flushSync

Forces React to flush all pending updates synchronously, ensuring the DOM is immediately updated.

import { flushSync } from 'react-dom';

flushSync(() => {
  setState(newValue);
});
// DOM is now updated

flushSync can significantly hurt performance. Use sparingly and only when necessary.

Reference

`flushSync(callback?)`

Call flushSync to force React to flush any pending work and update the DOM synchronously.

/src/shared/ReactDOMFlushSync.js:17-50

flushSync(() => {
  setCount(count + 1);
});

Parameters

callback

() => void

Optional callback function containing state updates. React will immediately flush these updates and apply them to the DOM before flushSync returns.

flushSync(() => {
  setState(newValue);
});
// State is now committed to DOM

// Or call without callback to flush pending updates
flushSync();

Returns

void

undefined

Returns nothing. All updates are flushed synchronously before the function returns.

When to Use flushSync

1. Third-Party Integrations

When integrating with third-party libraries that need immediate DOM access:

import { flushSync } from 'react-dom';
import { useState, useEffect } from 'react';
import thirdPartyLibrary from 'some-library';

function Chart({ data }) {
  const [container, setContainer] = useState(null);
  
  useEffect(() => {
    if (!container) return;
    
    // Ensure React has updated the DOM before library accesses it
    flushSync(() => {
      setContainer(container);
    });
    
    const chart = thirdPartyLibrary.init(container);
    chart.render(data);
    
    return () => chart.destroy();
  }, [container, data]);
  
  return <div ref={setContainer} />;
}

2. Print Functionality

Ensure all updates are applied before printing:

import { flushSync } from 'react-dom';
import { useState } from 'react';

function Report() {
  const [showDetails, setShowDetails] = useState(false);
  
  const handlePrint = () => {
    // Expand all sections before printing
    flushSync(() => {
      setShowDetails(true);
    });
    // DOM is now updated, safe to print
    window.print();
  };
  
  return (
    <div>
      <button onClick={handlePrint}>Print Report</button>
      {showDetails && <DetailedReport />}
    </div>
  );
}

3. Focus Management

When you need to focus an element immediately after rendering:

import { flushSync } from 'react-dom';
import { useState, useRef } from 'react';

function Dialog({ onClose }) {
  const [isOpen, setIsOpen] = useState(false);
  const inputRef = useRef(null);
  
  const openDialog = () => {
    flushSync(() => {
      setIsOpen(true);
    });
    // Input is now in DOM, safe to focus
    inputRef.current?.focus();
  };
  
  if (!isOpen) {
    return <button onClick={openDialog}>Open Dialog</button>;
  }
  
  return (
    <div>
      <input ref={inputRef} />
      <button onClick={onClose}>Close</button>
    </div>
  );
}

4. Measuring Layout

When you need to measure an element immediately after an update:

import { flushSync } from 'react-dom';
import { useState, useRef } from 'react';

function ExpandableSection({ children }) {
  const [isExpanded, setIsExpanded] = useState(false);
  const contentRef = useRef(null);
  
  const toggle = () => {
    flushSync(() => {
      setIsExpanded(!isExpanded);
    });
    
    // Measure the new height immediately
    if (contentRef.current) {
      const height = contentRef.current.offsetHeight;
      console.log('New height:', height);
    }
  };
  
  return (
    <div>
      <button onClick={toggle}>Toggle</button>
      {isExpanded && <div ref={contentRef}>{children}</div>}
    </div>
  );
}

5. Drag and Drop

Ensure immediate updates during drag operations:

import { flushSync } from 'react-dom';
import { useState } from 'react';

function DraggableList({ items }) {
  const [list, setList] = useState(items);
  const [draggedIndex, setDraggedIndex] = useState(null);
  
  const handleDrop = (targetIndex) => {
    if (draggedIndex === null) return;
    
    const newList = [...list];
    const [removed] = newList.splice(draggedIndex, 1);
    newList.splice(targetIndex, 0, removed);
    
    // Update list immediately for smooth drag feedback
    flushSync(() => {
      setList(newList);
      setDraggedIndex(null);
    });
  };
  
  return (
    <div>
      {list.map((item, index) => (
        <div
          key={item.id}
          draggable
          onDragStart={() => setDraggedIndex(index)}
          onDrop={() => handleDrop(index)}
          onDragOver={(e) => e.preventDefault()}
        >
          {item.name}
        </div>
      ))}
    </div>
  );
}

Performance Impact

How React Normally Works

By default, React batches updates for performance:

function Counter() {
  const [count, setCount] = useState(0);
  
  const handleClick = () => {
    // These are batched together
    setCount(count + 1);
    setCount(count + 2);
    setCount(count + 3);
    // React renders once with final value
  };
  
  return <button onClick={handleClick}>{count}</button>;
}

With flushSync (Slower)

function Counter() {
  const [count, setCount] = useState(0);
  
  const handleClick = () => {
    flushSync(() => {
      setCount(count + 1);
    });
    // React renders here (render #1)
    
    flushSync(() => {
      setCount(count + 2);
    });
    // React renders here (render #2)
    
    flushSync(() => {
      setCount(count + 3);
    });
    // React renders here (render #3)
    // Three renders instead of one!
  };
  
  return <button onClick={handleClick}>{count}</button>;
}

Performance Comparison

import { flushSync } from 'react-dom';
import { useState } from 'react';

function PerformanceTest() {
  const [count, setCount] = useState(0);
  
  const normalUpdate = () => {
    const start = performance.now();
    
    // Batched - fast
    for (let i = 0; i < 1000; i++) {
      setCount(i);
    }
    
    const duration = performance.now() - start;
    console.log('Normal:', duration, 'ms'); // ~1ms
  };
  
  const syncUpdate = () => {
    const start = performance.now();
    
    // Synchronous - slow
    for (let i = 0; i < 1000; i++) {
      flushSync(() => {
        setCount(i);
      });
    }
    
    const duration = performance.now() - start;
    console.log('Sync:', duration, 'ms'); // ~500ms!
  };
  
  return (
    <div>
      <button onClick={normalUpdate}>Normal Update</button>
      <button onClick={syncUpdate}>Sync Update</button>
      <div>Count: {count}</div>
    </div>
  );
}

Common Pitfalls

1. Using in Event Handlers (Usually Unnecessary)

// Bad: flushSync not needed here
function handleClick() {
  flushSync(() => {
    setCount(count + 1);
  });
}

// Good: React batches automatically
function handleClick() {
  setCount(count + 1);
}

2. Using in Effects

// Bad: Can cause issues
useEffect(() => {
  flushSync(() => {
    setState(value);
  });
}, [value]);

// Good: Let React batch naturally
useEffect(() => {
  setState(value);
}, [value]);

3. Nesting flushSync Calls

// Bad: Very slow!
flushSync(() => {
  flushSync(() => {
    setState(value);
  });
});

// Good: Single flushSync
flushSync(() => {
  setState(value);
});

4. Using for Animation

// Bad: Causes jank
for (let i = 0; i < 100; i++) {
  flushSync(() => {
    setPosition(i);
  });
}

// Good: Use CSS animations or requestAnimationFrame
function animate() {
  let i = 0;
  function step() {
    setPosition(i++);
    if (i < 100) {
      requestAnimationFrame(step);
    }
  }
  requestAnimationFrame(step);
}

Alternatives to flushSync

1. Use Effects Instead

// Instead of this:
flushSync(() => {
  setState(value);
});
measureElement();

// Consider this:
useEffect(() => {
  measureElement();
}, [value]);

2. Use Layout Effects

import { useLayoutEffect } from 'react';

// Runs synchronously after DOM updates
useLayoutEffect(() => {
  const width = divRef.current.offsetWidth;
  setWidth(width);
}, [dependency]);

3. Use Refs for Immediate Values

import { useRef } from 'react';

function Component() {
  const countRef = useRef(0);
  
  const handleClick = () => {
    // Immediate update without re-render
    countRef.current += 1;
    console.log(countRef.current); // Updated immediately
  };
}

4. Use Callback Refs

function Component() {
  const [height, setHeight] = useState(0);
  
  const measureRef = useCallback((node) => {
    if (node !== null) {
      setHeight(node.offsetHeight);
    }
  }, []);
  
  return <div ref={measureRef}>Content</div>;
}

TypeScript

import { flushSync } from 'react-dom';
import { useState } from 'react';

function Component() {
  const [count, setCount] = useState<number>(0);
  
  const handleUpdate = (): void => {
    // Type-safe usage
    flushSync(() => {
      setCount(count + 1);
    });
    
    // Or without callback
    flushSync();
  };
  
  return <button onClick={handleUpdate}>{count}</button>;
}

Testing

import { render, screen, waitFor } from '@testing-library/react';
import { flushSync } from 'react-dom';
import userEvent from '@testing-library/user-event';

test('updates synchronously', () => {
  let count = 0;
  
  function Counter() {
    const [value, setValue] = useState(0);
    
    return (
      <button
        onClick={() => {
          flushSync(() => {
            setValue(value + 1);
          });
          count = value + 1;
        }}
      >
        {value}
      </button>
    );
  }
  
  render(<Counter />);
  const button = screen.getByRole('button');
  
  userEvent.click(button);
  
  // With flushSync, count is updated synchronously
  expect(count).toBe(1);
  expect(button).toHaveTextContent('1');
});

Debugging

Enable logging to see when flushSync is called:

import { flushSync } from 'react-dom';

function logFlushSync(callback) {
  console.log('[flushSync] Start');
  const start = performance.now();
  
  flushSync(callback);
  
  const duration = performance.now() - start;
  console.log(`[flushSync] End (${duration.toFixed(2)}ms)`);
}

// Usage
logFlushSync(() => {
  setState(newValue);
});

Strict Mode Compatibility

flushSync works in Strict Mode but may cause warnings if used in render:

function Component() {
  // Warning: flushSync called during render!
  flushSync(() => {
    setState(value);
  });
  
  return <div>Content</div>;
}

// Move to event handlers or effects instead
function Component() {
  const handleClick = () => {
    flushSync(() => {
      setState(value);
    });
  };
  
  return <button onClick={handleClick}>Click</button>;
}

Best Practices

Use sparingly - Most apps don’t need flushSync
Measure performance - Profile before and after
Consider alternatives - Effects, layout effects, refs
Document usage - Explain why it’s necessary
Avoid in loops - Extremely slow
Test without it first - May not be needed

Browser Compatibility

Works in all browsers that support React:

Chrome 90+
Firefox 88+
Safari 14.1+
Edge 90+

useLayoutEffect - Run effects synchronously after DOM updates
useEffect - Run effects asynchronously
startTransition - Mark updates as non-urgent (opposite of flushSync)
createRoot - Enable concurrent features

React Core

Hooks

ReactDOM

Server APIs

flushSync

flushSync

Reference

`flushSync(callback?)`

Parameters

Returns

When to Use flushSync

1. Third-Party Integrations

2. Print Functionality

3. Focus Management

4. Measuring Layout

5. Drag and Drop

Performance Impact

How React Normally Works

With flushSync (Slower)

Performance Comparison

Common Pitfalls

1. Using in Event Handlers (Usually Unnecessary)

2. Using in Effects

3. Nesting flushSync Calls

4. Using for Animation

Alternatives to flushSync

1. Use Effects Instead

2. Use Layout Effects

3. Use Refs for Immediate Values

4. Use Callback Refs

TypeScript

Testing

Debugging

Strict Mode Compatibility

Best Practices

Browser Compatibility

React Core

Hooks

ReactDOM

Server APIs

​flushSync

​Reference

​flushSync(callback?)

​Parameters

​Returns

​When to Use flushSync

​1. Third-Party Integrations

​2. Print Functionality

​3. Focus Management

​4. Measuring Layout

​5. Drag and Drop

​Performance Impact

​How React Normally Works

​With flushSync (Slower)

​Performance Comparison

​Common Pitfalls

​1. Using in Event Handlers (Usually Unnecessary)

​2. Using in Effects

​3. Nesting flushSync Calls

​4. Using for Animation

​Alternatives to flushSync

​1. Use Effects Instead

​2. Use Layout Effects

​3. Use Refs for Immediate Values

​4. Use Callback Refs

​TypeScript

​Testing

​Debugging

​Strict Mode Compatibility

​Best Practices

​Browser Compatibility

​Related APIs

flushSync

Reference

`flushSync(callback?)`

Parameters

Returns

When to Use flushSync

1. Third-Party Integrations

2. Print Functionality

3. Focus Management

4. Measuring Layout

5. Drag and Drop

Performance Impact

How React Normally Works

With flushSync (Slower)

Performance Comparison

Common Pitfalls

1. Using in Event Handlers (Usually Unnecessary)

2. Using in Effects

3. Nesting flushSync Calls

4. Using for Animation

Alternatives to flushSync

1. Use Effects Instead

2. Use Layout Effects

3. Use Refs for Immediate Values

4. Use Callback Refs

TypeScript

Testing

Debugging

Strict Mode Compatibility

Best Practices

Browser Compatibility

Related APIs