Function keeps returning undefined (DynamoDB)

Question

I'm working with AWS DynamoDB and trying to query a table for email addresses. I want to see how many of the same emails exist in the table. But for some reason when I run the function below I get undefined.

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 think it may have to do something with the function finishing before the table is fully queried. But I don't know how I would go about solving this (async/await?).

Parzh · Accepted Answer · 2020-09-06 12:59:53Z

2

There's a race condition here: checkEmail returns earlier than docClient.query invokes its callback, since docClient.query is asynchronous, but checkEmail isn't.

What you have to do is define checkEmail to work asynchronously as well, — in this case, by using Promises and async / await syntax. As Richard Dunn suggested in the comment, AWS does provide such promise in this case, so it could be used seamlessly:

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;
    }
}

Now, as the function is asynchronous now, you don't just call it, you also wait until it finishes the job — and only then acquire the result:

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

edited Sep 6, 2020 at 12:59

answered Sep 4, 2020 at 14:30

Parzh

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1 Comment

Richard Dunn Over a year ago

The entire AWS SDK exposes promises for every callback. Just use: docClient.query(params).promise() - no need to reinvent the wheel. Also, async and await are redundant here, you can just return a promise from a function and it's the same thing.

ObsoleteAwareProduce · Accepted Answer · 2020-09-04 14:38:00Z

The callback function runs after the function returns. Change the function to this:

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'll also need to change wherever you're calling the function from; from this:

checkEmail(someValue);

to one of these:

//Change the function you're calling the function from to an ASYNC function:
async function myFunction() {
    await checkEmail(someValue);
    //DO SOMETHING
}

or, if you can't do that:

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

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