go-bananas/web/eslint.config.js

import js from '@eslint/js'
import reactHooks from 'eslint-plugin-react-hooks'
import reactRefresh from 'eslint-plugin-react-refresh'
import globals from 'globals'
import tseslint from 'typescript-eslint'

/**
 * ESLint flat configuration.
 *
 * Two decisions are worth stating:
 *
 * 1. Type-aware linting is on (`projectService`), because the rules that catch
 *    real bugs in a codebase using `unknown` at its boundaries need type
 *    information. This is also why TypeScript is pinned to 6.x: see ADR 0004.
 *
 * 2. `--max-warnings 0` in the npm script. A warning that is tolerated is a
 *    warning nobody reads, so the build is the place it stops.
 */
export default tseslint.config(
  {
    ignores: ['dist', 'coverage', 'node_modules', '*.config.js', '*.config.ts'],
  },

  js.configs.recommended,
  ...tseslint.configs.recommendedTypeChecked,
  ...tseslint.configs.stylisticTypeChecked,

  {
    languageOptions: {
      parserOptions: {
        projectService: true,
        tsconfigRootDir: import.meta.dirname,
      },
      globals: { ...globals.browser, ...globals.es2022 },
    },
    plugins: {
      'react-hooks': reactHooks,
      'react-refresh': reactRefresh,
    },
    rules: {
      ...reactHooks.configs.recommended.rules,
      'react-refresh/only-export-components': ['warn', { allowConstantExport: true }],

      // --- correctness -----------------------------------------------------
      '@typescript-eslint/no-floating-promises': 'error',
      '@typescript-eslint/no-misused-promises': [
        'error',
        // A promise-returning handler is the normal React pattern; the void
        // operator is the fix for the cases that are actually a mistake.
        { checksVoidReturn: { arguments: false, attributes: false } },
      ],
      '@typescript-eslint/await-thenable': 'error',
      '@typescript-eslint/no-unnecessary-condition': 'off',
      '@typescript-eslint/restrict-template-expressions': [
        'error',
        { allowNumber: true, allowBoolean: true },
      ],
      '@typescript-eslint/switch-exhaustiveness-check': 'error',
      // `||` is used deliberately where an empty string should fall through,
      // which is exactly what `??` does not do.
      '@typescript-eslint/prefer-nullish-coalescing': 'off',
      // A type alias reads better than an interface for everything in this
      // codebase, and the two are interchangeable in practice.
      '@typescript-eslint/consistent-type-definitions': ['error', 'type'],
      '@typescript-eslint/no-unnecessary-type-assertion': 'error',
      '@typescript-eslint/consistent-type-imports': [
        'error',
        { prefer: 'type-imports', fixStyle: 'inline-type-imports' },
      ],

      // --- readability -----------------------------------------------------
      // Both return-type annotation rules are off by design.
      //
      // `tsc` with `strict` already enforces a type at every boundary: a
      // component with a wrongly-typed prop is a compile error whether or not
      // its return type is written out. What the ESLint rules add is annotation
      // churn — one per callback, and `(): JSX.Element =>` on components that
      // legitimately start returning null and then need the annotation widened.
      // That is a maintenance cost with no defect class behind it.
      //
      // What *is* enforced instead: every component declares a typed props
      // interface, every exported non-component function carries a JSDoc
      // `@returns`, and `strict` plus `noUncheckedIndexedAccess` run on every
      // build. See docs/standards.md.
      '@typescript-eslint/explicit-function-return-type': 'off',
      '@typescript-eslint/explicit-module-boundary-types': 'off',
      // An empty function is a legitimate no-op: a ResizeObserver stub, an
      // intentionally ignored error, a deliberate empty effect body.
      '@typescript-eslint/no-empty-function': ['error', { allow: ['arrowFunctions', 'methods'] }],
      // A leading or trailing underscore is the convention for an intentionally
      // unused binding, and no-unused-vars above is configured to honour it. The
      // two rules have to agree or every intentionally-ignored value is a
      // naming error.
      '@typescript-eslint/naming-convention': [
        'error',
        {
          selector: 'variable',
          format: ['camelCase', 'UPPER_CASE', 'PascalCase'],
          leadingUnderscore: 'allow',
          trailingUnderscore: 'allow',
        },
        { selector: 'parameter', format: ['camelCase'], leadingUnderscore: 'allow' },
        { selector: 'typeLike', format: ['PascalCase'] },
      ],
      '@typescript-eslint/no-unused-vars': [
        'error',
        { argsIgnorePattern: '^_', varsIgnorePattern: '^_', caughtErrors: 'none' },
      ],
      '@typescript-eslint/consistent-type-definitions': ['error', 'type'],
      'no-console': ['warn', { allow: ['warn', 'error'] }],
      eqeqeq: ['error', 'always', { null: 'ignore' }],
      'prefer-const': 'error',
      'object-shorthand': 'error',
    },
  },

  // Tests are allowed to reach into internals and to use `any` in a double.
  {
    files: ['**/*.test.ts', '**/*.test.tsx', 'src/test/**'],
    rules: {
      '@typescript-eslint/no-explicit-any': 'off',
      '@typescript-eslint/no-unsafe-assignment': 'off',
      '@typescript-eslint/no-unsafe-member-access': 'off',
      '@typescript-eslint/no-unsafe-call': 'off',
      '@typescript-eslint/unbound-method': 'off',
      'no-console': 'off',
    },
  },
)