How to access env values for react in mern app deployed on heroku?

Question

I have deployed a mern stack application on heroku. Where the directory structure of my app is like this: -

|__client/  ** THIS IS EVERYTHING FROM THE REACT SIDE **
    |__ node_modules/
        |__ tons of stuff...
    |__ public/
        |__ index.html
        |__ favicon.ico
        |__ etc.
    |__ src/
        |__ index.js
        |__ main/
            |__ App.js
            |__ etc.
|__ models/
    |__ user.js
    |__ todo.js
    |__ etc.
|__ node_modules/
    |__ stuff...
|__ routes
    |__ userRoutes.js
    |__ todoRoutes.js
    |__ etc.
|__ .gitignore
|__ package.json
|__ server.js
|__ etc.

I have some environment variables set in heroku for backend that is working absolutely fine. I am serving my react build folder from the express app. My react app is having some env variables that I am not able to access. Is there any way I can access env in heroku from /client directory?

This is my package.json for backend

{
    "name": "mern-to-heroku",
    "version": "1.0.0",
    "main": "index.js",
    "license": "MIT",
    "dependencies": {
        "body-parser": "^1.18.2",
        "express": "^4.16.2",
        "mongoose": "^4.13.6",
        "morgan": "^1.9.0"
    },
    "scripts": {
        "start": "node server.js",
        "heroku-postbuild": "cd client && npm install --only=dev && npm install && npm run build"
    },
    "engines": {
        "node": "8.9.2"
    }
}

Answer 1

Env. are only available at build-time ( npm run build ) of your react app and will be then replaced with their value. They are not "available" at run-time of your application, so they can not be changed, after your react-app has been build.

That means you will have to build you app on heroku itself if the environment variables are only available on heroku.

Example: If you are using the deploy-to-heroku button, you can add to app.json (of course, needs to be customized for your setup):

 "scripts": {
    "postdeploy": "npm run build"
 },

To build you react-app with the new env. after it has been deployed to heroku