build(eslint-config-fluid): remove ESLint 8/9 compatibility layer (#26272)

Author: tylerbutler

common/build/eslint-config-fluid/.eslint-print-configs/README.md

@@ -19,7 +19,7 @@
 		"format": "npm run prettier:fix",
 		"prettier": "prettier --check . --cache --ignore-path ../../../.prettierignore",
 		"prettier:fix": "prettier --write . --cache --ignore-path ../../../.prettierignore",
-		"print-configs": "cross-env ESLINT_USE_FLAT_CONFIG=true tsx scripts/print-configs.ts printed-configs",
+		"print-configs": "tsx scripts/print-configs.ts printed-configs",
 		"test": "echo TODO: add tests"
 	},
 	"dependencies": {
@@ -47,7 +47,6 @@
 		"@fluidframework/build-common": "^2.0.3",
 		"@types/sort-json": "^2.0.3",
 		"concurrently": "^9.2.1",
-		"cross-env": "^10.1.0",
 		"eslint": "~9.39.1",
 		"jiti": "^2.6.1",
 		"mocha-multi-reporters": "^1.5.1",

common/build/eslint-config-fluid/.eslint-print-configs/minimal.mjs

@@ -16,108 +16,89 @@ import fs from "node:fs/promises";
 import path from "node:path";
 import { fileURLToPath } from "node:url";
 
-import { loadESLint } from "eslint";
+import { ESLint } from "eslint";
+import type { Linter } from "eslint";
 import sortJson from "sort-json";
 
+// Import flat configs directly from flat.mjs
+import { recommended, strict, minimalDeprecated } from "../flat.mjs";
+
 const __filename = fileURLToPath(import.meta.url);
 const __dirname = path.dirname(__filename);
 
-// Determine which config files to use based on ESLINT_USE_FLAT_CONFIG
-// NOTE: loadESLint() returns the appropriate ESLint class based on ESLINT_USE_FLAT_CONFIG,
-// but we still need to manually determine which config file format to use.
-// Legacy ESLint cannot read flat config files (.mjs), and FlatESLint cannot read legacy config files (.js/.cjs).
-const useFlatConfig = process.env.ESLINT_USE_FLAT_CONFIG === "true";
-
-// During the hybrid ESLint 8/9 migration, we need to maintain both legacy (.js/.cjs) and flat (.mjs) configs.
-// The .eslint-print-configs directory contains standalone flat configs that can be loaded by ESLint 9
-// without requiring the legacy config infrastructure.
-const configDir = useFlatConfig ? "../.eslint-print-configs" : "..";
-
-// File extensions differ between legacy (.js) and flat (.mjs) config formats.
-// Using .mjs for flat configs ensures they're treated as ES modules.
-const configExt = useFlatConfig ? ".mjs" : ".js";
+type FlatConfigArray = Linter.Config[];
 
 interface ConfigToPrint {
 	name: string;
-	configPath: string;
+	config: FlatConfigArray;
 	sourceFilePath: string;
 }
 
-// Legacy configs use "index.js" for default and "minimal-deprecated.js" for minimal.
-// Flat configs use "recommended.mjs" for default and "minimal.mjs" for minimal.
-const defaultConfigFile = useFlatConfig ? "recommended.mjs" : "index.js";
-const minimalConfigFile = `minimal${useFlatConfig ? "" : "-deprecated"}${configExt}`;
-const strictBiomeConfigFile = `strict${useFlatConfig ? "" : "-biome"}${configExt}`;
-
 const configsToPrint: ConfigToPrint[] = [
 	{
 		name: "default",
-		configPath: path.join(__dirname, configDir, defaultConfigFile),
+		config: recommended,
 		sourceFilePath: path.join(__dirname, "..", "src", "file.ts"),
 	},
 	{
 		name: "minimal",
-		configPath: path.join(__dirname, configDir, minimalConfigFile),
+		config: minimalDeprecated,
 		sourceFilePath: path.join(__dirname, "..", "src", "file.ts"),
 	},
 	{
 		name: "react",
-		configPath: path.join(__dirname, configDir, defaultConfigFile),
+		config: recommended,
 		sourceFilePath: path.join(__dirname, "..", "src", "file.tsx"),
 	},
 	{
 		name: "recommended",
-		configPath: path.join(__dirname, configDir, `recommended${configExt}`),
+		config: recommended,
 		sourceFilePath: path.join(__dirname, "..", "src", "file.ts"),
 	},
 	{
 		name: "strict",
-		configPath: path.join(__dirname, configDir, `strict${configExt}`),
+		config: strict,
 		sourceFilePath: path.join(__dirname, "..", "src", "file.ts"),
 	},
 	{
 		name: "strict-biome",
-		configPath: path.join(__dirname, configDir, strictBiomeConfigFile),
+		// strict-biome uses the same flat config as strict; biome integration is handled separately
+		config: strict,
 		sourceFilePath: path.join(__dirname, "..", "src", "file.ts"),
 	},
 	{
 		name: "test",
-		configPath: path.join(__dirname, configDir, `recommended${configExt}`),
+		config: recommended,
 		sourceFilePath: path.join(__dirname, "..", "src", "test", "file.ts"),
 	},
 ];
 
 /**
- * Generates the applied ESLint config for a specific file and config path.
+ * Generates the applied ESLint config for a specific file and config.
  */
-async function generateConfig(filePath: string, configPath: string): Promise<string> {
-	console.log(`Generating config for ${filePath} using ${configPath}`);
+async function generateConfig(filePath: string, config: FlatConfigArray): Promise<string> {
+	console.log(`Generating config for ${filePath}`);
 
-	// loadESLint() respects ESLINT_USE_FLAT_CONFIG and returns the appropriate ESLint class.
-	// However, it's the caller's responsibility to provide config files in the correct format.
-	const ESLint = await loadESLint();
+	// ESLint 9's default ESLint class uses flat config format.
+	// Use overrideConfigFile: true to prevent loading eslint.config.js,
+	// and pass the config directly via overrideConfig.
 	const eslint = new ESLint({
-		overrideConfigFile: configPath,
+		overrideConfigFile: true,
+		overrideConfig: config,
 	});
 
-	const config = (await eslint.calculateConfigForFile(filePath)) as unknown;
-	if (!config) {
+	const resolvedConfig = (await eslint.calculateConfigForFile(filePath)) as unknown;
+	if (!resolvedConfig) {
 		console.warn("Warning: ESLint returned undefined config for " + filePath);
 		return "{}\n";
 	}
 
 	// Serialize and parse to create a clean copy without any circular references or non-serializable values
-	const cleanConfig = JSON.parse(JSON.stringify(config));
+	const cleanConfig = JSON.parse(JSON.stringify(resolvedConfig));
 
-	// Remove properties that contain environment-specific paths
-	if (useFlatConfig) {
-		// For flat configs, remove languageOptions which has environment-specific paths and large globals
-		if (cleanConfig.languageOptions) {
-			delete cleanConfig.languageOptions;
-		}
-	} else {
-		// For legacy configs, remove the parser property
-		delete cleanConfig.parser;
+	// Remove languageOptions which contains environment-specific paths and large globals
+	if (cleanConfig.languageOptions) {
+		delete cleanConfig.languageOptions;
 	}
 
 	// Convert numeric severities to string equivalents in rules
@@ -162,7 +143,7 @@ async function generateConfig(filePath: string, configPath: string): Promise<str
 	await fs.mkdir(outputPath, { recursive: true });
 	const expectedFiles = new Set<string>();
 
-	for (const { name, configPath, sourceFilePath } of configsToPrint) {
+	for (const { name, config, sourceFilePath } of configsToPrint) {
 		const outputFilePath = path.join(outputPath, `${name}.json`);
 		expectedFiles.add(`${name}.json`);
 
@@ -173,7 +154,7 @@ async function generateConfig(filePath: string, configPath: string): Promise<str
 			// File doesn't exist yet, which is OK - we'll create it
 		}
 
-		const newContent = await generateConfig(sourceFilePath, configPath);
+		const newContent = await generateConfig(sourceFilePath, config);
 
 		// Only write the file if the content has changed
 		if (newContent !== originalContent) {