javascript – How to use nginx to fetch the environment variables

How do I pick the values of the env variables which i am fetching from the secrets (defined in the pipeline) during the deployment in my frontend which uses javascript. I have a docker container running Nginx.

I got to know that we can replace the variables with values through nginx.conf.
nginx docker image already runs envsubst and the pipeline i am using is already picking the secrets ,but i am not sure how i use them in nginx.conf and after picking how do i substitute in the .js files where it is referencing.
Any suggestions would be appreciated, thank you.