chore: migrate to ESLint v9 and Flat Config (#1753)

Upgraded eslint to v9 and replaced the legacy .eslintrc.js with the new eslint.config.js format.

Applied minor fixes to 4 files (renaming unused catch variables to _e and one small refactor) to satisfy slightly stricter baseline rules from updated plugins.

Mostly doing this due to our upgrade in TS to latest v5 version which the old versions of eslint ts plugins no longer support.

Author: taeold

.eslintignore

@@ -0,0 +1,110 @@
+const { FlatCompat } = require("@eslint/eslintrc");
+const js = require("@eslint/js");
+const path = require("path");
+
+const compat = new FlatCompat({
+    baseDirectory: __dirname,
+    recommendedConfig: js.configs.recommended,
+    allConfig: js.configs.all
+});
+
+module.exports = [
+    {
+        ignores: [
+            "lib/",
+            "dev/",
+            "node_modules/",
+            "coverage/",
+            "docgen/",
+            "v1/",
+            "v2/",
+            "logger/",
+            "dist/",
+            "spec/fixtures/",
+            "scripts/**/*.js",
+            "protos/",
+            ".prettierrc.js",
+            "eslint.config.js",
+            "scripts/bin-test/sources/esm-ext/index.mjs",
+        ],
+    },
+    ...compat.extends(
+        "eslint:recommended",
+        "plugin:@typescript-eslint/recommended",
+        "plugin:@typescript-eslint/recommended-requiring-type-checking",
+        "plugin:jsdoc/recommended",
+        "google",
+        "prettier"
+    ),
+    {
+        languageOptions: {
+            parser: require("@typescript-eslint/parser"),
+            parserOptions: {
+                project: "tsconfig.json",
+                tsconfigRootDir: __dirname,
+            },
+            ecmaVersion: 2022
+        },
+        plugins: {
+            "prettier": require("eslint-plugin-prettier"),
+        },
+        rules: {
+            "jsdoc/newline-after-description": "off",
+            "jsdoc/require-jsdoc": ["warn", { publicOnly: true }],
+            "jsdoc/check-tag-names": ["warn", { definedTags: ["alpha", "remarks", "typeParam", "packageDocumentation", "hidden"] }],
+            "no-restricted-globals": ["error", "name", "length"],
+            "prefer-arrow-callback": "error",
+            "prettier/prettier": "off",
+            "require-atomic-updates": "off", // This rule is so noisy and isn't useful: https://github.com/eslint/eslint/issues/11899
+            "require-jsdoc": "off", // This rule is deprecated and superseded by jsdoc/require-jsdoc.
+            "valid-jsdoc": "off", // This is deprecated but included in recommended configs.
+            "no-prototype-builtins": "warn",
+            "no-useless-escape": "warn",
+            "prefer-promise-reject-errors": "warn",
+        },
+    },
+    {
+        files: ["**/*.ts"],
+        rules: {
+            "jsdoc/require-param-type": "off",
+            "jsdoc/require-returns-type": "off",
+            // Google style guide allows us to omit trivial parameters and returns
+            "jsdoc/require-param": "off",
+            "jsdoc/require-returns": "off",
+
+            "@typescript-eslint/no-invalid-this": "error",
+            "@typescript-eslint/no-unused-vars": ["error", { argsIgnorePattern: "^_", caughtErrorsIgnorePattern: "^_" }], // Unused vars should not exist.
+            "@typescript-eslint/no-misused-promises": "warn", // rule does not work with async handlers for express.
+            "no-invalid-this": "off", // Turned off in favor of @typescript-eslint/no-invalid-this.
+            "no-unused-vars": "off", // Off in favor of @typescript-eslint/no-unused-vars.
+            eqeqeq: ["error", "always", { null: "ignore" }],
+            camelcase: ["error", { properties: "never" }], // snake_case allowed in properties iif to satisfy an external contract / style
+
+            // Ideally, all these warning should be error - let's fix them in the future.
+            "@typescript-eslint/no-unsafe-argument": "warn",
+            "@typescript-eslint/no-unsafe-assignment": "warn",
+            "@typescript-eslint/no-unsafe-call": "warn",
+            "@typescript-eslint/no-unsafe-member-access": "warn",
+            "@typescript-eslint/no-unsafe-return": "warn",
+            "@typescript-eslint/restrict-template-expressions": "warn",
+            "@typescript-eslint/no-explicit-any": "warn",
+            "@typescript-eslint/no-redundant-type-constituents": "warn",
+            "@typescript-eslint/no-base-to-string": "warn",
+            "@typescript-eslint/no-duplicate-type-constituents": "warn",
+            "@typescript-eslint/no-require-imports": "warn",
+            "@typescript-eslint/no-empty-object-type": "warn",
+            "@typescript-eslint/prefer-promise-reject-errors": "warn",
+        },
+    },
+    {
+        files: ["**/*.spec.ts", "**/*.spec.js", "spec/helper.ts", "scripts/bin-test/**/*.ts", "integration_test/**/*.ts"],
+        languageOptions: {
+            globals: {
+                mocha: true,
+            },
+        },
+        rules: {
+            "@typescript-eslint/no-unused-expressions": "off",
+        }
+    },
+];

.eslintrc.js

@@ -13,8 +13,8 @@ export const pubsubTests: any = functions
     let testId: string;
     try {
       testId = m.json.testId;
-    } catch (e) {
-      /* Ignored. Covered in another test case that `event.data.json` works. */
+    } catch (_e) {
+      // Ignored. Covered in another test case that `event.data.json` works.
     }
 
     return new TestSuite<PubsubMessage>("pubsub onPublish")