coding-standards

Development

Provides coding standards for React Native — performance patterns, consistency rules, and clean React architecture. Use when writing, modifying, or reviewing code.

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Source SKILL.md: https://github.com/Expensify/App/blob/HEAD/.claude/skills/coding-standards/SKILL.md

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Expensify Coding Standards

Coding standards for the Expensify App. Each standard is a standalone file in rules/ with reasoning, examples, and applicability conditions.

Category	Prefix	Focus
Performance	`PERF-*`	Render optimization, memo patterns, useEffect hygiene, data selection
Consistency	`CONSISTENCY-*`	Platform checks, magic values, unused props, ESLint discipline, localization, file naming, JSDoc
Clean React Patterns	`CLEAN-REACT-PATTERNS-*`	Composition, component ownership, state structure, prop typing, function components
UI	`UI-*`	Loading indicators, scrollable pages, styling conventions

Quick Reference

Performance

PERF-1 — No spread in renderItem
PERF-2 — Return early before expensive work
PERF-3 — Use OnyxListItemProvider in renderItem
PERF-5 — Shallow over deep comparisons
PERF-6 — Derive state from props
PERF-7 — Reset via key prop
PERF-8 — Handle events in handlers
PERF-9 — No useEffect chains
PERF-10 — No useEffect parent communication
PERF-11 — Optimize data selection
PERF-12 — Prevent memory leaks
PERF-13 — Hoist iterator-independent calls
PERF-14 — Use useSyncExternalStore
PERF-15 — Clean up async Effects
PERF-16 — Guard double initialization
PERF-17 — Pass raw source, index on demand (no pre-built digest)
PERF-18 — Use usePreMountDestination for RHP pre-mounting

Consistency

CONSISTENCY-1 — No platform-specific checks in components
CONSISTENCY-2 — No magic numbers/strings
CONSISTENCY-3 — No code duplication
CONSISTENCY-4 — No unused props
CONSISTENCY-5 — Justify ESLint disables
CONSISTENCY-6 — Proper error handling
CONSISTENCY-7 — Localize all user-visible copy
CONSISTENCY-8 — Localize numbers, amounts, dates and phone numbers
CONSISTENCY-9 — Name files after what they export
CONSISTENCY-10 — Follow the JSDoc style guidelines
CONSISTENCY-11 — Track future work in an issue, not a TODO comment
CONSISTENCY-12 — Name callbacks for what they do, not the event they handle
CONSISTENCY-13 — Document component props with a JSDoc block comment
CONSISTENCY-14 — Non-trivial new files start with a header description

Clean React Patterns

CLEAN-REACT-PATTERNS-0 — React Compiler compliance
CLEAN-REACT-PATTERNS-1 — Composition over configuration
CLEAN-REACT-PATTERNS-2 — Components own their behavior
CLEAN-REACT-PATTERNS-3 — Context-free component contracts
CLEAN-REACT-PATTERNS-4 — No side-effect spaghetti
CLEAN-REACT-PATTERNS-5 — Keep state narrow
CLEAN-REACT-PATTERNS-6 — Import the exported prop type instead of ComponentProps
CLEAN-REACT-PATTERNS-7 — Do not inline prop types on exported components
CLEAN-REACT-PATTERNS-8 — Use function components, not class components
CLEAN-REACT-PATTERNS-9 — Use TypeScript types, not propTypes or defaultProps

UI

UI-1 — Use the correct loading indicator based on navigation context
UI-2 — New pages must be scrollable
UI-3 — Do not use inline style objects

Usage

During development: When writing or modifying src/ files, consult the relevant standard files for detailed conditions, examples, and exceptions.

During review: The code-inline-reviewer agent loads all standards from this directory. See .claude/agents/code-inline-reviewer.md.