How do I go about waiting for two DynamoDB calls to finish before executing new function?

Question

I'm trying to execute two asynchronous functions to DynamoDB.
I will need the return data from both before continuing with the following step which is sending an email that contains their data.
How do I tackle this issue?

I'm using the following code:

var productParams = {
      TableName: productsTable,
      FilterExpression: 'client = :this_client',
      ExpressionAttributeValues: { ':this_client': "someclient" }
    };

    dynamoClient.scan(productParams, function (err, data) {
      if (err) {
        console.error("Unable to query. Error:", JSON.stringify(err, null, 2));
      } else {
        console.log("Query succeeded.");
        data.Items.forEach(item => {
          products.push(item)
        });
      }
    });

    var retailerParams = {
      TableName: retailersTable,
      FilterExpression: 'leadTime = :this_leadTime',
      ExpressionAttributeValues: { ':this_leadTime': 42 }
    };

    dynamoClient.scan(retailerParams, function (err, data) {
      if (err) {
        console.error("Unable to query. Error:", JSON.stringify(err, null, 2));
      } else {
        console.log("Query succeeded.");
        data.Items.forEach(item => {
          retailers.push(item)
        });
      }
    });

    var email = {
      "Source": "somemail@gmail.com",
      "Template": "some_template",
      "Destination": {
        "ToAddresses": ["somemail@gmail.com"]
      },
      "TemplateData": `{somedata}`
    }
    await ses.sendTemplatedEmail(email).promise();

Answer 1

You can convert both DynamoDB calls into promises (by chaining scan calls with .promise() calls) and await them using Promise.all before sending the email:

var productParams = {
  TableName: productsTable,
  FilterExpression: 'client = :this_client',
  ExpressionAttributeValues: { ':this_client': "someclient" }
};

const productsPromise = dynamoClient.scan(productParams).promise()
  .then(data => {
    data.Items.forEach(item => {
      products.push(item)
    });
  })
  .catch(err => {
    console.error("Unable to query. Error:", JSON.stringify(err, null, 2));
  });

var retailerParams = {
  TableName: retailersTable,
  FilterExpression: 'leadTime = :this_leadTime',
  ExpressionAttributeValues: { ':this_leadTime': 42 }
};

const retailersPromise = dynamoClient.scan(retailerParams).promise()
  .then(data => {
    console.log("Query succeeded.");
    data.Items.forEach(item => {
      retailers.push(item)
    });
  })
  .catch(err => {
    console.error("Unable to query. Error:", JSON.stringify(err, null, 2));
  });


await Promise.all([
  productsPromise,
  retailersPromise
]);

var email = {
  "Source": "somemail@gmail.com",
  "Template": "some_template",
  "Destination": {
    "ToAddresses": ["somemail@gmail.com"]
  },
  "TemplateData": `{somedata}`
}

await ses.sendTemplatedEmail(email).promise();