eslint.config.mjs

import { fileURLToPath } from 'node:url';

import { includeIgnoreFile } from '@eslint/compat';
import js from '@eslint/js';
import prettier from 'eslint-config-prettier';
import preferLet from 'eslint-plugin-prefer-let';
import storybook from 'eslint-plugin-storybook';
import svelte from 'eslint-plugin-svelte';
import eslintPluginUnicorn from 'eslint-plugin-unicorn';
import { defineConfig } from 'eslint/config';
import globals from 'globals';
import ts from 'typescript-eslint';

import svelteConfig from './svelte/svelte.config.js';

const gitignorePath = fileURLToPath(new URL('.gitignore', import.meta.url));

export default defineConfig(
  {
    languageOptions: {
      globals: globals.builtin,
    },
    plugins: {
      unicorn: eslintPluginUnicorn,
    },
    rules: unicornRules(),
  },
  includeIgnoreFile(gitignorePath),
  // `crates/` (Rust workspace crates) is not in `.gitignore` but should not be
  // linted. `svelte/static/` contains vendored files like `mockServiceWorker.js`
  // (auto-generated by MSW) that should not be linted either.
  { ignores: ['crates/', 'svelte/static/'] },
  js.configs.recommended,
  ...ts.configs.recommended,
  ...svelte.configs.recommended,
  ...svelte.configs.prettier,
  ...storybook.configs['flat/recommended'],
  prettier,
  {
    plugins: {
      'prefer-let': preferLet,
    },

    languageOptions: {
      globals: {
        ...globals.browser,
        ...globals.node,
        // Defined at build time by Vite (see `vite.config.ts`) and declared
        // ambiently in `src/app.d.ts`. ESLint's `no-undef` rule does not see
        // ambient `.d.ts` declarations, so we have to spell it out here.
        __TEST__: 'readonly',
      },

      parserOptions: {
        tsconfigRootDir: fileURLToPath(new URL('.', import.meta.url)),
      },
    },

    rules: {
      // it's fine to use `return` without a value and rely on the implicit `undefined` return value
      'getter-return': 'off',

      'prefer-const': 'off',
      'prefer-let/prefer-let': 'error',
    },
  },
  {
    files: ['**/*.svelte', '**/*.svelte.ts', '**/*.svelte.js'],

    languageOptions: {
      parserOptions: {
        projectService: true,
        extraFileExtensions: ['.svelte'],
        parser: ts.parser,
        svelteConfig,
        tsconfigRootDir: fileURLToPath(new URL('svelte', import.meta.url)),
      },
    },
  },
);

/**
 * Returns the Unicorn rules that are appropriate for crates.io.
 *
 * Even Unicorn's `unopinionated` preset enables many rules that encode style
 * preferences or otherwise do not fit this project. Defining the rules here
 * lets us enable only checks that we consider relevant, and prevents dependency
 * upgrades from silently changing the project's lint policy. New Unicorn rules
 * should therefore be reviewed and added deliberately instead of inherited
 * through a built-in preset.
 */
function unicornRules() {
  return {
    'unicorn/better-dom-traversing': 'error',
    'unicorn/consistent-compound-words': 'error',
    'unicorn/consistent-date-clone': 'error',
    'unicorn/consistent-existence-index-check': 'error',
    'unicorn/dom-node-dataset': 'error',
    'unicorn/error-message': 'error',
    'unicorn/escape-case': 'error',
    'unicorn/expiring-todo-comments': 'error',
    'unicorn/explicit-length-check': ['error', { 'non-zero': 'not-equal' }],
    'unicorn/import-style': 'error',
    'unicorn/new-for-builtins': 'error',
    'unicorn/no-abusive-eslint-disable': 'error',
    'unicorn/no-accessor-recursion': 'error',
    'unicorn/no-anonymous-default-export': 'error',
    'unicorn/no-array-fill-with-reference-type': 'error',
    'unicorn/no-for-each': 'error',
    'unicorn/no-array-from-fill': 'error',
    'unicorn/no-array-method-this-argument': 'error',
    'unicorn/no-array-reverse': 'error',
    'unicorn/no-array-sort': 'error',
    'unicorn/no-await-in-promise-methods': 'error',
    'unicorn/no-blob-to-file': 'error',
    'unicorn/no-canvas-to-image': 'error',
    'unicorn/no-console-spaces': 'error',
    'unicorn/no-document-cookie': 'error',
    'unicorn/no-empty-file': 'error',
    'unicorn/no-exports-in-scripts': 'error',
    'unicorn/no-hex-escape': 'error',
    'unicorn/no-instanceof-builtins': 'error',
    'unicorn/no-invalid-fetch-options': 'error',
    'unicorn/no-invalid-remove-event-listener': 'error',
    'unicorn/no-lonely-if': 'error',
    'unicorn/no-magic-array-flat-depth': 'error',
    'unicorn/no-named-default': 'error',
    'unicorn/no-negated-condition': 'error',
    'unicorn/no-negation-in-equality-check': 'error',
    'unicorn/no-new-array': 'error',
    'unicorn/no-new-buffer': 'error',
    'unicorn/no-object-as-default-parameter': 'error',
    'unicorn/no-process-exit': 'error',
    'unicorn/no-single-promise-in-promise-methods': 'error',
    'unicorn/no-static-only-class': 'error',
    'unicorn/no-thenable': 'error',
    'unicorn/no-this-assignment': 'error',
    'unicorn/no-typeof-undefined': 'error',
    'unicorn/no-unnecessary-array-flat-depth': 'error',
    'unicorn/no-unnecessary-array-splice-count': 'error',
    'unicorn/no-unnecessary-await': 'error',
    'unicorn/no-unnecessary-nested-ternary': 'error',
    'unicorn/no-unnecessary-polyfills': 'error',
    'unicorn/no-unnecessary-slice-end': 'error',
    'unicorn/no-unreadable-array-destructuring': 'error',
    'unicorn/no-unreadable-iife': 'error',
    'unicorn/no-unused-array-method-return': 'error',
    'unicorn/no-useless-collection-argument': 'error',
    'unicorn/no-useless-error-capture-stack-trace': 'error',
    'unicorn/no-useless-fallback-in-spread': 'error',
    'unicorn/no-useless-iterator-to-array': 'error',
    'unicorn/no-useless-length-check': 'error',
    'unicorn/no-useless-promise-resolve-reject': 'error',
    'unicorn/no-useless-spread': 'error',
    'unicorn/no-useless-switch-case': 'error',
    'unicorn/no-useless-undefined': 'error',
    'unicorn/no-zero-fractions': 'error',
    'unicorn/number-literal-case': 'error',
    'unicorn/numeric-separators-style': 'error',
    'unicorn/prefer-add-event-listener': 'error',
    'unicorn/prefer-array-find': 'error',
    'unicorn/prefer-array-flat': 'error',
    'unicorn/prefer-array-flat-map': 'error',
    'unicorn/prefer-array-index-of': 'error',
    'unicorn/prefer-array-last-methods': 'error',
    'unicorn/prefer-array-some': 'error',
    'unicorn/prefer-at': 'error',
    'unicorn/prefer-bigint-literals': 'error',
    'unicorn/prefer-blob-reading-methods': 'error',
    'unicorn/prefer-class-fields': 'error',
    'unicorn/prefer-classlist-toggle': 'error',
    'unicorn/prefer-code-point': 'error',
    'unicorn/prefer-date-now': 'error',
    'unicorn/prefer-default-parameters': 'error',
    'unicorn/prefer-dom-node-append': 'error',
    'unicorn/prefer-dom-node-remove': 'error',
    'unicorn/prefer-dom-node-text-content': 'error',
    'unicorn/prefer-event-target': 'error',
    'unicorn/prefer-global-this': 'error',
    'unicorn/prefer-includes': 'error',
    'unicorn/prefer-iterator-to-array-at-end': 'error',
    'unicorn/prefer-keyboard-event-key': 'error',
    'unicorn/prefer-logical-operator-over-ternary': 'error',
    'unicorn/prefer-math-abs': 'error',
    'unicorn/prefer-math-min-max': 'error',
    'unicorn/prefer-math-trunc': 'error',
    'unicorn/prefer-modern-dom-apis': 'error',
    'unicorn/prefer-modern-math-apis': 'error',
    // We don't want to go all-in on ES6 modules for Node.js code yet.
    'unicorn/prefer-module': 'off',
    'unicorn/prefer-native-coercion-functions': 'error',
    'unicorn/prefer-negative-index': 'error',
    'unicorn/prefer-node-protocol': 'error',
    // This rule seems unnecessary.
    'unicorn/prefer-number-properties': 'off',
    'unicorn/prefer-object-from-entries': 'error',
    'unicorn/prefer-optional-catch-binding': 'error',
    'unicorn/prefer-prototype-methods': 'error',
    'unicorn/prefer-queue-microtask': 'error',
    'unicorn/prefer-reflect-apply': 'error',
    'unicorn/prefer-regexp-test': 'error',
    'unicorn/prefer-response-static-json': 'error',
    'unicorn/prefer-set-has': 'error',
    'unicorn/prefer-set-size': 'error',
    'unicorn/prefer-simple-condition-first': 'error',
    // This rule has false positives on non-array `push()` methods.
    'unicorn/prefer-single-call': 'off',
    'unicorn/prefer-split-limit': 'error',
    'unicorn/prefer-string-match-all': 'error',
    'unicorn/prefer-string-pad-start-end': 'error',
    'unicorn/prefer-string-raw': 'error',
    'unicorn/prefer-string-repeat': 'error',
    'unicorn/prefer-string-replace-all': 'error',
    'unicorn/prefer-string-slice': 'error',
    'unicorn/prefer-string-starts-ends-with': 'error',
    'unicorn/prefer-string-trim-start-end': 'error',
    'unicorn/prefer-structured-clone': 'error',
    // Switch statements in JavaScript are quite error-prone.
    'unicorn/prefer-switch': 'off',
    // Guard clauses and multi-line returns are clearer than ternaries.
    'unicorn/prefer-ternary': 'off',
    // Svelte component `<script>` blocks instantiate synchronously, so top-level
    // await is not appropriate there.
    'unicorn/prefer-top-level-await': 'off',
    'unicorn/prefer-type-error': 'error',
    'unicorn/relative-url-style': 'error',
    'unicorn/require-array-join-separator': 'error',
    'unicorn/require-css-escape': 'error',
    'unicorn/require-module-attributes': 'error',
    'unicorn/require-module-specifiers': 'error',
    'unicorn/require-number-to-fixed-digits-argument': 'error',
    'unicorn/require-passive-events': 'error',
    'unicorn/text-encoding-identifier-case': 'error',
    'unicorn/throw-new-error': 'error',
  };
}