Correct Usage of event.persist() in React

A common React mistake is misusing event.persist() when working with SyntheticEvents, either using it unnecessarily or incorrectly. Let’s clarify its proper usage.

The Problem

// ❌ Unnecessary usage
function handleClick(event) {
  event.persist(); // Not needed for synchronous code
  console.log(event.target.value);
}

// ❌ Incorrect async usage
async function handleChange(event) {
  event.persist();
  await fetchData();
  console.log(event.target.value); // May still fail
}

When You Actually Need event.persist()

React’s SyntheticEvent objects are pooled for performance. Normally, event properties are nullified after the event handler completes. event.persist() removes the event from the pool, allowing you to access it asynchronously.

Correct Use Case

function handleEvent(event) {
  // ✅ Only when you need to reference the event after the handler
  event.persist();

  setTimeout(() => {
    console.log(event.type); // Works because we persisted
  }, 1000);
}

Better Alternatives

1. Extract Values Immediately (Recommended)

function handleChange(event) {
  const { value } = event.target; // ✅ Capture sync
  setTimeout(() => {
    console.log(value); // Use captured value
  }, 1000);
}

2. For Forms, Use State

const [formData, setFormData] = useState();

function handleChange(event) {
  setFormData({
    ...formData,
    [event.target.name]: event.target.value
  });
  // No need to persist - values are in state
}

3. Use Refs for Persistent Access

const eventRef = useRef();

function handleClick(event) {
  eventRef.current = {
    type: event.type,
    target: event.target
  };

  setTimeout(() => {
    console.log(eventRef.current); // Access later
  }, 1000);
}

Common Misuses

1. Persisting in synchronous code:

   function handleClick(event) {
     event.persist(); // ❌ Unnecessary
     console.log(event.type); // Works without persist
   }

2. Assuming full DOM event functionality:

   event.persist();
   // ❌ Still can't use all native event methods
   event.target.removeEventListener(); // Won't work

3. Memory leaks from over-persisting:

   // ❌ Persisting all events can cause memory issues
   componentDidMount() {
     window.addEventListener('scroll', e => {
       e.persist();
       this.saveEvent(e); // Now React can't recycle it
     });
   }

Best Practices

1. Avoid persist() – Extract values synchronously when possible
2. Use sparingly – Only when you truly need the full event object later
3. Clean up persisted events – If storing references
4. Consider alternative architectures – Like storing needed data in state/refs

Performance Impact

• Each persist() prevents React from reusing that event object
• Increases memory usage for persisted events
• Only noticeable when persisting many events (scrolling, mousemove, etc.)

TypeScript Note

When using TypeScript, you may need to type persisted events:

const handleEvent = (event: React.PersistableEvent) => {
  event.persist();
  // ...async usage
};