feat: migrate to ESLint v9 with flat config (#4508)

Author: quiet-node

.eslintignore

@@ -0,0 +1,111 @@
+// SPDX-License-Identifier: Apache-2.0
+
+import { defineConfig } from 'eslint/config';
+import js from '@eslint/js';
+import tsPlugin from '@typescript-eslint/eslint-plugin';
+import tsParser from '@typescript-eslint/parser';
+import prettierConfig from 'eslint-config-prettier';
+import simpleImportSort from 'eslint-plugin-simple-import-sort';
+import globals from 'globals';
+
+// Custom inline plugin for header checking
+const headerRule = {
+  create(context) {
+    return {
+      Program(node) {
+        const comments = context.sourceCode.getAllComments();
+        const hasHeader = comments[0]?.value.trim() === 'SPDX-License-Identifier: Apache-2.0';
+
+        if (!hasHeader) {
+          context.report({ node, message: 'Missing SPDX license header' });
+        }
+      },
+    };
+  },
+};
+
+export default defineConfig([
+  // Global ignores
+  {
+    ignores: ['**/node_modules/**', '**/dist/**'],
+  },
+
+  // Base recommended configs
+  js.configs.recommended,
+
+  // Main configuration for all JS/TS files
+  {
+    files: ['**/*.js', '**/*.mjs', '**/*.cjs', '**/*.ts'],
+    plugins: {
+      '@typescript-eslint': tsPlugin,
+      'simple-import-sort': simpleImportSort,
+      local: {
+        rules: {
+          header: headerRule,
+        },
+      },
+    },
+    languageOptions: {
+      ecmaVersion: 'latest',
+      sourceType: 'module',
+      parser: tsParser,
+      globals: {
+        ...globals.node,
+        ...globals.es2021,
+        __ENV: 'readonly',
+        NodeJS: 'readonly',
+      },
+    },
+    rules: {
+      // Merge recommended TypeScript rules
+      ...tsPlugin.configs.recommended.rules,
+      // Merge prettier config rules (disables conflicting rules)
+      ...prettierConfig.rules,
+      // Custom rules
+      '@typescript-eslint/ban-ts-comment': 'off',
+      '@typescript-eslint/no-explicit-any': 'off',
+      '@typescript-eslint/no-var-requires': 'warn',
+      '@typescript-eslint/no-unused-vars': 'warn',
+      'no-trailing-spaces': 'error',
+      'no-useless-escape': 'warn',
+      'prefer-const': 'error',
+      'comma-dangle': [
+        'error',
+        {
+          arrays: 'always-multiline',
+          objects: 'always-multiline',
+          imports: 'always-multiline',
+          exports: 'always-multiline',
+          functions: 'always-multiline',
+        },
+      ],
+      'simple-import-sort/imports': 'error',
+      'simple-import-sort/exports': 'off',
+      'local/header': 'error',
+    },
+  },
+
+  // Config for eslint config files themselves
+  {
+    files: ['eslint.config.js', '.eslintrc.{js,cjs}'],
+    languageOptions: {
+      sourceType: 'script',
+      globals: {
+        ...globals.node,
+      },
+    },
+  },
+
+  // Test files - relaxed rules
+  {
+    files: ['**/*.spec.ts', '**/*.test.ts', '**/tests/**/*.ts'],
+    languageOptions: {
+      globals: {
+        ...globals.mocha,
+      },
+    },
+    rules: {
+      '@typescript-eslint/no-unused-expressions': 'off',
+    },
+  },
+]);

.eslintrc.js

@@ -2,6 +2,7 @@
   "name": "root",
   "version": "0.73.0-SNAPSHOT",
   "devDependencies": {
+    "@eslint/js": "^9.38.0",
     "@hashgraph/hedera-local": "^2.39.0",
     "@open-rpc/schema-utils-js": "^2.1.2",
     "@types/chai-as-promised": "^8.0.2",
@@ -20,9 +21,9 @@
     "chai-exclude": "^2.1.1",
     "eslint": "^9.34.0",
     "eslint-config-prettier": "^10.1.8",
-    "eslint-plugin-header": "^3.1.1",
     "eslint-plugin-n": "^17.21.3",
     "eslint-plugin-simple-import-sort": "^12.1.1",
+    "globals": "^16.4.0",
     "husky": "^9.1.7",
     "lint-staged": "^16.1.6",
     "mocha": "^11.7.1",