This can be achieved by adding a small helper script at the root of your Node.js application. The file must be named env.sh. The following shows the code snippet for the env.sh file:

    #!/usr/bin/env bash

    set -eo pipefail

    function cleanup {
      rm -f client.crt client.key
      echo ""
      echo "-----> Exiting."
      exit
    }
    trap cleanup SIGINT SIGTERM

     if [[ -z "${KAFKA_CLIENT_CERT}" ]]; then
       echo "KAFKA_CLIENT_CERT is not set. Aborting"
       exit 1
     fi

    if [[ -z "${KAFKA_CLIENT_CERT_KEY}" ]]; then
      echo "KAFKA_CLIENT_CERT_KEY is not set. Aborting"
      exit 1
    fi

    if [[ -z "${KAFKA_URL}" ]]; then
      echo "KAFKA_URL is not set. Aborting"
      exit 1
    fi

    # Setup cert and cert key
    rm -f client.crt client.key
      echo -n "$KAFKA_CLIENT_CERT" > client.crt
      echo -n "$KAFKA_CLIENT_CERT_KEY" > client.key

The Heroku profile will have the following code:

web: ./env.sh && npm start