Use ESLint and Stylelint to check for potential errors and enforce a consistent coding style

* ESLint checks JS, JSON, and some CSS
* Stylelint checks CSS
* Stylistic plugins for both enforce coding style
* Adds GitHub workflow to run checks on push and PR
* Adds pre-commit hook to run checks that devs can install
* Update CONTRIBUTING.md

Author: AnSq

.github/workflows/lint.yml

@@ -0,0 +1,32 @@
+name: Lint
+
+on:
+  push:
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v6
+        with:
+          node-version: '24'
+          cache: 'npm'
+
+      - name: Install dependencies
+        run: npm install
+
+      - parallel:
+        - name: ESLint
+          run: npm run lint:js
+
+        - name: Stylelint
+          run: npm run lint:css

CONTRIBUTING.md

@@ -32,6 +32,15 @@ If you encounter a bug, graphical glitch, or something isn't working as expected
 
 **Create a Pull Request:** Do you have a feature you would like added, or a task that you would like added that could benefit other Tenno? Fork the repository, and create a pull request!
 
+### Coding Style
+
+This project uses [ESLint](https://eslint.org/) and [Stylelint](https://stylelint.io/) to enforce a consistent coding style.
+A GitHub workflow will check your code automatically when you push changes or create a pull request.
+You can also run the checks locally with `npm run lint`, and [a git pre-commit hook is provided](./pre-commit) that you can install.
+Extensions may also be available for your editor.
+
+### AI Policy
+
 You must disclose any use of AI that went in to preparing your pull request by **both**:
 
 1.  Marking the appropriate box in the pull request template, *and*

eslint.config.js

@@ -0,0 +1,134 @@
+import js from "@eslint/js";
+import globals from "globals";
+import json from "@eslint/json";
+import css from "@eslint/css";
+import { defineConfig } from "eslint/config";
+import stylistic from "@stylistic/eslint-plugin";
+
+export default defineConfig([
+    { ignores: [".coverage/**/*"] },
+    {
+        linterOptions: {
+            reportUnusedInlineConfigs: "warn",
+        },
+    },
+    {
+        files: ["sources/**/*.{js,mjs,cjs}", "*.config.*js"],
+        plugins: { js },
+        languageOptions: {
+            globals: globals.browser,
+        },
+        extends: [
+            "js/recommended",
+            stylistic.configs.customize({
+                indent: 4,
+                semi: true,
+                quotes: "double",
+                jsx: false,
+                arrowParens: true,
+                quoteProps: "consistent",
+            }),
+        ],
+        rules: {
+            // "important" stuff
+            "curly": ["error", "all"],
+            "eqeqeq": ["error", "always"],
+            "no-var": "error",
+            "prefer-const": "error",
+            "radix": "error",
+            // niche stuff
+            "array-callback-return": "error",
+            "no-duplicate-imports": "error",
+            "no-self-compare": "error",
+            "no-template-curly-in-string": "warn",
+            "no-unreachable-loop": "error",
+            "no-use-before-define": ["error", "nofunc"],
+            "block-scoped-var": "error",
+            "consistent-return": "error",
+            "default-param-last": "error",
+            "dot-notation": "warn",
+            "func-style": ["error", "declaration"],
+            "no-empty-function": "error",
+            "no-extend-native": "error",
+            "no-labels": "error",
+            "no-implicit-coercion": "error",
+            "no-lone-blocks": "error",
+            "no-lonely-if": "error",
+            "no-new-wrappers": "error",
+            "no-object-constructor": "error",
+            "no-proto": "error",
+            "no-return-assign": ["error", "always"],
+            "no-sequences": ["error", { "allowInParentheses": false }],
+            "no-shadow": "error",
+            "no-throw-literal": "error",
+            "no-unused-expressions": "error",
+            "no-useless-concat": "error",
+            "no-useless-rename": "error",
+            "no-void": "error",
+            "prefer-arrow-callback": "error",
+            "prefer-numeric-literals": "error",
+            "unicode-bom": "error",
+            // eval
+            "no-eval": "error",
+            "no-implied-eval": "error",
+            "no-new-func": "error",
+            "no-script-url": "error",
+            // complexity
+            "complexity": ["warn", 20],
+            "id-length": ["warn", { "min": 1, "max": 30 }],
+            "max-depth": ["warn", 4],
+            "max-lines-per-function": ["warn", {
+                "max": 60,
+                "skipBlankLines": true,
+                "skipComments": true,
+            }],
+            "max-nested-callbacks": ["warn", 4],
+            "max-params": ["warn", 4],
+            // stylistic
+            "@stylistic/quotes": ["error", "double", { "avoidEscape": true }],
+            "@stylistic/brace-style": ["error", "1tbs", { "allowSingleLine": true }],
+            "@stylistic/no-multiple-empty-lines": ["error", { "max": 2 }],
+            "@stylistic/max-statements-per-line": ["error", { "max": 2 }],
+            "@stylistic/object-curly-spacing": ["error", "always", { "emptyObjects": "never" }],
+        },
+    },
+    {
+        files: ["sources/tests/*.js", "*.config.*js"],
+        languageOptions: {
+            globals: globals.node,
+        },
+    },
+    {
+        files: ["sources/**/*.json"],
+        plugins: { json },
+        language: "json/json",
+        extends: ["json/recommended"],
+    },
+    {
+        files: ["sources/**/*.css"],
+        plugins: { css },
+        language: "css/css",
+        extends: ["css/recommended"],
+        languageOptions: {
+            tolerant: true,
+        },
+        rules: {
+            /* allowUnknownVariables suppresses errors from variables defined in other files.
+            Stylelint does proper checking for this kind of error with its `referenceFiles` feature. */
+            "css/no-invalid-properties": ["error", { "allowUnknownVariables": true }],
+            "css/use-baseline": ["warn", {
+                "available": "widely",
+                "allowProperties": [
+                    "backdrop-filter", // baseline 2024
+                ],
+                "allowPropertyValues": {
+                    "background-attachment": ["fixed"], // only unavailable in iOS
+                },
+                "allowAtRules": [
+                    "starting-style", // baseline 2024
+                ],
+            }],
+            "css/no-important": "warn",
+        },
+    },
+]);