function is defined _inside_ a component so it is different
on every re-render:

    
    
    function ChatIndicator() {  
    
    
      const isOnline = useSyncExternalStore(subscribe, getSnapshot);  
    
    
        
    
    
      // 🚩 Always a different function, so React will resubscribe on every re-render  
    
    
      function subscribe() {  
    
    
        // ...  
    
    
      }  
    
    
      
    
    
      // ...  
    
    
    }

React will resubscribe to your store if you pass a different ` subscribe `
function between re-renders. If this causes performance issues and you’d like
to avoid resubscribing, move the ` subscribe ` function outside:

    
    
    function ChatIndicator() {  
    
    
      const isOnline = useSyncExternalStore(subscribe, getSnapshot);  
    
    
      // ...  
    
    
    }  
    
    
      
    
    
    // ✅ Always the same function, so React won't need to resubscribe  
    
    
    function subscribe() {  
    
    
      // ...  
    
    
    }

Alternatively, wrap ` subscribe ` into [ ` useCallback `
](/reference/react/useCallback) to only resubscribe when some argument
changes:

    
    
    function ChatIndicator({ userId }) {  
    
    
      const isOnline = useSyncExternalStore(subscribe, getSnapshot);  
    
    
        
    
    
      // ✅ Same function as long as userId doesn't change  
    
    
      const subscribe = useCallback(() => {  
    
    
        // ...  
    
    
      }, [userId]);  
    
    
      
    
    
      // ...  
    
    
    }

[ Previous  useState  ](/reference/react/useState) [ Next  useTransition
](/reference/react/useTransition)

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##  On this page

  * Overview 
  * Reference 
  * ` useSyncExternalStore(subscribe, getSnapshot, getServerSnapshot?) `
  * Usage 
  * Subscribing to an external store 
  * Subscribing to a browser API 
  * Extracting the logic to a custom Hook 
  * Adding support for server rendering 
  * Troubleshooting 
  * I’m getting an error: “The result of ` getSnapshot ` should be cached” 
  * My ` subscribe ` function gets called after every re-render