Expensify Coding Standards

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

Categories

Category Prefix Focus

Performance PERF-*

Render optimization, memo patterns, useEffect hygiene, data selection

Consistency CONSISTENCY-*

Platform checks, magic values, unused props, ESLint discipline

Clean React Patterns CLEAN-REACT-PATTERNS-*

Composition, component ownership, state structure

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

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

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

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 .