed. Modify the function as follows:

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

function checkEmail(email) {
    return new Promise(function(resolve, reject) {
        var params = {
            TableName: "Users",
            IndexName: "email",
            KeyConditionExpression: "email=:email",
            ExpressionAttributeValues: {
                ":email": email
            }
        }
        docClient.query(params, function (err, data) {
            if (err) {
                reject(err); //Error
            }
            else {
                resolve(data.ScannedCount); //OK
            }
        });
    });
}

You also need to update the section where you are calling the function from, like so:

checkEmail(someValue);

Replace it with one of the following methods:

//Change the function that calls this function to an ASYNC function:
async function myFunction() {
    await checkEmail(someValue);
    //DO SOMETHING
}

Alternatively, if that's not possible:

checkEmail(someValue).then(function(returnValue) {
    //DO SOMETHING HERE
});
//But NOT here

Answer 4

Once the function is complete, the callback function will be executed. Modify the function as follows:

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

function checkEmail(email) {
    return new Promise(function(resolve, reject) {
        var params = {
            TableName: "Users",
            IndexName: "email",
            KeyConditionExpression: "email=:email",
            ExpressionAttributeValues: {
                ":email": email
            }
        }
        docClient.query(params, function (err, data) {
            if (err) {
                reject(err); //Error
            }
            else {
                resolve(data.ScannedCount); //OK
            }
        });
    });
}

You also need to update the section where you are calling the function from, like so:

checkEmail(someValue);

Replace it with one of the following methods:

//Change the function that calls this function to an ASYNC function:
async function myFunction() {
    await checkEmail(someValue);
    //DO SOMETHING
}

Alternatively, if that's not possible:

checkEmail(someValue).then(function(returnValue) {
    //DO SOMETHING HERE
});
//But NOT here

Getting "undefined" as a return value from a function in DynamoDB

Answer №1

Answer №2

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