The Purpose of the useTransition Hook in React 18

Introduction to useTransition

The useTransition hook is one of the key features introduced in React 18 as part of the Concurrent Rendering features. It allows you to mark certain state updates as non-urgent, giving React the ability to pause these updates if there are more urgent updates (like user input) that need to be processed first.

import { useTransition } from 'react';

function MyComponent() {
  const [isPending, startTransition] = useTransition();
  
  // Use these values in your component...
}

The Problem useTransition Solves

Before React 18, all state updates were treated with the same priority. This could lead to unresponsive user interfaces when performing expensive operations:

// Before React 18 - All updates have the same priority
function FilterableList({ items }) {
  const [filterText, setFilterText] = useState('');
  const filteredItems = items.filter(item => 
    item.text.includes(filterText)
  );
  
  return (
    <div>
      <input 
        value={filterText}
        onChange={e => {
          // This update could block the UI if items is large
          setFilterText(e.target.value);
        }}
      />
      
      <ul>
        {filteredItems.map(item => (
          <li key={item.id}>{item.text}</li>
        ))}
      </ul>
    </div>
  );
}

In this example, typing in the input could feel sluggish if the list is large, because React needs to re-render the entire list on every keystroke.

How useTransition Works

The useTransition hook returns an array with two elements:

1. isPending: A boolean that tells you whether there’s a pending transition
2. startTransition: A function that lets you mark a state update as a transition

import { useState, useTransition } from 'react';

function FilterableList({ items }) {
  const [filterText, setFilterText] = useState('');
  const [isPending, startTransition] = useTransition();
  const filteredItems = items.filter(item => 
    item.text.includes(filterText)
  );
  
  return (
    <div>
      <input 
        value={filterText}
        onChange={e => {
          // Urgent: Update the input immediately
          const value = e.target.value;
          
          // Non-urgent: Filter the list inside a transition
          startTransition(() => {
            setFilterText(value);
          });
        }}
      />
      
      {isPending && <div>Updating list...</div>}
      
      <ul>
        {filteredItems.map(item => (
          <li key={item.id}>{item.text}</li>
        ))}
      </ul>
    </div>
  );
}

Key Benefits of useTransition

1. Improved User Experience

By prioritizing updates, React ensures that the UI remains responsive even during intensive operations:

function SearchResults() {
  const [query, setQuery] = useState('');
  const [results, setResults] = useState([]);
  const [isPending, startTransition] = useTransition();
  
  const handleSearch = (e) => {
    const newQuery = e.target.value;
    
    // Update the input immediately
    setQuery(newQuery);
    
    // Fetch and update results in a transition
    startTransition(() => {
      fetchResults(newQuery).then(data => {
        setResults(data);
      });
    });
  };
  
  return (
    <div>
      <input 
        value={query} 
        onChange={handleSearch} 
        placeholder="Search..."
      />
      
      {isPending ? (
        <div>Loading results...</div>
      ) : (
        <ul>
          {results.map(result => (
            <li key={result.id}>{result.title}</li>
          ))}
        </ul>
      )}
    </div>
  );
}

2. Avoiding Unnecessary Loading States

With useTransition, you can avoid showing loading indicators for fast transitions:

function TabContainer() {
  const [activeTab, setActiveTab] = useState('home');
  const [isPending, startTransition] = useTransition();
  
  const selectTab = (tab) => {
    startTransition(() => {
      setActiveTab(tab);
    });
  };
  
  return (
    <div>
      <nav>
        <button 
          className={activeTab === 'home' ? 'active' : ''}
          onClick={() => selectTab('home')}
        >
          Home
        </button>
        <button 
          className={activeTab === 'about' ? 'active' : ''}
          onClick={() => selectTab('about')}
        >
          About
        </button>
        <button 
          className={activeTab === 'contact' ? 'active' : ''}
          onClick={() => selectTab('contact')}
        >
          Contact
        </button>
      </nav>
      
      {isPending && <div>Changing tab...</div>}
      
      <div className="tab-content">
        {activeTab === 'home' && <HomeTab />}
        {activeTab === 'about' && <AboutTab />}
        {activeTab === 'contact' && <ContactTab />}
      </div>
    </div>
  );
}

3. Keeping Previous UI Visible During Transitions

React will keep the previous UI visible while the new UI is being prepared:

function ProductPage() {
  const [productId, setProductId] = useState(1);
  const [product, setProduct] = useState(null);
  const [isPending, startTransition] = useTransition();
  
  useEffect(() => {
    fetchProduct(productId).then(data => {
      setProduct(data);
    });
  }, [productId]);
  
  const nextProduct = () => {
    startTransition(() => {
      setProductId(prevId => prevId + 1);
    });
  };
  
  if (!product) return <div>Loading...</div>;
  
  return (
    <div className={isPending ? 'pending' : ''}>
      <h1>{product.name}</h1>
      <p>{product.description}</p>
      <button onClick={nextProduct} disabled={isPending}>
        Next Product
      </button>
      
      {isPending && <div className="transition-indicator">Loading next product...</div>}
    </div>
  );
}

useTransition vs. useDeferredValue

React 18 also introduced another hook called useDeferredValue, which is related to useTransition but serves a slightly different purpose:

import { useState, useDeferredValue } from 'react';

function SearchResults() {
  const [query, setQuery] = useState('');
  const deferredQuery = useDeferredValue(query);
  
  // This will use the deferred value for expensive calculations
  const results = searchItems(deferredQuery);
  
  const isStale = query !== deferredQuery;
  
  return (
    <div>
      <input 
        value={query} 
        onChange={e => setQuery(e.target.value)} 
      />
      
      <div className={isStale ? 'stale' : ''}>
        {results.map(result => (
          <div key={result.id}>{result.name}</div>
        ))}
      </div>
    </div>
  );
}

The key differences are:

1. useTransition is for wrapping state updates, while useDeferredValue is for deferring a value
2. useTransition gives you an isPending boolean, while with useDeferredValue you need to compare the original and deferred values
3. useTransition requires you to wrap the state update, while useDeferredValue can be used with values from props or other sources

Practical Examples of useTransition

Example 1: Filtering a Large List

import { useState, useTransition } from 'react';

function FilterableList() {
  const [items] = useState(generateLargeList(10000));
  const [filterText, setFilterText] = useState('');
  const [isPending, startTransition] = useTransition();
  
  const filteredItems = items.filter(item =>
    item.name.toLowerCase().includes(filterText.toLowerCase())
  );
  
  function handleFilterChange(e) {
    const value = e.target.value;
    
    // Update the input immediately
    setFilterText(value);
    
    // Filter the list in a transition
    startTransition(() => {
      setFilterText(value);
    });
  }
  
  return (
    <div>
      <input
        value={filterText}
        onChange={handleFilterChange}
        placeholder="Filter list..."
      />
      
      {isPending ? (
        <div>Filtering...</div>
      ) : (
        <ul>
          {filteredItems.map(item => (
            <li key={item.id}>{item.name}</li>
          ))}
        </ul>
      )}
    </div>
  );
}

function generateLargeList(count) {
  return Array(count).fill().map((_, i) => ({
    id: i,
    name: `Item ${i + 1}`
  }));
}

Example 2: Navigating Between Pages

import { useState, useTransition } from 'react';

const pages = {
  home: { title: 'Home', content: <HomePage /> },
  about: { title: 'About', content: <AboutPage /> },
  contact: { title: 'Contact', content: <ContactPage /> },
  products: { title: 'Products', content: <ProductsPage /> }
};

function App() {
  const [currentPage, setCurrentPage] = useState('home');
  const [isPending, startTransition] = useTransition();
  
  function navigate(page) {
    startTransition(() => {
      setCurrentPage(page);
    });
  }
  
  const { title, content } = pages[currentPage];
  
  return (
    <div>
      <header>
        <h1>My Website</h1>
        <nav>
          {Object.keys(pages).map(page => (
            <button
              key={page}
              onClick={() => navigate(page)}
              disabled={isPending}
              className={currentPage === page ? 'active' : ''}
            >
              {pages[page].title}
            </button>
          ))}
        </nav>
      </header>
      
      <main className={isPending ? 'pending' : ''}>
        {isPending && <div className="loading-indicator">Loading page...</div>}
        <h2>{title}</h2>
        {content}
      </main>
    </div>
  );
}

Example 3: Data Visualization with Complex Calculations

import { useState, useTransition } from 'react';
import { Chart } from 'chart-library';

function DataDashboard() {
  const [data, setData] = useState([]);
  const [filter, setFilter] = useState('all');
  const [isPending, startTransition] = useTransition();
  
  useEffect(() => {
    fetchData().then(rawData => {
      setData(rawData);
    });
  }, []);
  
  function applyFilter(newFilter) {
    // Update the filter selector immediately
    setFilter(newFilter);
    
    // Process data in a transition
    startTransition(() => {
      setFilter(newFilter);
    });
  }
  
  // Expensive calculation
  const processedData = useMemo(() => {
    if (!data.length) return [];
    
    // Simulate expensive data processing
    console.log('Processing data with filter:', filter);
    return processData(data, filter);
  }, [data, filter]);
  
  return (
    <div>
      <div className="controls">
        <select 
          value={filter} 
          onChange={e => applyFilter(e.target.value)}
          disabled={isPending}
        >
          <option value="all">All Data</option>
          <option value="lastWeek">Last Week</option>
          <option value="lastMonth">Last Month</option>
          <option value="lastYear">Last Year</option>
        </select>
      </div>
      
      {isPending ? (
        <div className="chart-skeleton">
          <div>Updating chart...</div>
        </div>
      ) : (
        <Chart data={processedData} />
      )}
    </div>
  );
}

function processData(data, filter) {
  // Simulate expensive calculation
  const start = performance.now();
  while (performance.now() - start < 200) {
    // Artificial delay to simulate complex processing
  }
  
  // Actual data processing logic
  switch (filter) {
    case 'lastWeek':
      return data.filter(item => isWithinLastWeek(item.date));
    case 'lastMonth':
      return data.filter(item => isWithinLastMonth(item.date));
    case 'lastYear':
      return data.filter(item => isWithinLastYear(item.date));
    default:
      return data;
  }
}

Advanced Patterns with useTransition

Pattern 1: Coordinating Multiple Transitions

function Dashboard() {
  const [isPending, startTransition] = useTransition();
  const [activeView, setActiveView] = useState('overview');
  const [timeRange, setTimeRange] = useState('week');
  
  function updateDashboard(view, range) {
    // Group multiple state updates in a single transition
    startTransition(() => {
      setActiveView(view);
      setTimeRange(range);
    });
  }
  
  return (
    <div>
      <DashboardControls 
        activeView={activeView}
        timeRange={timeRange}
        onUpdate={updateDashboard}
        disabled={isPending}
      />
      
      {isPending ? (
        <DashboardSkeleton />
      ) : (
        <DashboardContent 
          view={activeView} 
          timeRange={timeRange} 
        />
      )}
    </div>
  );
}

Pattern 2: Nested Transitions

function NestedTransitions() {
  const [outerIsPending, startOuterTransition] = useTransition();
  const [section, setSection] = useState('main');
  
  function SectionContent({ section }) {
    const [innerIsPending, startInnerTransition] = useTransition();
    const [tab, setTab] = useState('first');
    
    function changeTab(newTab) {
      startInnerTransition(() => {
        setTab(newTab);
      });
    }
    
    return (
      <div>
        <TabSelector 
          activeTab={tab} 
          onChange={changeTab} 
          disabled={innerIsPending}
        />
        
        {innerIsPending ? (
          <div>Loading tab...</div>
        ) : (
          <TabContent tab={tab} section={section} />
        )}
      </div>
    );
  }
  
  function changeSection(newSection) {
    startOuterTransition(() => {
      setSection(newSection);
    });
  }
  
  return (
    <div>
      <SectionSelector 
        activeSection={section} 
        onChange={changeSection} 
        disabled={outerIsPending}
      />
      
      {outerIsPending ? (
        <div>Loading section...</div>
      ) : (
        <SectionContent section={section} />
      )}
    </div>
  );
}

Pattern 3: Transition with Suspense

import { Suspense, useState, useTransition } from 'react';

// Data fetching component that suspends
const DataComponent = ({ id }) => {
  const data = useData(id); // Custom hook that might suspend
  return <div>{data.name}</div>;
};

function App() {
  const [id, setId] = useState(1);
  const [isPending, startTransition] = useTransition();
  
  return (
    <div>
      <button
        onClick={() => {
          startTransition(() => {
            setId(id + 1);
          });
        }}
        disabled={isPending}
      >
        Next Item
      </button>
      
      {isPending && <div>Loading next item...</div>}
      
      <Suspense fallback={<div>Loading data...</div>}>
        <DataComponent id={id} />
      </Suspense>
    </div>
  );
}

Performance Considerations

When to Use useTransition

// GOOD: Use for expensive operations
startTransition(() => {
  setFilteredItems(items.filter(complexFilterFunction));
});

// GOOD: Use for navigation
startTransition(() => {
  setCurrentPage('dashboard');
});

// BAD: Don't use for immediate feedback
// This should be an urgent update
startTransition(() => {
  setIsButtonPressed(true);
});

Balancing Transitions and User Feedback

function SearchComponent() {
  const [query, setQuery] = useState('');
  const [results, setResults] = useState([]);
  const [isPending, startTransition] = useTransition();
  const [isStale, setIsStale] = useState(false);
  
  const handleSearch = (e) => {
    const newQuery = e.target.value;
    setQuery(newQuery);
    
    // Mark results as stale immediately
    setIsStale(true);
    
    // Update results in a transition
    startTransition(() => {
      const newResults = searchDatabase(newQuery);
      setResults(newResults);
      setIsStale(false);
    });
  };
  
  return (
    <div>
      <input 
        value={query} 
        onChange={handleSearch} 
      />
      
      <div className={isStale ? 'stale-results' : ''}>
        {isPending && <div>Updating results...</div>}
        
        <ul>
          {results.map(result => (
            <li key={result.id}>{result.title}</li>
          ))}
        </ul>
      </div>
    </div>
  );
}

Common Pitfalls and Solutions

Pitfall 1: Overusing Transitions

// BAD: Using transitions for everything
function BadComponent() {
  const [count, setCount] = useState(0);
  const [isPending, startTransition] = useTransition();
  
  return (
    <div>
      <p>Count: {count}</p>
      <button
        onClick={() => {
          // This is a simple update that doesn't need a transition
          startTransition(() => {
            setCount(count + 1);
          });
        }}
      >
        Increment
      </button>
    </div>
  );
}

// GOOD: Only use transitions for expensive updates
function GoodComponent() {
  const [count, setCount] = useState(0);
  const [items, setItems] = useState([]);
  const [isPending, startTransition] = useTransition();
  
  return (
    <div>
      <p>Count: {count}</p>
      <button
        onClick={() => {
          // Simple update - no transition needed
          setCount(count + 1);
          
          // Expensive update - use transition
          startTransition(() => {
            setItems(generateLargeArray(count));
          });
        }}
      >
        Increment and Update Items
      </button>
      
      {isPending ? (
        <div>Updating items...</div>
      ) : (
        <ItemList items={items} />
      )}
    </div>
  );
}

Pitfall 2: Not Handling Stale Closures

// BAD: Potential stale closure issue
function BadComponent() {
  const [query, setQuery] = useState('');
  const [isPending, startTransition] = useTransition();
  
  function handleChange(e) {
    const value = e.target.value;
    
    // This creates a closure over the current value
    startTransition(() => {
      // By the time this runs, 'value' might be stale
      fetchResults(value).then(results => {
        // ...
      });
    });
  }
  
  // ...
}

// GOOD: Using function updates to avoid stale closures
function GoodComponent() {
  const [query, setQuery] = useState('');
  const [results, setResults] = useState([]);
  const [isPending, startTransition] = useTransition();
  
  // Store the latest query in a ref
  const latestQueryRef = useRef('');
  
  function handleChange(e) {
    const value = e.target.value;
    setQuery(value);
    latestQueryRef.current = value;
    
    startTransition(() => {
      // Use the ref to get the latest value
      fetchResults(latestQueryRef.current).then(data => {
        setResults(data);
      });
    });
  }
  
  // ...
}

Pitfall 3: Ignoring the isPending State

// BAD: Not using isPending
function BadComponent() {
  const [tab, setTab] = useState('home');
  const [isPending, startTransition] = useTransition();
  
  return (
    <div>
      <nav>
        <button onClick={() => {
          startTransition(() => {
            setTab('home');
          });
        }}>Home</button>
        <button onClick={() => {
          startTransition(() => {
            setTab('about');
          });
        }}>About</button>
      </nav>
      
      {/* No indication that a transition is happening */}
      <div>
        {tab === 'home' && <HomeContent />}
        {tab === 'about' && <AboutContent />}
      </div>
    </div>
  );
}

// GOOD: Properly using isPending
function GoodComponent() {
  const [tab, setTab] = useState('home');
  const [isPending, startTransition] = useTransition();
  
  return (
    <div>
      <nav>
        <button 
          onClick={() => {
            startTransition(() => {
              setTab('home');
            });
          }}
          disabled={isPending}
          className={tab === 'home' ? 'active' : ''}
        >
          Home
        </button>
        <button 
          onClick={() => {
            startTransition(() => {
              setTab('about');
            });
          }}
          disabled={isPending}
          className={tab === 'about' ? 'active' : ''}
        >
          About
        </button>
      </nav>
      
      <div className={isPending ? 'content-pending' : ''}>
        {isPending && <div className="loading-indicator">Loading...</div>}
        
        {tab === 'home' && <HomeContent />}
        {tab === 'about' && <AboutContent />}
      </div>
    </div>
  );
}

Interview Tips

• Explain that useTransition is a React 18 hook that helps prioritize state updates
• Highlight that it improves user experience by keeping the UI responsive during expensive operations
• Mention that it works by marking certain updates as “transitions” which can be interrupted by more urgent updates
• Explain the difference between useTransition and useDeferredValue
• Be ready to discuss real-world scenarios where useTransition would be beneficial
• Emphasize that useTransition is part of React’s Concurrent Rendering features
• Mention that it returns an isPending boolean that can be used to show loading states
• Discuss how it can be combined with Suspense for more advanced patterns