rules--js-cache-function-results

Cache Repeated Function Calls

Use a module-level Map to cache function results when the same function is called repeatedly with the same inputs during render.

Incorrect (redundant computation):

function ProjectList({ projects }: { projects: Project[] }) {
  return (
    <div>
      {projects.map(project => {
        // slugify() called 100+ times for same project names
        const slug = slugify(project.name)
        
        return <ProjectCard key={project.id} slug={slug} />
      })}
    </div>
  )
}

Correct (cached results):

// Module-level cache
const slugifyCache = new Map<string, string>()

function cachedSlugify(text: string): string {
  if (slugifyCache.has(text)) {
    return slugifyCache.get(text)!
  }
  const result = slugify(text)
  slugifyCache.set(text, result)
  return result
}

function ProjectList({ projects }: { projects: Project[] }) {
  return (
    <div>
      {projects.map(project => {
        // Computed only once per unique project name
        const slug = cachedSlugify(project.name)
        
        return <ProjectCard key={project.id} slug={slug} />
      })}
    </div>
  )
}

Simpler pattern for single-value functions:

let isLoggedInCache: boolean | null = null

function isLoggedIn(): boolean {
  if (isLoggedInCache !== null) {
    return isLoggedInCache
  }
  
  isLoggedInCache = document.cookie.includes('auth=')
  return isLoggedInCache
}

// Clear cache when auth changes
function onAuthChange() {
  isLoggedInCache = null
}

Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.

Reference: How we made the Vercel Dashboard twice as fast