How to Test useDeferredValue Hook in React

Testing React’s useDeferredValue hook requires an understanding of concurrent rendering and how React schedules low-priority UI updates. This article provides a direct, step-by-step guide on how to write unit tests for components utilizing useDeferredValue using React Testing Library and Jest, focusing on verifying both the immediate and deferred states of your UI.

Understanding the Testing Challenge

The useDeferredValue hook accepts a value and returns a new copy of that value that will defer to more urgent updates. During an urgent update (like typing in an input), the deferred value remains at its previous state. Once the urgent render completes, React schedules a low-priority render in the background with the new deferred value.

To test this behavior, your tests must assert two distinct phases: 1. The urgent phase: The input value updates immediately, but the deferred value remains unchanged. 2. The deferred phase: The deferred value eventually catches up and renders the new state.

The Component under Test

Consider this simple search component that uses useDeferredValue to delay a heavy list filtering process:

import { useState, useDeferredValue } from 'react';

export function SearchComponent() {
  const [text, setText] = useState('');
  const deferredText = useDeferredValue(text);

  return (
    <div>
      <input
        type="text"
        placeholder="Search..."
        value={text}
        onChange={(e) => setText(e.target.value)}
      />
      <p data-testid="immediate-value">Immediate: {text}</p>
      <p data-testid="deferred-value">Deferred: {deferredText}</p>
    </div>
  );
}

Writing the Test with React Testing Library

To test this component, you need @testing-library/react and @testing-library/user-event. Because useDeferredValue relies on React’s concurrent scheduler, you must use asynchronous utilities like waitFor or findBy to catch the deferred update.

Here is how to write the test case:

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

test('defers updating the deferred value during rapid input', async () => {
  const user = userEvent.setup();
  render(<SearchComponent />);

  const input = screen.getByPlaceholderText('Search...');
  const immediateText = screen.getByTestId('immediate-value');
  const deferredText = screen.getByTestId('deferred-value');

  // 1. Initial State
  expect(immediateText).toHaveTextContent('Immediate:');
  expect(deferredText).toHaveTextContent('Deferred:');

  // 2. Trigger an update
  await user.type(input, 'React');

  // 3. Verify immediate state is updated
  expect(immediateText).toHaveTextContent('Immediate: React');

  // 4. Verify deferred state eventually matches
  await waitFor(() => {
    expect(deferredText).toHaveTextContent('Deferred: React');
  });
});

Key Tactics for Testing Concurrent Hooks