Returning Promises from Netlify Function

I am working on a serverless function to be hosted on Netlify to subscribe users to a mailchimp email list.

I am getting the following obscure error:

lambda response was undefined. check your function code again

Here is my function:

const handler = async function (event, context) {
    try {
        let body = JSON.parse(event.body);

        mailchimp.setConfig({
            apiKey: 'XXXXXXXXX',
            server: 'us20',
        });

        const submit = async () => {
            const response = await mailchimp.lists.addListMember("XXXXXXXX", {
                email_address: body.email.toLowerCase(),
                status: 'subscribed'
            });

            if (response.errors !== undefined && response.errors.length) { 
                throw new Error(response.errors);
            }
        }

        submit().then(response => {
            console.log(response);

            return {
                statusCode: 200,
                body: JSON.stringify({ response }),
            }
        }).catch(errors => {
            return {
                statusCode: 500,
                body: JSON.stringify({ errors }),
            }
        });
    } catch (error) {
        // output to netlify function log
        console.log(error);

        return {
            statusCode: 500,
            // Could be a custom message or object i.e. JSON.stringify(err)
            body: JSON.stringify({ msg: error.message }),
        }
    }
}

module.exports = { handler }

I think the issue may be because nothing is being returned after calling submit(), but I am not sure how best to return it. I still can't quite get my head around promises.

I am really hoping someone can point me in the right direction.

Many thanks David

You are on the right track, nothing is returned in the submit function so the response will always be undefined. Also You are using .then with async/await syntax which is ok, async/await is just a cleaner way of using promises so you typically use one or the other. And nesting catch blocks here is unnecessary. I would rewrite it to something like this:

const handler = async function (event, context) {
  try {
      let body = JSON.parse(event.body);

      mailchimp.setConfig({
          apiKey: 'XXXXXXXXX',
          server: 'us20',
      });

      const submit = async () => {
        const response = await mailchimp.lists.addListMember("XXXXXXXX", {
            email_address: body.email.toLowerCase(),
            status: 'subscribed'
        });
        if (response.errors !== undefined && response.errors.length) {
            throw new Error(response.errors);
        }
        return response;
      }

      const response = await submit();
      return {
        statusCode: 200,
        body: JSON.stringify({ response }),
      }
  } catch (error) {
      // output to netlify function log
      console.log(error);

      return {
          statusCode: 500,
          // Could be a custom message or object i.e. JSON.stringify(err)
          body: JSON.stringify({ msg: error.message }),
      }
  }
}

If you are still struggling with promises, Id recommend reading the docs for them as well as the docs for async/await.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function