Extending ESLint config not working

Created by: conor-cafferkey-sociomantic

Describe the bug

Environment

  System:
    OS: macOS 10.14.6
    CPU: (4) x64 Intel(R) Core(TM) i7-7660U CPU @ 2.50GHz
  Binaries:
    Node: 8.15.0 - ~/.nvm/versions/node/v8.15.0/bin/node
    Yarn: 1.17.3 - /usr/local/bin/yarn
    npm: 6.4.1 - ~/.nvm/versions/node/v8.15.0/bin/npm
  Browsers:
    Chrome: 76.0.3809.100
    Firefox: 67.0
    Safari: 12.1.2
  npmPackages:
    react: ^16.9.0 => 16.9.0
    react-dom: ^16.9.0 => 16.9.0
    react-scripts: 3.1.0 => 3.1.0
  npmGlobalPackages:
    create-react-app: Not Found

Steps to reproduce

yarn create react-app testapp (or npx create-react-app testapp)
cd testapp
yarn add eslint-config-airbnb (or npm install eslint-config-airbnb)
Add the following to package.json:

"eslintConfig": { "extends": [ "react-app", "airbnb"] }

EXTEND_ESLINT=true yarn start ( or EXTEND_ESLINT=true npm start)

Expected behavior

eslint-config-airbnb should be used
app should not compile

Actual behavior

eslint-config-airbnb is not used
app compiles (it shouldn’t!)

Problem

webpack.config.js checks thateslintConfig.extends exists: https://github.com/facebook/create-react-app/blob/c0b4173d1bba37fde2d699fe9914d16f823bcbe5/packages/react-scripts/config/webpack.config.js#L355 and – if it does – that it includes 'react-app': https://github.com/facebook/create-react-app/blob/c0b4173d1bba37fde2d699fe9914d16f823bcbe5/packages/react-scripts/config/webpack.config.js#L356

It seems that the property extends never exists on eslintConfig(??)

Deleting the above two lines yields the expected behavior (eslint-config-airbnb is picked up and we see lint errors / Failed to compile message)