In ```webpack.config.prod.js``` L391 L405 L420 L437, ```loader```is used instead of ```use```. See: [webpack.config.prod.js](https://github.com/facebook/create-react-app/blob/77f8df7ad73daced49b20a7b59776b90d311b5ff/packages/react-scripts/config/webpack.config.prod.js#L391) . In file ```webpack.config.dev.js``` L318 L326 L340 L346 , it's correct usage ```use```. See: [webpack.config.dev.js](https://github.com/facebook/create-react-app/blob/77f8df7ad73daced49b20a7b59776b90d311b5ff/packages/react-scripts/config/webpack.config.dev.js#L340) Webpack4 [documentation](https://webpack.js.org/configuration/module/#rule-loader) here: ``` Rule.loader is a shortcut to Rule.use: [ { loader } ]. See Rule.use and UseEntry.loader for details. ```