Linting & Formatting Highlights Summary

Every highlight bullet from the 8 pages in this section, gathered on one page and grouped by the page it came from.

Linting Basics

• SDK 57 scaffolds ESLint 9 flat config via npx expo lint - no legacy .eslintrc
• eslint-config-expo/flat bundles React, hooks, import, and Expo-aware globals out of the box
• The lint npm script delegates to expo lint so CI and laptops share the same entry point
• Put global ignores in their own config object - mixing ignores with rules changes ESLint semantics
• Prefix intentionally unused params with _ to satisfy @typescript-eslint/no-unused-vars
• Metro and Babel config files need Node globals - eslint-config-expo handles most of them automatically
• Pair npm run lint with tsc --noEmit before EAS builds - types and lint catch different failure classes

Prettier & Import Sorting

• One root .prettierrc covers apps/mobile and packages/ui - Metro does not care about import order, but humans do
• Use prettier --check in CI and --write locally; never format in CI without committing
• @ianvs/prettier-plugin-sort-imports groups react, react-native, expo, then relative paths
• eslint-config-prettier disables ESLint formatting rules so lint and format stay separate
• .prettierignore must skip .expo/, ios/, android/, and lock files
• Import sorting via Prettier plugin or ESLint import/order - pick one, not both fighting

TypeScript Strict Mode

• Extend expo/tsconfig.base - SDK 57 already sets jsx and moduleResolution Metro expects
• strict: true enables eight checks; noUncheckedIndexedAccess is separate and high-value for RN
• Type route params and API JSON at boundaries - not every StyleSheet line at once
• npx tsc --noEmit in CI catches cross-file breaks ESLint cannot see
• Enable strictNullChecks first when tightening a brownfield app
• Pair compiler strictness with @typescript-eslint/no-floating-promises for async screens

React Hooks Lint Rules

• eslint-config-expo ships react-hooks/rules-of-hooks and exhaustive-deps - verify both are on before tuning
• rules-of-hooks is always error; exhaustive-deps stays warn until the team trusts suppressions
• AppState, useFocusEffect, and NetInfo subscriptions are the top mobile stale-closure sources
• Never call hooks inside FlatList renderItem - extract a row component so lint and React agree
• Prefer functional setState and AbortController cleanup over blanket eslint-disable comments
• TanStack Query hook results belong in deps; the stable queryClient ref usually does not

CI Quality Gates

• Standardize scripts: lint (expo lint), typecheck, format:check, test - CI calls the same npm scripts as local
• npm ci + Node 20 for deterministic GitHub Actions installs
• Run tsc --noEmit before EAS build jobs - native compile is too late to catch TS errors
• EAS Workflows use eas/checkout and eas/install_node_modules for cloud quality gates
• Branch protection requires status checks so red lint cannot merge to main
• Monorepos: turbo run lint typecheck test fans out across apps and packages

Custom ESLint Rules for RN

• Start with built-in no-restricted-syntax and no-restricted-imports before writing custom rule modules
• Ban console.log in src/ but allow structured logging wrappers in lib/logger.ts
• eslint-plugin-react-native-a11y catches missing accessibilityLabel on TextInput - not a device-test substitute
• import/no-restricted-paths enforces feature → feature boundaries without a custom AST visitor
• Local rules live in tools/eslint/rules/ and load via @typescript-eslint/utils createRule
• Scope test-only relaxations with files globs - never weaken a11y rules globally

Dead Code & Dependency Analysis

• Knip finds unused files, exports, and dependencies from the module graph
• depcheck lists package.json deps Metro never imports - fast first pass
• Every unused native module is install size and autolinking surface - delete aggressively
• Ignore .expo/, ios/, android/, and config-only packages in knip.json
• False positives: expo plugins in app.config, jest setupFiles, side-effect imports
• Run knip in CI monthly or on dependency PRs - not every commit if noisy

Linting Best Practices

• Start with npx expo lint and eslint-config-expo flat config
• Run Prettier and ESLint as separate concerns with eslint-config-prettier
• Enable TypeScript strict incrementally - do not flip everything in one PR
• Treat react-hooks/exhaustive-deps seriously on mobile lifecycle code
• Gate CI on lint, typecheck, format:check, and test scripts
• Run Knip and depcheck quarterly - native deps bloat binaries