infinite-canvas/.agents/skills/vercel-react-best-practices/rules/rerender-lazy-state-init.md at fb79a1550838dcd65fb343bca9ea88c9b0274dd8

Files

T

HouYunFei fb79a15508 docs(vercel-react-best-practices): 添加 Vercel React 最佳实践规则文档

- 添加 sections 定义文件，包含性能优化各领域分类
- 添加规则模板文件，规范文档结构和标签定义
- 添加异步操作优化规则，包括防止瀑布流、并行化、延迟等待等
- 添加包大小优化规则，包括避免桶式导入、动态导入、预加载等
- 添加服务端性能优化规则，包括 API 路

2026-05-21 15:22:11 +08:00

2.0 KiB

Raw Blame History

title, impact, impactDescription, tags

title	impact	impactDescription	tags
Use Lazy State Initialization	MEDIUM	wasted computation on every render	react, hooks, useState, performance, initialization

Use Lazy State Initialization

Pass a function to useState for expensive initial values. Without the function form, the initializer runs on every render even though the value is only used once.

Incorrect (runs on every render):

function FilteredList({ items }: { items: Item[] }) {
  // buildSearchIndex() runs on EVERY render, even after initialization
  const [searchIndex, setSearchIndex] = useState(buildSearchIndex(items))
  const [query, setQuery] = useState('')
  
  // When query changes, buildSearchIndex runs again unnecessarily
  return <SearchResults index={searchIndex} query={query} />
}

function UserProfile() {
  // JSON.parse runs on every render
  const [settings, setSettings] = useState(
    JSON.parse(localStorage.getItem('settings') || '{}')
  )
  
  return <SettingsForm settings={settings} onChange={setSettings} />
}

Correct (runs only once):

function FilteredList({ items }: { items: Item[] }) {
  // buildSearchIndex() runs ONLY on initial render
  const [searchIndex, setSearchIndex] = useState(() => buildSearchIndex(items))
  const [query, setQuery] = useState('')
  
  return <SearchResults index={searchIndex} query={query} />
}

function UserProfile() {
  // JSON.parse runs only on initial render
  const [settings, setSettings] = useState(() => {
    const stored = localStorage.getItem('settings')
    return stored ? JSON.parse(stored) : {}
  })
  
  return <SettingsForm settings={settings} onChange={setSettings} />
}

Use lazy initialization when computing initial values from localStorage/sessionStorage, building data structures (indexes, maps), reading from the DOM, or performing heavy transformations.

For simple primitives (useState(0)), direct references (useState(props.value)), or cheap literals (useState({})), the function form is unnecessary.

2.0 KiB Raw Blame History

Use Lazy State Initialization

2.0 KiB

Raw Blame History