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CS156

Config Vars Frontend - How to pass config vars to the frontend

Suppose you have a variable that you want to configure via dokku config:set ... and you want to access this value in the backend.

It’s a bit of a puzzle to figure out all of the pieces needed to make that work.

The frontend is built by the com.github.eirslett.frontend-maven-plugin which is configured in the pom.xml. On dokku, this happens during the build phase.

Normally, config vars are not available during the build phase, but you can override this by doing this:

  • First, make sure the variable is prefixed with REACT_APP_
    • For example, not START_QTR, but REACT_APP_START_QTR
  • One time setup: 
    dokku docker-options:add courses-qa build '--build-arg REACT_APP_START_QTR'
  • Add the variable to the Dockerfile as an ARG 
    ARG REACT_APP_START_QTR
  • Then, set up a script to get all of the variables that start with REACT_APP and put them in a .env file in the frontend directory during the build phase: 
    RUN (cd /home/app/frontend; ./scripts/create_dotenv_dokku.sh; pwd; ls -al; cat ./scripts/create_dotenv_dokku.sh; cat ./.env; cd ..)

The script ./scripts/create_dotenv_dokku.sh looks like this:

#!/usr/bin/env sh
# Copy REACT_APP_ variables to a local .env

rm -rf .env
touch .env
env | grep '^REACT_APP_' | while read -r line ; do
  echo "${line} " >> .env
done

Then, in package.json prefix npm run build or npm run start with env-cmd like this:

 "build": "env-cmd react-scripts build",

In the React code, you can then access the variables like this:

process.env.REACT_APP_START_QTR

Relevant documentation: