React in heroku does not recognize the environment - reactjs

Question

I am creating an application in react with graphql for the API and rails as the backend. I have just configured the environment variables so that depending on whether the application is running in production or in development the url that is consumed in the front changes.
But when I deploy in heroku and check the console in the browser, the url of the api is still the development one http://localhost:3000. I don't know why React is not recognizing the production environment in heroku.
I just created a file constant.js:
const prod = {
url: {
API_URL: 'https://hackathon-one-api.herokuapp.com/graphql'
}
}
const dev = {
url: {
API_URL: 'http://localhost:3000/graphql'
}
}
console.log(process.env.NODE_ENV)
export const config = process.env.NODE_ENV === 'development' ? dev : prod;
And in my index.js (where is imported):
// Environments variables
import { config } from '../src/constant';
// 1
import { ApolloProvider } from 'react-apollo';
import { ApolloClient } from 'apollo-client';
import { createHttpLink } from 'apollo-link-http';
import { setContext } from 'apollo-link-context';
import { InMemoryCache } from 'apollo-cache-inmemory';
// 2
const httpLink = createHttpLink({
uri: config.url.API_URL //here!
});
// 3
const authLink = setContext((_, { headers }) => {
// get the authentication token from local storage if it exists
const token = localStorage.getItem('token');
// return the headers to the context so httpLink can read them
return {
headers: {
...headers,
authorization: token ? `Bearer ${token}` : "",
}
}
});
// 4
export const client = new ApolloClient({
link: authLink.concat(httpLink),
cache: new InMemoryCache()
})