Configuring environment variables for static web site on AWS S3

You can't set environment variables in S3. S3 is just a static object store, not a dynamic content server. To evaluate variable values would be the very definition of dynamic content, and S3 only serves static content.

Note that AngularJS code runs in the browser, not on a server, so those keys you want to keep out of your code are going to be viewable in the browser by anyone that knows how to look. There's no way for you to hide those values if you want to call SendGrid from a client-side framework.