chore: bump eslint to 10.3.0 and migrate to flat config

eslint.config.mjs

@@ -0,0 +1,153 @@
+/*!
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { defineConfig } from 'eslint/config';
+import eslint from '@eslint/js';
+import tseslint from 'typescript-eslint';
+import globals from 'globals';
+
+export default defineConfig([
+  eslint.configs.recommended,
+  ...tseslint.configs.recommended,
+  {
+    files: ['src/**/*.ts', 'test/**/*.ts'],
+    languageOptions: {
+      globals: {
+        ...globals.node,
+      },
+    },
+    rules: {
+      // Following checks are temporarily disabled. We shall incrementally enable them in the
+      // future, fixing any violations as we go.
+      '@typescript-eslint/no-non-null-assertion': 0,
+
+      // Disabled checks
+      '@typescript-eslint/no-explicit-any': 0,
+      '@typescript-eslint/no-use-before-define': 0,
+      '@typescript-eslint/no-var-requires': 0,
+      '@typescript-eslint/no-require-imports': 0,
+      '@typescript-eslint/no-unused-expressions': 0,
+
+      // Required checks
+      'indent': ['error', 2],
+      'keyword-spacing': ['error'],
+      'max-len': [
+        'error',
+        {
+          'code': 120,
+          'ignoreUrls': true
+        }
+      ],
+      'object-curly-spacing': [2, 'always'],
+      '@typescript-eslint/explicit-function-return-type': [
+        'error',
+        {
+          'allowExpressions': true,
+          'allowTypedFunctionExpressions': true,
+          'allowHigherOrderFunctions': true
+        }
+      ],
+      'no-unused-vars': 'off', // Must be disabled to enable the next rule
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        {
+          'varsIgnorePattern': '^_',
+          'argsIgnorePattern': '^_',
+          'caughtErrors': 'none'
+        }
+      ],
+      'quotes': ['error', 'single', {'avoidEscape': true}],
+      '@typescript-eslint/naming-convention': [
+        'error',
+        {
+          "selector": "variable",
+          "format": ["camelCase", "UPPER_CASE"]
+        },
+        {
+          "selector": "parameter",
+          "format": ["camelCase"],
+          "leadingUnderscore": "allow"
+        },
+
+        {
+          "selector": "memberLike",
+          "format": ["camelCase"]
+        },
+
+        {
+          "selector": "typeLike",
+          "format": ["PascalCase"]
+        },
+
+        // Ignore properties that require quotes (HTTP headers, names that include spaces or dashes etc.).
+        {
+          "selector": [
+            "classProperty",
+            "objectLiteralProperty",
+            "typeProperty",
+            "classMethod",
+            "objectLiteralMethod",
+            "typeMethod",
+            "accessor",
+            "enumMember"
+          ],
+          "format": null,
+          "modifiers": ["requiresQuotes"]
+        },
+
+        // Ignore destructured property names.
+        {
+          "selector": "variable",
+          "modifiers": ["destructured"],
+          "format": null
+        },
+
+        // Following types are temporarily disabled. We shall incrementally enable them in the
+        // future, fixing any violations as we go.
+        {
+          "selector": [
+            "classProperty",
+            "objectLiteralProperty",
+            "typeProperty",
+            "enumMember"
+          ],
+          "format": null
+        }
+      ],
+    }
+  },
+  {
+    files: ['test/**/*.ts'],
+    languageOptions: {
+      globals: {
+        ...globals.mocha,
+      },
+    },
+  },
+  {
+    ignores: [
+      'lib/',
+      'node_modules/',
+      'temp/',
+      'docgen/',
+      'dist/',
+      'coverage/',
+      '**/*.d.ts',
+      '**/*.js',
+      'firebase-admin-*.tgz'
+    ]
+  }
+]);