create-react-app + Relay Modern without Eject

setup_relay.js

#!/usr/bin/env node
// by nic - 2017
// It's 10 lines of code to prevent you to call 'yarn eject'
// Just call this script before start/build as in this example: https://github.com/nic/relay/blob/master/package.json#L23-L24

const fs      = require('fs'),
      file    = require('path').resolve('./node_modules/babel-preset-react-app/index.js'),
      texts   = fs.readFileSync(file, 'utf8'),
      plugin  = 'babel-plugin-relay',
      pattern = 'const plugins = [';

if (!texts.includes(pattern)) throw new Error(`Failed to inject babel-plugin-relay.`);

if (!texts.includes(plugin))
  fs.writeFileSync(file, texts.replace(pattern, `${pattern}\n  require.resolve('${plugin}'),`), 'utf8');