Allow extending config used in eslint-loader (facebook#7036)

Author: mrmckeb

config/webpack.config.js

@@ -33,6 +33,7 @@ const getClientEnvironment = require('./env');
 const ModuleNotFoundPlugin = require('react-dev-utils/ModuleNotFoundPlugin');
 const ForkTsCheckerWebpackPlugin = require('react-dev-utils/ForkTsCheckerWebpackPlugin');
 const typescriptFormatter = require('react-dev-utils/typescriptFormatter');
+const eslint = require('eslint');
 // @remove-on-eject-begin
 const getCacheIdentifier = require('react-dev-utils/getCacheIdentifier');
 // @remove-on-eject-end
@@ -323,9 +324,30 @@ module.exports = function(webpackEnv) {
                 formatter: require.resolve('react-dev-utils/eslintFormatter'),
                 eslintPath: require.resolve('eslint'),
                 // @remove-on-eject-begin
-                baseConfig: {
-                  extends: [require.resolve('eslint-config-react-app')],
-                },
+                baseConfig: (() => {
+                  const eslintCli = new eslint.CLIEngine();
+                  let eslintConfig;
+                  try {
+                    eslintConfig = eslintCli.getConfigForFile(paths.appIndexJs);
+                  } catch (e) {
+                    // A config couldn't be found.
+                  }
+
+                  // We allow overriding the config, only if it extends our config
+                  // (`extends` can be a string or array of strings).
+                  if (
+                    process.env.EXTEND_ESLINT &&
+                    eslintConfig &&
+                    eslintConfig.extends &&
+                    eslintConfig.extends.includes('react-app')
+                  ) {
+                    return eslintConfig;
+                  } else {
+                    return {
+                      extends: [require.resolve('eslint-config-react-app')],
+                    };
+                  }
+                })(),
                 ignore: false,
                 useEslintrc: false,
                 // @remove-on-eject-end

scripts/eject.js

@@ -239,10 +239,12 @@ inquirer
     };
 
     // Add ESlint config
-    console.log(`  Adding ${cyan('ESLint')} configuration`);
-    appPackage.eslintConfig = {
-      extends: 'react-app',
-    };
+    if (!appPackage.eslintConfig) {
+      console.log(`  Adding ${cyan('ESLint')} configuration`);
+      appPackage.eslintConfig = {
+        extends: 'react-app',
+      };
+    }
 
     fs.writeFileSync(
       path.join(appPath, 'package.json'),