Progressive mailchimp subscription on Netlify

.env

MAILCHIMP_API_KEY="...-us19"
MAILCHIMP_LIST_ID="..."

form-handler.js

import { parse } from 'querystring'
const axios = require('axios');
const mailChimpAPI = process.env.MAILCHIMP_API_KEY;
const mailChimpListID = process.env.MAILCHIMP_LIST_ID;


exports.handler = (event, context, callback) => {
  let body = {}
  console.log(event)
  try {
    body = JSON.parse(event.body)
  } catch (e) {
    body = parse(event.body)
  }

  if (!body.email) {
    console.log('missing email')
    return callback(null, {
      statusCode: 400,
      body: JSON.stringify({
        error: 'missing email'
      })
    })
  }

  if (!mailChimpAPI) {
    console.log('missing mailChimpAPI key')
    return callback(null, {
      statusCode: 400,
      body: JSON.stringify({
        error: 'missing mailChimpAPI key'
      })
    })
  }

  if (!mailChimpListID) {
    console.log('missing mailChimpListID key')
    return callback(null, {
      statusCode: 400,
      body: JSON.stringify({
        error: 'missing mailChimpListID key'
      })
    })
  }

  const data = {
    email_address: body.email,
    status: "pending",
    merge_fields: {}
  };

  const subscriber = JSON.stringify(data);
  console.log("Sending data to mailchimp", subscriber);

  // Subscribe an email

  axios(
    {
      method: 'post',
      url: `https://us19.api.mailchimp.com/3.0/lists/${mailChimpListID}/members/`, //change region (us19) based on last values of ListId.
      data: subscriber,
      auth: {
        username: 'apikey', // any value will work 
        password: mailChimpAPI
      }
    }
  ).then(function(response){
    console.log(`status:${response.status}` )
    console.log(`data:${response.data}` )
    console.log(`headers:${response.headers}` )

    if (response.headers['content-type'] === 'application/x-www-form-urlencoded') {
      // Do redirect for non JS enabled browsers
      return callback(null, {
        statusCode: 302,
        headers: {
          Location: '/thanks.html',
          'Cache-Control': 'no-cache',
        },
        body: JSON.stringify({})
      });
    }

    // Return data to AJAX request
    return callback(null, {
      statusCode: 200,
      body: JSON.stringify({ emailAdded: true })
    })
  }).catch(function(error) {
    if (error.response) {
      // The request was made and the server responded with a status code
      // that falls out of the range of 2xx
      console.log(error.response.data);
      console.log(error.response.status);
      console.log(error.response.headers);
    } else if (error.request) {
      // The request was made but no response was received
      console.log(error.request);
    } else {
      // Something happened in setting up the request that triggered an Error
      console.log('Error', error.message);
    }
    console.log(error.config);
  });
};

package.json

{
  "scripts": {
    "start": "netlify-lambda serve source/lambda",
    "build": "netlify-lambda build source/lambda"
  },
  "dependencies": {
    "axios": "^0.19.0",
    "dotenv": "^8.1.0",
    "netlify-lambda": "^1.6.3"
  }
}

subscribe.html

<div id="message"></div>
<form id="newsletter" class="subscribe" action="/.netlify/functions/form-handler" method="post">
  <input type="email" id="inputEmail" name='email' placeholder="Enter email to subscribe for FREE" class="email" required autofocus>
  <button class="button" type="submit">Subscribe</button>
</form>

<script>
  var form = document.getElementById('newsletter');

  form.addEventListener("submit", function(e) {
    e.preventDefault();
    email = document.getElementById('inputEmail').value;
    submitEmail(email)
  });

  function submitEmail(email) {
    fetch('/.netlify/functions/form-handler', {
        method: 'post',
        body: JSON.stringify({
          email: email
        })
      }).then(function(response) {
        return response.json();
      }).then(function(data) {
        console.log('data from function', data);
        messageDiv = document.getElementById('message');
        messageDiv.innerText = 'Confirmation email has been sent!'
      });
  };
</script>

This function was open source from https://www.fbacatalog.com project.
To find more details on this code and our porject -- read up dev.to community post https://dev.to/skatkov/jamstack-progressive-mailchimp-sign-up-form-with-netlify-13m3

Thanks for your interest!