Use native ESLint behaviour when extending (#8276)

Author: mrmckeb

docusaurus/docs/advanced-configuration.md

@@ -25,5 +25,5 @@ You can adjust various development and production settings by setting environmen
 | NODE_PATH               |   ✅ Used   |  ✅ Used   | Same as [`NODE_PATH` in Node.js](https://nodejs.org/api/modules.html#modules_loading_from_the_global_folders), but only relative folders are allowed. Can be handy for emulating a monorepo setup by setting `NODE_PATH=src`.                                                                                                                                                                                                                                                                                                                                                                                                                                            |
 | INLINE_RUNTIME_CHUNK    | 🚫 Ignored  |  ✅ Used   | By default, Create React App will embed the runtime script into `index.html` during the production build. When set to `false`, the script will not be embedded and will be imported as usual. This is normally required when dealing with CSP.                                                                                                                                                                                                                                                                                                                                                                                                                           |
 | IMAGE_INLINE_SIZE_LIMIT | 🚫 Ignored  |  ✅ Used   | By default, images smaller than 10,000 bytes are encoded as a data URI in base64 and inlined in the CSS or JS build artifact. Set this to control the size limit in bytes. Setting it to 0 will disable the inlining of images.                                                                                                                                                                                                                                                                                                                                                                                                                                          |
-| EXTEND_ESLINT           |   ✅ Used   |  ✅ Used   | When set to `true`, ESLint configs that extend `eslint-config-react-app` will be used by `eslint-loader`. Any rules that are set to `"error"` will stop the application from building.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
+| EXTEND_ESLINT           |   ✅ Used   |  ✅ Used   | When set to `true`, user provided ESLint configs will be used by `eslint-loader`. Note that any rules set to `"error"` will stop the application from building.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
 | TSC_COMPILE_ON_ERROR    |   ✅ Used   |  ✅ Used   | When set to `true`, you can run and properly build TypeScript projects even if there are TypeScript type check errors. These errors are printed as warnings in the terminal and/or browser console.                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |

docusaurus/docs/setting-up-your-editor.md

@@ -35,7 +35,6 @@ Note that any rules set to `"error"` will stop the project from building.
 There are a few things to remember:
 
 1. We highly recommend extending the base config, as removing it could introduce hard-to-find issues.
-1. `.eslintignore` files will be respected
 1. When working with TypeScript, you'll need to provide an `overrides` object for rules that should _only_ target TypeScript files.
 
 In the below example:

packages/react-scripts/config/webpack.config.js

@@ -33,7 +33,6 @@ const ModuleNotFoundPlugin = require('react-dev-utils/ModuleNotFoundPlugin');
 const ForkTsCheckerWebpackPlugin = require('react-dev-utils/ForkTsCheckerWebpackPlugin');
 const typescriptFormatter = require('react-dev-utils/typescriptFormatter');
 // @remove-on-eject-begin
-const eslint = require('eslint');
 const getCacheIdentifier = require('react-dev-utils/getCacheIdentifier');
 // @remove-on-eject-end
 const postcssNormalize = require('postcss-normalize');
@@ -46,6 +45,8 @@ const shouldUseSourceMap = process.env.GENERATE_SOURCEMAP !== 'false';
 // makes for a smoother build process.
 const shouldInlineRuntimeChunk = process.env.INLINE_RUNTIME_CHUNK !== 'false';
 
+const isExtendingEslintConfig = process.env.EXTEND_ESLINT === 'true';
+
 const imageInlineSizeLimit = parseInt(
   process.env.IMAGE_INLINE_SIZE_LIMIT || '10000'
 );
@@ -346,28 +347,13 @@ module.exports = function(webpackEnv) {
                 eslintPath: require.resolve('eslint'),
                 resolvePluginsRelativeTo: __dirname,
                 // @remove-on-eject-begin
-                ignore: process.env.EXTEND_ESLINT === 'true',
-                baseConfig: (() => {
-                  // We allow overriding the config only if the env variable is set
-                  if (process.env.EXTEND_ESLINT === 'true') {
-                    const eslintCli = new eslint.CLIEngine();
-                    let eslintConfig;
-                    try {
-                      eslintConfig = eslintCli.getConfigForFile(
-                        paths.appIndexJs
-                      );
-                    } catch (e) {
-                      console.error(e);
-                      process.exit(1);
-                    }
-                    return eslintConfig;
-                  } else {
-                    return {
+                ignore: isExtendingEslintConfig,
+                baseConfig: isExtendingEslintConfig
+                  ? undefined
+                  : {
                       extends: [require.resolve('eslint-config-react-app')],
-                    };
-                  }
-                })(),
-                useEslintrc: false,
+                    },
+                useEslintrc: isExtendingEslintConfig,
                 // @remove-on-eject-end
               },
               loader: require.resolve('eslint-loader'),