Proper Usage of event.persist() in React

A common mistake when working with React’s Synthetic Events is misusing the event.persist() method, which can lead to memory leaks or unexpected behavior.

The Wrong Way (Common Mistakes)

// ❌ Incorrect usage examples:

// 1. Calling persist() unnecessarily
function handleClick(event) {
  event.persist(); // Not needed if using event synchronously
  console.log(event.target.value);
}

// 2. Using persist() with async but not storing the event
function handleChange(event) {
  event.persist(); // Pointless without capturing the event
  setTimeout(() => {
    console.log(event.target.value); // Still nullified!
  }, 1000);
}

// 3. Persisting events in loops
function handleMultiple(events) {
  events.forEach(event => {
    event.persist(); // Memory leak risk
    // ...
  });
}

When You Actually Need event.persist()

React pools Synthetic Events for performance, nullifying them after the event callback. Use persist() only when:

1. Accessing the event asynchronously
2. Storing the event object for later use
3. Using the event in a closure that executes after the callback

The Right Way

// ✅ Correct usage examples:

// 1. Proper async usage with persist()
function handleAsyncClick(event) {
  event.persist();

  fetch('/api').then(() => {
    console.log(event.type); // Works because we persisted
  });
}

// 2. Alternative to persist() - extract values immediately
function handleChangeBetter(event) {
  const { value } = event.target; // Extract synchronously

  setTimeout(() => {
    console.log(value); // Better than persisting whole event
  }, 1000);
}

// 3. With React 17+ (event pooling removed)
function handleClickReact17(event) {
  // No need for persist() in React 17+
  setTimeout(() => {
    console.log(event.type); // Works without persist
  }, 1000);
}

Modern Alternatives (React 17+)

Since React 17, the event pooling optimization was removed, making event.persist() largely unnecessary:

// React 17+ - no pooling means no need for persist()
async function handleSubmit(event) {
  await submitForm();
  console.log(event.target); // Works fine
}

Best Practices

1. Avoid persist() when possible – prefer extracting values synchronously
2. In React 17+, you can almost always skip persist()
3. For class components, store needed values in instance properties
4. For function components, use state or refs to store event data
5. Clean up persisted events if storing them long-term

Performance Considerations

• Each persist() prevents event object recycling
• Storing many persisted events can cause memory pressure
• Extracting values is more efficient than persisting whole events

Debugging Tips

If you’re unsure if you need persist():

1. Check React version (17+ needs it less)
2. Try accessing event properties in your async code
3. Use error boundaries to catch null reference errors
4. Profile memory usage if persisting many events

Remember: event.persist() is a escape hatch, not a standard practice. Always prefer extracting event properties synchronously when possible.