hide privateKey from nodeJS on Heroku?

I’ve integrate BrainTree Drop-in UI for checkout payment in nodeJS. It’s only a demonstration that I needed to create. I had no issues, it was easy enough. The only thing that I would like to do is to hide merchantId, publicKey and privateKey from the code. I would like to add them directly in Heroku Config Vars. I know how to do all this in Python, but I have no idea how to do it in JavaScript. I show you the code changing the keys:

const express = require('express');
const router = express.Router();
const braintree = require('braintree');

router.post('/', (req, res, next) => {
  const gateway = new braintree.BraintreeGateway({
    environment: braintree.Environment.Sandbox,
    // Use your own credentials from the sandbox Control Panel here
    merchantId: 'h43jgh5g3gl4543',
    publicKey: 'hj45j4h5lh45hl4h5l',
    privateKey: 'b5hbhbb45bhbh4kfndnfdkkfnd'
  });

  // Use the payment method nonce here
  const nonceFromTheClient = req.body.paymentMethodNonce;
  // Create a new transaction for $10
  const newTransaction = gateway.transaction.sale({
    amount: '10.00',
    paymentMethodNonce: nonceFromTheClient,
    options: {
      // This option requests the funds from the transaction
      // once it has been authorized successfully
      submitForSettlement: true
    }
  }, (error, result) => {
      if (result) {
        res.send(result);
      } else {
        res.status(500).send(error);
      }
  });
});

module.exports = router;

How can I assign those values to Variables that I can then pass to Heroku Config Vars?

Thank you very much for your help!