Support eslint's new flat config

.github/workflows/nodejs.yml

@@ -19,7 +19,7 @@ jobs:
       fail-fast: false
       matrix:
         node-version: [12.x, 14.x, 16.x, 18.x, 19.x]
-        eslint-version: [7, 8]
+        eslint-version: [7, "8.40", 8]
         jest-version: [27, 28, 29]
         jest-watch-typeahead-version: [1, 2]
         exclude:

integrationTests/__fixtures__/flat-config/__eslint__/file.js

@@ -0,0 +1,3 @@
+const a = 1;
+
+console.log('a', a);

integrationTests/__fixtures__/flat-config/eslint.config.js

@@ -0,0 +1,7 @@
+module.exports = [
+  {
+    rules: {
+      'no-console': 'error',
+    },
+  },
+];

integrationTests/__fixtures__/flat-config/jest.config.js

@@ -0,0 +1,4 @@
+module.exports = {
+  runner: '../../../',
+  testMatch: ['**/__eslint__/**/*.js'],
+};

integrationTests/__fixtures__/legacy-config-at-eslint.config.js/__eslint__/file.js

@@ -0,0 +1,3 @@
+const a = 1;
+
+console.log('a', a);

integrationTests/__fixtures__/legacy-config-at-eslint.config.js/eslint.config.js

@@ -0,0 +1,5 @@
+module.exports = {
+  rules: {
+    'no-console': 'error',
+  },
+};

integrationTests/__fixtures__/legacy-config-at-eslint.config.js/jest-runner-eslint.config.js

@@ -0,0 +1,3 @@
+module.exports = {
+  config: './eslint.config.js',
+};

integrationTests/__fixtures__/legacy-config-at-eslint.config.js/jest.config.js

@@ -0,0 +1,4 @@
+module.exports = {
+  runner: '../../../',
+  testMatch: ['**/__eslint__/**/*.js'],
+};

integrationTests/__snapshots__/flat-config.test.js.snap

@@ -0,0 +1,19 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`Works with the new flat config format 1`] = `
+"FAIL __eslint__/file.js
+  ✕ no-console 
+
+
+/mocked-path-to-jest-runner-mocha/integrationTests/__fixtures__/flat-config/__eslint__/file.js
+  3:1  error  Unexpected console statement  no-console
+
+✖ 1 problem (1 error, 0 warnings)
+
+Test Suites: 1 failed, 1 total
+Tests:       1 failed, 1 total
+Snapshots:   0 total
+Time:        
+Ran all test suites.
+"
+`;

integrationTests/__snapshots__/legacy-config-at-eslint.config.js.test.js.snap

@@ -0,0 +1,22 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`Does not try to use flat config on eslint versions that don't support it 1`] = `
+"PASS __eslint__/file.js
+  ● Console
+
+  console.warn
+    
+    /mocked-path-to-jest-runner-mocha/integrationTests/__fixtures__/legacy-config-at-eslint.config.js/__eslint__/file.js
+      3:1  warning  Unexpected console statement  no-console
+    
+    ✖ 1 problem (0 errors, 1 warning)
+
+  ✓ ESLint 
+
+Test Suites: 1 passed, 1 total
+Tests:       1 passed, 1 total
+Snapshots:   0 total
+Time:        
+Ran all test suites.
+"
+`;

integrationTests/flat-config.test.js

@@ -0,0 +1,10 @@
+const { version } = require('eslint/package.json');
+const semver = require('semver');
+const runJest = require('./runJest');
+
+(semver.satisfies(version, '>=8.41') ? it : it.skip)(
+  'Works with the new flat config format',
+  async () => {
+    expect(await runJest('flat-config')).toMatchSnapshot();
+  },
+);

integrationTests/legacy-config-at-eslint.config.js.test.js

@@ -0,0 +1,12 @@
+const { version } = require('eslint/package.json');
+const semver = require('semver');
+const runJest = require('./runJest');
+
+(semver.satisfies(version, '<8.41') ? it : it.skip)(
+  "Does not try to use flat config on eslint versions that don't support it",
+  async () => {
+    expect(
+      await runJest('legacy-config-at-eslint.config.js'),
+    ).toMatchSnapshot();
+  },
+);

src/runner/runESLint.js

@@ -1,6 +1,32 @@
 const { ESLint } = require('eslint');
 const getESLintOptions = require('../utils/getESLintOptions');
 
+let FlatESLint;
+let shouldUseFlatConfig;
+
+try {
+  // Use a dynamic require here rather than a global require because this
+  // import path does not exist in eslint v7 which this library still
+  // supports
+  //
+  // ESlint exposes the new FlatESLint API under `eslint/use-at-your-own-risk` by
+  // using it's [export configuration](https://tinyurl.com/2s45zh9b).  However,
+  // the `import/no-unresolved` rule is [not aware of
+  // `exports`](https://tinyurl.com/469djpx3) and causes a false error here.  So,
+  // let's ignore that rule for this import.
+  //
+  // eslint-disable-next-line global-require, import/no-unresolved
+  const eslintExperimental = require('eslint/use-at-your-own-risk');
+  FlatESLint = eslintExperimental.FlatESLint;
+  shouldUseFlatConfig = eslintExperimental.shouldUseFlatConfig;
+} catch {
+  /* no-op */
+}
+
+if (shouldUseFlatConfig === undefined) {
+  shouldUseFlatConfig = () => Promise.resolve(false);
+}
+
 /*
  * This function exists because there are issues with the `pass`, `skip`, and
  * `fail` functions from `create-jest-runner`:
@@ -96,8 +122,41 @@ const getComputedFixValue = ({ fix, quiet, fixDryRun }) => {
   return undefined;
 };
 
+const getESLintConstructor = async () => {
+  if (await shouldUseFlatConfig()) {
+    return FlatESLint;
+  }
+
+  return ESLint;
+};
+
+// Remove options that are not constructor args.
+const getESLintConstructorArgs = async cliOptions => {
+  // these are not constructor args for either the legacy or the flat ESLint
+  // api
+  const { fixDryRun, format, maxWarnings, quiet, ...legacyConstructorArgs } =
+    cliOptions;
+
+  if (await shouldUseFlatConfig()) {
+    // these options are supported by the legacy ESLint api but aren't
+    // supported by the ESLintFlat api
+    const {
+      extensions,
+      ignorePath,
+      rulePaths,
+      resolvePluginsRelativeTo,
+      useEslintrc,
+      overrideConfig,
+      ...flatConstructorArgs
+    } = legacyConstructorArgs;
+    return flatConstructorArgs;
+  }
+
+  return legacyConstructorArgs;
+};
+
 let cachedValues;
-const getCachedValues = (config, extraOptions) => {
+const getCachedValues = async (config, extraOptions) => {
   if (!cachedValues) {
     const { cliOptions: baseCliOptions } = getESLintOptions(config);
     const cliOptions = {
@@ -106,20 +165,20 @@ const getCachedValues = (config, extraOptions) => {
       ...extraOptions,
     };
 
-    // these are not constructor args, so remove them
-    const { fixDryRun, format, maxWarnings, quiet, ...eslintOptions } =
-      cliOptions;
-
-    const cli = new ESLint(eslintOptions);
+    const ESLintConstructor = await getESLintConstructor();
+    const cli = new ESLintConstructor(
+      await getESLintConstructorArgs(cliOptions),
+    );
 
     cachedValues = {
       isPathIgnored: cli.isPathIgnored.bind(cli),
       lintFiles: (...args) => cli.lintFiles(...args),
       formatter: async (...args) => {
-        const formatter = await cli.loadFormatter(format);
+        const formatter = await cli.loadFormatter(cliOptions.format);
         return formatter.format(...args);
       },
       cliOptions,
+      ESLintConstructor,
     };
   }
 
@@ -139,10 +198,8 @@ const runESLint = async ({ testPath, config, extraOptions }) => {
     config.setupFilesAfterEnv.forEach(require);
   }
 
-  const { isPathIgnored, lintFiles, formatter, cliOptions } = getCachedValues(
-    config,
-    extraOptions,
-  );
+  const { isPathIgnored, lintFiles, formatter, cliOptions, ESLintConstructor } =
+    await getCachedValues(config, extraOptions);
 
   if (await isPathIgnored(testPath)) {
     return mkTestResults({
@@ -163,13 +220,13 @@ const runESLint = async ({ testPath, config, extraOptions }) => {
   const report = await lintFiles([testPath]);
 
   if (cliOptions.fix && !cliOptions.fixDryRun) {
-    await ESLint.outputFixes(report);
+    await ESLintConstructor.outputFixes(report);
   }
 
   const end = Date.now();
 
   const message = await formatter(
-    cliOptions.quiet ? ESLint.getErrorResults(report) : report,
+    cliOptions.quiet ? ESLintConstructor.getErrorResults(report) : report,
   );
 
   if (report[0]?.errorCount > 0) {