Submit Hubspot Form with serverless

cloud-function.serverless.js

const fetch = require('node-fetch');
/**
 * Responds to any HTTP request.
 *
 * @param {!express:Request} req HTTP request context.
 * @param {!express:Response} res HTTP response context.
 */
exports.hubspot = async (req, res) => {
  res.set('Access-Control-Allow-Origin', '*');
  if (req.method === 'OPTIONS') {
    // Send response to OPTIONS requests
    res.set('Access-Control-Allow-Methods', 'POST');
    res.set('Access-Control-Allow-Headers', 'Content-Type');
    res.set('Access-Control-Max-Age', '3600');
    res.status(204).send('');
  } else {
    // Object destructing for request object
    let {body, method, headers} = req;
    // If is not POST return 400
    if (method !== 'POST') return res.status(400).send({message: `This endpoint not allow ${method} method.`});
    // Parse to JSON
    const form = (typeof body === 'string') ? JSON.parse(body) : body;
    // Create body
    // Format read here: https://developers.hubspot.com/docs/methods/forms/submit_form
    const data = {
      submittedAt: new Date().getTime(),
      fields: Object.entries(form).map(([name, value]) => ({name, value})), // Thanks to @hseguro
      context: {
        pageUri: headers.referer,
        pageName: process.env.website || ''
      }
    };
    // Set URL
    const url = `https://api.hsforms.com/submissions/v3/integration/submit/${process.env.portal}/${process.env.form}?hapikey=${process.env.key}`;
    // Fetch to Hubspot
    const response = await fetch(url,
      {
        method: 'POST',
        body: JSON.stringify(data),
        headers: {'Content-Type': 'application/json'}
      }
    );
    // Parse
    const json = await response.json();
    // Return Hubspot status code and body of response
    res.status(response.status).send(json);
  }
}

package.json

{
  "name": "hubspot-form",
  "version": "0.0.1",
  "dependencies": {
    "node-fetch": "^2.6.0"
  }
}

Format to sent in the body

// Ex.
{
    "email": "[email protected]",
    "firstname": "John",
    "lastname": "Doe",
    "phone": "+1098438509345"
}

  if (method !== 'POST') return res.status(400).send({ message: `This endpoint not allow ${method} method.` });
  ...
  const data = {
    submittedAt: new Date().getTime(),
    fields: Object.entries(form).map(([name, value]) => ({ name, value })),
    context: {
      pageUri: headers.referer,
      pageName: process.env.website || ''
    }
  };

Updated with CORS and improvements on the code.

Thanks you @hseguro!