vercel-react-only-best-practices

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React performance optimization guidelines from Vercel Engineering. This skill should be used when writing, reviewing, or refactoring React code to ensure optimal performance patterns. Triggers on tasks involving React components, data fetching, bundle optimization, or performance improvements.

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Vercel React Best Practices

Comprehensive performance optimization guide for React applications, maintained by Vercel. Contains 40+ rules across 7 categories, prioritized by impact to guide automated refactoring and code generation.

When to Apply

Reference these guidelines when:

Writing new React components
Implementing client-side data fetching
Reviewing code for performance issues
Refactoring existing React code
Optimizing bundle size or load times

Rule Categories by Priority


async-
bundle-
client-
rerender-
rendering-
js-
advanced-

Priority	Category	Impact	Prefix
1	Eliminating Waterfalls	CRITICAL	`async-`
2	Bundle Size Optimization	CRITICAL	`bundle-`
3	Client-Side Data Fetching	MEDIUM-HIGH	`client-`
4	Re-render Optimization	MEDIUM	`rerender-`
5	Rendering Performance	MEDIUM	`rendering-`
6	JavaScript Performance	LOW-MEDIUM	`js-`
7	Advanced Patterns	LOW	`advanced-`

Quick Reference

1. Eliminating Waterfalls (CRITICAL)

```
async-defer-await
```
- Move await into branches where actually used
```
async-parallel
```
- Use Promise.all() for independent operations
```
async-dependencies
```
- Use better-all for partial dependencies
```
async-suspense-boundaries
```
- Use Suspense to stream content

2. Bundle Size Optimization (CRITICAL)

```
bundle-barrel-imports
```
- Import directly, avoid barrel files
```
bundle-conditional
```
- Load modules only when feature is activated
```
bundle-preload
```
- Preload on hover/focus for perceived speed

3. Client-Side Data Fetching (MEDIUM-HIGH)

```
client-swr-dedup
```
- Use SWR for automatic request deduplication
```
client-event-listeners
```
- Deduplicate global event listeners
```
client-passive-event-listeners
```
- Use passive listeners for scroll
```
client-localstorage-schema
```
- Version and minimize localStorage data

4. Re-render Optimization (MEDIUM)

```
rerender-defer-reads
```
- Don't subscribe to state only used in callbacks
```
rerender-memo
```
- Extract expensive work into memoized components
```
rerender-memo-with-default-value
```
- Hoist default non-primitive props
```
rerender-dependencies
```
- Use primitive dependencies in effects
```
rerender-derived-state
```
- Subscribe to derived booleans, not raw values
```
rerender-derived-state-no-effect
```
- Derive state during render, not effects
```
rerender-functional-setstate
```
- Use functional setState for stable callbacks
```
rerender-lazy-state-init
```
- Pass function to useState for expensive values
```
rerender-simple-expression-in-memo
```
- Avoid memo for simple primitives
```
rerender-move-effect-to-event
```
- Put interaction logic in event handlers
```
rerender-transitions
```
- Use startTransition for non-urgent updates
```
rerender-use-ref-transient-values
```
- Use refs for transient frequent values

5. Rendering Performance (MEDIUM)

```
rendering-animate-svg-wrapper
```
- Animate div wrapper, not SVG element
```
rendering-content-visibility
```
- Use content-visibility for long lists
```
rendering-hoist-jsx
```
- Extract static JSX outside components
```
rendering-svg-precision
```
- Reduce SVG coordinate precision
```
rendering-hydration-no-flicker
```
- Use inline script for client-only data
```
rendering-hydration-suppress-warning
```
- Suppress expected mismatches
```
rendering-activity
```
- Use Activity component for show/hide
```
rendering-conditional-render
```
- Use ternary, not && for conditionals
```
rendering-usetransition-loading
```
- Prefer useTransition for loading state

6. JavaScript Performance (LOW-MEDIUM)

```
js-batch-dom-css
```
- Group CSS changes via classes or cssText
```
js-index-maps
```
- Build Map for repeated lookups
```
js-cache-property-access
```
- Cache object properties in loops
```
js-cache-function-results
```
- Cache function results in module-level Map
```
js-cache-storage
```
- Cache localStorage/sessionStorage reads
```
js-combine-iterations
```
- Combine multiple filter/map into one loop
```
js-length-check-first
```
- Check array length before expensive comparison
```
js-early-exit
```
- Return early from functions
```
js-hoist-regexp
```
- Hoist RegExp creation outside loops
```
js-min-max-loop
```
- Use loop for min/max instead of sort
```
js-set-map-lookups
```
- Use Set/Map for O(1) lookups
```
js-tosorted-immutable
```
- Use toSorted() for immutability

7. Advanced Patterns (LOW)

```
advanced-event-handler-refs
```
- Store event handlers in refs
```
advanced-init-once
```
- Initialize app once per app load
```
advanced-use-latest
```
- useLatest for stable callback refs

How to Use

Read individual rule files for detailed explanations and code examples:

rules/async-parallel.md
rules/bundle-barrel-imports.md

Each rule file contains:

Brief explanation of why it matters
Incorrect code example with explanation
Correct code example with explanation
Additional context and references

Full Compiled Document

For the complete guide with all rules expanded:

AGENTS.md