Synthetic events in React

What are Synthetic Events?

Synthetic Events are React's wrapper around native browser events. React creates a unified event system that works the same across all browsers.

javascript

function Button() {
  function handleClick(event) {
    console.log(event); // SyntheticEvent, not native Event
  }

  return <button onClick={handleClick}>Click me</button>;
}

Why are they needed?

Cross-browser compatibility

Different browsers have different event implementations. Synthetic Events unify the API:

javascript

function Input() {
  function handleChange(event) {
    // event.target.value works the same everywhere
    console.log(event.target.value);
  }

  return <input onChange={handleChange} />;
}

Performance

React uses event delegation: all handlers are attached to the root element, not to each element separately.

javascript

// Instead of 1000 handlers on each button
// React creates one handler at the root
function List({ items }) {
  return (
    <ul>
      {items.map(item => (
        <li key={item.id}>
          <button onClick={() => console.log(item.id)}>
            {item.name}
          </button>
        </li>
      ))}
    </ul>
  );
}

Synthetic Event API

Synthetic Event has the same interface as native events:

javascript

function Form() {
  function handleSubmit(event) {
    event.preventDefault(); // Works like native event
    event.stopPropagation();
    
    console.log(event.type); // 'submit'
    console.log(event.target); // element reference
    console.log(event.currentTarget); // element with handler
  }

  return (
    <form onSubmit={handleSubmit}>
      <button type="submit">Submit</button>
    </form>
  );
}

Event Pooling (before React 17)

In React 16 and earlier, events were reused for performance:

javascript

// React 16 and earlier
function handleClick(event) {
  console.log(event.type); // 'click'
  
  setTimeout(() => {
    console.log(event.type); // null, event cleared!
  }, 0);
}

To preserve an event, you used event.persist():

javascript

function handleClick(event) {
  event.persist(); // Preserve event
  
  setTimeout(() => {
    console.log(event.type); // 'click', works!
  }, 0);
}

React 17+:

In React 17, event pooling was removed. Events are no longer cleared, event.persist() is not needed.

Accessing native event

You can get the native event through nativeEvent:

javascript

function handleClick(event) {
  console.log(event); // SyntheticEvent
  console.log(event.nativeEvent); // Native browser Event
  
  // Useful for browser-specific features
  console.log(event.nativeEvent.path);
}

Differences from native events

Naming

React uses camelCase instead of lowercase:

javascript

// HTML
<button onclick="handleClick()">Click</button>

// React
<button onClick={handleClick}>Click</button>

Returning false

In HTML you can return false to prevent default action:

html

<!-- HTML -->
<a href="#" onclick="console.log('clicked'); return false">
  Link
</a>

In React you must explicitly call preventDefault():

javascript

// React
function Link() {
  function handleClick(event) {
    event.preventDefault();
    console.log('clicked');
  }

  return <a href="#" onClick={handleClick}>Link</a>;
}

Supported events

React supports all standard DOM events:

Mouse events

javascript

onClick
onContextMenu
onDoubleClick
onDrag
onDragEnd
onDragEnter
onDragExit
onDragLeave
onDragOver
onDragStart
onDrop
onMouseDown
onMouseEnter
onMouseLeave
onMouseMove
onMouseOut
onMouseOver
onMouseUp

Keyboard events

javascript

function Input() {
  function handleKeyDown(event) {
    if (event.key === 'Enter') {
      console.log('Enter pressed');
    }
  }

  return <input onKeyDown={handleKeyDown} />;
}

Form events

javascript

function Form() {
  return (
    <form
      onSubmit={e => e.preventDefault()}
      onChange={e => console.log('changed')}
      onFocus={e => console.log('focused')}
      onBlur={e => console.log('blurred')}
    >
      <input type="text" />
    </form>
  );
}

Focus events

javascript

onFocus
onBlur

Touch events

javascript

onTouchStart
onTouchMove
onTouchEnd
onTouchCancel

Event delegation

React 16 and earlier

Events were attached to document:

javascript

// React attached handlers to document
document.addEventListener('click', handleAllClicks);

React 17+

Events are attached to the React root node:

javascript

// React attaches to app root
const root = document.getElementById('root');
root.addEventListener('click', handleAllClicks);

This is important when using multiple React versions on one page or integrating with other libraries.

Event handling specifics

onChange vs onInput

In React onChange works like native onInput — fires on every change:

javascript

function Input() {
  const [value, setValue] = useState('');

  return (
    <input
      value={value}
      // Fires on every keystroke
      onChange={e => setValue(e.target.value)}
    />
  );
}

onScroll

onScroll doesn't bubble in React (as in DOM), but React provides it for convenience:

javascript

function ScrollableDiv() {
  function handleScroll(event) {
    console.log(event.target.scrollTop);
  }

  return (
    <div onScroll={handleScroll} style={{ height: 200, overflow: 'auto' }}>
      {/* Lots of content */}
    </div>
  );
}

Direct native handler attachment

Sometimes you need to attach a native handler directly:

javascript

function Component() {
  const ref = useRef(null);

  useEffect(() => {
    const element = ref.current;
    
    function handleNativeClick(event) {
      console.log('Native click', event);
    }

    element.addEventListener('click', handleNativeClick);

    return () => {
      element.removeEventListener('click', handleNativeClick);
    };
  }, []);

  return <div ref={ref}>Click me</div>;
}

Warning:

Be careful with direct handlers — don't forget to remove them in cleanup function.

Event capturing

React supports the capture phase:

javascript

function Parent() {
  return (
    <div
      onClickCapture={() => console.log('1. Parent capture')}
      onClick={() => console.log('3. Parent bubble')}
    >
      <button
        onClickCapture={() => console.log('2. Button capture')}
        onClick={() => console.log('4. Button bubble')}
      >
        Click
      </button>
    </div>
  );
}

// Clicking button outputs:
// 1. Parent capture
// 2. Button capture
// 4. Button bubble
// 3. Parent bubble

TypeScript typing

typescript

import { MouseEvent, ChangeEvent, FormEvent } from 'react';

function Component() {
  function handleClick(event: MouseEvent<HTMLButtonElement>) {
    console.log(event.currentTarget.name);
  }

  function handleChange(event: ChangeEvent<HTMLInputElement>) {
    console.log(event.target.value);
  }

  function handleSubmit(event: FormEvent<HTMLFormElement>) {
    event.preventDefault();
  }

  return (
    <form onSubmit={handleSubmit}>
      <input onChange={handleChange} />
      <button onClick={handleClick}>Submit</button>
    </form>
  );
}

Common mistakes

Asynchronous event usage (React 16)

javascript

// Wrong in React 16
function handleClick(event) {
  setTimeout(() => {
    console.log(event.type); // null!
  }, 0);
}

// Correct
function handleClick(event) {
  const eventType = event.type; // Save value
  setTimeout(() => {
    console.log(eventType); // 'click'
  }, 0);
}

Passing event to setState

javascript

// Wrong
function handleChange(event) {
  setState(prevState => ({
    ...prevState,
    value: event.target.value // event might be cleared
  }));
}

// Correct
function handleChange(event) {
  const newValue = event.target.value;
  setState(prevState => ({
    ...prevState,
    value: newValue
  }));
}

Conclusion

Synthetic Events:

Wrapper around native browser events
Provide cross-browser compatibility
Use delegation for performance
Have the same API as native events
In React 17+ are not cleared automatically
Use camelCase for naming
Native event accessible through nativeEvent

In interviews:

Important to be able to:

Explain what Synthetic Events are and why they're needed
Describe event delegation
Explain differences from native events
Describe changes in React 17 (event pooling)
Show how to access native event

Short Answer

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