Handling Synthetic Events with Async Operations in React

A common React pitfall occurs when trying to access event.target.value after an asynchronous operation because React’s SyntheticEvent gets nullified for performance reasons.

The Problem: Nullified Event

function handleChange(event) {
  // ❌ Problematic - event will be nullified by the time we use it
  setTimeout(() => {
    console.log(event.target.value); // Error: can't read property of null
  }, 1000);
}

Why This Happens

1. Event Pooling: React reuses SyntheticEvent objects for performance
2. Nullification: Events are nullified after the handler completes
3. Async Timing: By the time your async code runs, the event is gone

Solutions

1. Store Values Immediately

function handleChange(event) {
  // ✅ Correct - store value immediately
  const value = event.target.value;

  setTimeout(() => {
    console.log(value); // Works fine
  }, 1000);
}

2. Use event.persist()

function handleChange(event) {
  // ✅ Correct but less preferred - keeps event object
  event.persist();

  setTimeout(() => {
    console.log(event.target.value); // Works
  }, 1000);
}

Note: event.persist() removes the event from the pool and may impact performance

3. Modern Approach (React 17+)

async function handleChange(event) {
  // ✅ Modern approach - React 17+ doesn't pool events by default
  const value = event.target.value;

  await someAsyncOperation();
  console.log(value); // Still accessible
}

React 17+ changed event pooling behavior, but it’s still good practice to extract values early

4. Using Refs for Form Values

function MyComponent() {
  const inputRef = useRef();

  const handleSubmit = async () => {
    // ✅ Get current value directly from ref
    console.log(inputRef.current.value);
  };

  return (
    <input 
      ref={inputRef}
      onChange={(e) => console.log(e.target.value)} // For immediate changes
    />
  );
}

Best Practices

1. Extract values early – Store needed properties before async operations
2. Avoid persist() – Unless absolutely necessary
3. Use refs – For form elements when you need later access
4. Consider state – Store values in component state if needed elsewhere

Class Component Example

class MyComponent extends React.Component {
  handleChange = (event) => {
    const value = event.target.value;

    this.setState({ inputValue: value }, () => {
      // Access via state in callback
      console.log(this.state.inputValue);
    });
  };

  render() {
    return <input onChange={this.handleChange} />;
  }
}

Common Scenarios

1. Form submissions with API calls:

async function handleSubmit(event) {
  event.preventDefault();
  const formData = new FormData(event.target);

  // Process sync data first
  const email = formData.get('email');

  // Then make async call
  await submitForm(email);
}

2. Debounced inputs:

const debouncedSearch = debounce((value) => {
  console.log(value);
}, 300);

function handleChange(event) {
  const value = event.target.value;
  debouncedSearch(value);
}