Setting environment variable in react-native?

I am using react-native to build a cross-platform app, but I do not know how to set the environment variable so that I can have different constants for different environments.

Example:

development: 
  BASE_URL: '',
  API_KEY: '',
staging: 
  BASE_URL: '',
  API_KEY: '',
production:
  BASE_URL: '',
  API_KEY: '',