eslint-plugin-sonarjs

name	eslint-plugin-sonarjs
description	Local-only second-opinion lint pass via `eslint-plugin-sonarjs` — code-smell, cognitive-complexity, and bug-pattern rules layered on top of Biome (which owns formatting + the bulk of lint). NO SonarQube server, NO `sonar-scanner`, NO project token. Runs as `bun run check:sonar` (the workspace task) and as the standalone `.github/workflows/sonarjs.yml` workflow. Triggers on: sonar, sonarjs, eslint-plugin-sonarjs, cognitive-complexity, pseudo-random, no-nested-conditional, no-nested-functions, void-use, second-opinion lint, eslint.sonar.config.mjs.
license	MIT

Owns the second-opinion code-smell scan. Layered on top of Biome (which owns formatting + the bulk of lint) and Stylelint (which owns CSS). Sonarjs's job is the narrow set of patterns Biome doesn't cover: cognitive complexity, dead-branch detection, redundant-assignment, nested-conditional/function depth, and a small set of bug patterns.

This is a local lint pass. No SonarQube server, no sonar-scanner CLI, no host.url / project token. The full Sonar product would require a self-hosted SonarQube instance or SonarCloud account; Snapmatch deliberately stops at the eslint-plugin layer because the CLI gate already supplies the required signal. Firebase Hosting and browser-side Firestore do not create a need for a Sonar service.

When to invoke

• Authoring or editing apps/<name>/eslint.sonar.config.mjs.
• Diagnosing a bun run check:sonar (or .github/workflows/sonarjs.yml) failure.
• Deciding whether a sonarjs finding is a real bug, an intrinsic domain pattern, or a candidate for rule-policy adjustment.
• Adding a new sonarjs rule disable or a new rule override (must include rationale).
• Triaging a freshly-discovered category of finding (the user wants to add sonarjs to a new app, or upgrade the plugin and re-baseline).

Owns

• The eslint-plugin-sonarjs flat config file at apps/<name>/eslint.sonar.config.mjs.
• The calibrated rule policy: which rules are off, which thresholds are raised, which are scoped to specific file globs, and the rationale comment for every deviation from the recommended preset.
• The check:sonar workspace script and its --max-warnings 0 exit contract.
• The .github/workflows/sonarjs.yml workflow shape (PR-only, head_ref pinned, bun --filter=@snapmatch/example-game run check:sonar).
• The refactor patterns for the rules that fire most often in this codebase (table below).

Defers to

• biome — for formatter + the bulk of lint (.ts/.tsx/.js/.json). Sonarjs MUST NOT overlap with Biome — that's why the config loads ONLY sonarjs.configs.recommended, no @eslint/js recommended rules.
• stylelint — for everything .css. Sonarjs is JS/TS only.
• react-doctor — for React-specific patterns (heading weight, useState(prop), barrel imports, etc.). Sonarjs and react-doctor have separate ownership; both run in the gate.
• fallow — for cross-cutting structural checks (unused files/exports/types/deps, circular dependencies, boundary violations). Sonarjs is rule-based per-file; fallow is graph-based.
• turborepo — for when check:sonar runs in the gate sequence (it's a per-app turbo task; the root check / check:fast scripts depend on it).

Snapmatch stack rules

• Pillar 4 (CLI-gate-first) means: eslint --config eslint.sonar.config.mjs --max-warnings 0 exits non-zero on any finding, and any warning is a failure (zero-warning policy). Local pre-push gate (bun run check:fast) and CI both run it. The IDE is not the source of truth — the CLI is.
• NO Sonar service. Never reach for sonar-scanner-cli, sonarqube-scanner, or @sonar/scan; they all require a SonarQube/SonarCloud server. The local eslint-plugin gives ~95% of the rule signal with 0% of that infrastructure.
• NO Biome overlap. The flat config loads ONLY sonarjs.configs.recommended. Do NOT add @eslint/js recommended, typescript-eslint's recommended sets, or any other eslint plugin. Dual-linter drift with Biome is the failure mode you're trying to avoid; keeping the surface narrow is the protection.
• Every rule deviation from sonarjs.configs.recommended MUST carry a rationale comment in eslint.sonar.config.mjs. The current deviations and their reasons live there — read them before adding a new one.
• Disable comments are a code smell (per AGENTS.md). If you find yourself reaching for // eslint-disable-next-line sonarjs/<rule>, refactor instead — the patterns table below is the menu of canonical fixes.

Calibrated rule policy (for snapmatch's domain)

These deviations from sonarjs.configs.recommended are deliberate and live in apps/example-game/eslint.sonar.config.mjs with rationale comments. Read those before changing the policy.

Rule	Setting	Why this codebase deviates
sonarjs/pseudo-random	off (global)	Math.random drives card shuffles, particle jitter, effects variation, audio detune. Nothing security-sensitive. Swapping to crypto.getRandomValues would add ceremony without buying anything.
sonarjs/void-use	off (global)	Fire-and-forget Promises in sound/use-sound.ts (audio unlock listeners), sound/player.ts (one-shot SFX dispatch), state/db.ts (BroadcastChannel + IDB persistence callbacks), and effects/kinds/vortex.ts are intentional. void doThing() documents that the caller doesn't await; .catch(() => undefined) is noisier and just as untyped.
sonarjs/cognitive-complexity	["error", 50]	Game state machines (auto-assist.applyAutoAssist, hints.generateHints, deal.dealRound, AddingGame route component, drag pointer handlers, dive-in/round-complete ticker callbacks) are intrinsically branchy on comparator/operator/shape. The default 15 flagged 11 sites all in this category. Mechanically splitting them produces helper soup that's harder to read; 50 still flags egregious cases.
sonarjs/no-nested-functions	off for tests/**/*.ts	Playwright IDB seeding uses promise-wrapped indexedDB.open + transaction-completion callbacks (addInitScript(async () => { ... openDb().then(...) })). The depth limit can't accommodate IDB's required callback shape; flattening would mean inventing a Promise-returning IDB helper just for tests. App code keeps the rule.

Refactor patterns — apply these instead of disabling

When sonarjs fires on app code, prefer the canonical refactor over a disable comment. Each pattern below has a worked example committed to the repo.

Rule	Refactor	Worked example
sonarjs/no-nested-conditional	Extract a single-purpose helper (comparisonGlyph, dotClass, firstLocked); use a lookup table (COMPARATOR_VERB: Record<Comparator, string>); split a nested ?: chain into independent && blocks; wrap a multi-branch expression in an IIFE that returns the value.	apps/example-game/app/components/hint-tooltip/index.tsx (comparisonGlyph); apps/example-game/app/games/sample-game/auto-assist.ts (firstLocked); apps/example-game/app/games/sample-game/hints.ts (COMPARATOR_VERB); apps/example-game/app/routes/sample-game.tsx (independent && blocks).
sonarjs/no-redundant-assignments	Restructure the if/else ladder so the initial value of the variable IS the fallback (drop the redundant else var = initial;).	apps/example-game/app/components/round-complete-fx/index.tsx (alpha = 0 initial, then if (t >= 400 && t < 1600) alpha = ...).
sonarjs/no-unused-vars (e.g. unused destructured key)	Switch to the iterator that doesn't expose the unused half — Object.values() instead of for (const [_id, entry] of Object.entries()).	apps/example-game/app/sound/registry.test.ts.
sonarjs/concise-regex	Use the canonical character-class shortcut (\w for [a-zA-Z0-9_], \d for [0-9], \s for whitespace).	apps/example-game/app/env.ts (Twitter handle regex).
sonarjs/no-nested-functions (in app code)	Hoist the inner function declaration out of the closure; if it needs closure state, use a Promise-returning helper at module scope and call it. (Tests/ are exempt — see policy table.)	apps/example-game/tests/sample-game-poster-progression.app.spec.ts (openDb + awaitTx Promise wrappers).
sonarjs/cognitive-complexity (above the 50 threshold, when raised)	Extract sub-state-machines as their own useReducers, extract pure helpers to module scope, replace big switches with Record<Kind, Runner> dispatch tables. (Same pattern fallow's circular-deps fix for effects/runtime.ts used.)	apps/example-game/app/games/sample-game/effects/runtime.ts (KIND_RUNNERS dispatch table replacing a 10-case switch).

Anti-patterns

• Adding eslint-plugin-react, eslint-plugin-import, @typescript-eslint/recommended, or @eslint/js to eslint.sonar.config.mjs. Sonar config is sonar-only; adding more plugins re-introduces dual-linter drift with Biome. If you want React-specific signal, that's react-doctor's job.
• sonar-scanner, sonarqube-scanner, @sonar/scan, host.url, sonar.token. All require a SonarQube/SonarCloud service. Snapmatch uses Firebase Hosting and browser-side Firestore, but neither authorizes or requires separate Sonar infrastructure.
• Disabling a rule globally without a rationale comment. Future-you (or a future agent) will not know whether the disable is load-bearing. Always inline the WHY.
• Inline // eslint-disable-next-line sonarjs/<rule> for no-nested-conditional, no-nested-functions, or cognitive-complexity in app code. These have canonical refactors (see the table). Suppressions in app code accumulate; one-off becomes pattern.
• Adding sonarjs as a separate "check:sonar" step at the root package.json level instead of as a per-app turbo task. It's a per-workspace lint; let turborepo orchestrate it via the check:sonar task in turbo.json so cache + filter + future workspace fan-out work.
• Pinning the .github/workflows/sonarjs.yml checkout to the merge SHA (the default). Use ref: ${{ github.head_ref }} so the scan reflects the PR branch's actual state, not a synthetic merge with main. Same pattern as react-doctor.yml and fallow.yml.

Setup recipe (new app)

1. Add devDeps to apps/<name>/package.json:

   "eslint": "^10.3.0",
   "eslint-plugin-sonarjs": "^4.0.3",
   "typescript-eslint": "^8.59.2"
   

2. Copy apps/example-game/eslint.sonar.config.mjs (or scaffold from turbo/generators/templates/app/eslint.sonar.config.mjs).
3. Add the check:sonar script to apps/<name>/package.json:

   "check:sonar": "eslint --config eslint.sonar.config.mjs --no-warn-ignored --max-warnings 0 \"app/**/*.{ts,tsx}\" \"tests/**/*.ts\" \"scripts/**/*.ts\""
   

4. Add the task to turbo.json (already done — task name is check:sonar, no dependsOn):

   "check:sonar": { "outputs": [] }
   

5. The root check and check:fast scripts already include check:sonar in their turbo dependsOn. New apps automatically get gated.
6. Run bun --filter=@snapmatch/<name> run check:sonar once and triage findings: real bugs first, then refactor patterns from the table above, then policy adjustments (with rationale comments) for irreducible cases.

Plugin pin

eslint-plugin-sonarjs ^4.0.3 (LGPL-3.0). The plugin is bundled with the recommended preset; no peer-plugin dance required. The eslint and typescript-eslint peers must be installed at the consuming workspace's package.json (workspace hoisting won't satisfy ESLint's plugin-resolution path).