Sometimes, when running a create-react-app website, you may want to include additional or different scripts in your production build vs your development build. A very common use case would be to include Google Tag Manager or Google Analytics scripts in your production build. To do so, you can use the NODE_ENV value.
<head>
<% if (process.env.NODE_ENV === 'production') { %>
<!-- production -->
<script src="/my-prod-script.js"></script>
<% } %>
<% if (process.env.NODE_ENV === 'development') { %>
<!-- development -->
<script src="/my-prod-script.js"></script>
<% } %>
</head>