Accidentally using event.persist() incorrectly

When working with React events, you might encounter event.persist() as a solution to event pooling issues, but it’s often misunderstood or misused. Here’s what you need to know:

The Problem event.persist() Solves

In React 16 and earlier, React used event pooling for performance optimization:

• Event objects were reused after the event handler completed
• Event properties became null after the handler finished
• This caused issues when trying to access events asynchronously

Correct Usage of event.persist()

✅ Proper use case (React 16 and earlier):

handleChange = (event) => {
  event.persist(); // Correctly preserves the event
  setTimeout(() => {
    console.log(event.target.value); // Works
  }, 1000);
};

Common Misuses and Fixes

❌ Misuse 1: Using in React 17+

// React 17+ removed event pooling - no need for persist()
handleChange = (event) => {
  event.persist(); // ❌ Unnecessary in React 17+
  // ...
};

Fix: Simply remove persist() in React 17+.

❌ Misuse 2: Calling too late

handleChange = (event) => {
  setTimeout(() => {
    event.persist(); // ❌ Too late - event already nullified
    console.log(event.target.value); // Fails
  }, 1000);
};

Fix: Call persist() immediately at the start of the handler.

❌ Misuse 3: Using instead of proper state management

handleChange = (event) => {
  event.persist();
  // Storing event object in state - bad practice
  this.setState({ lastEvent: event });
};

Fix: Store only the values you need:

handleChange = (event) => {
  this.setState({ value: event.target.value }); // ✅ Good
};

Modern Alternatives (React 17+)

Since React 17 removed event pooling, you can:

1. Directly access event properties asynchronously
2. Extract values synchronously when needed for async operations

// React 17+ - no persist needed
handleChange = async (event) => {
  const value = event.target.value; // Extract synchronously
  const result = await fetchData(value); // Safe to use later
};

Key Takeaways

1. React 16 and earlier:

• Use event.persist() when accessing events asynchronously
• Call it immediately at the start of event handlers

1. React 17+:

• No need for event.persist() (event pooling removed)
• Simply extract values synchronously if needed later

1. Best practices:

• Avoid storing entire event objects in state
• Prefer extracting specific values synchronously
• Consider using refs for form elements if needed