When the useEffect runs in Lazy Loaded Component

In React, when you use React.lazy() to dynamically import a component and then render it within a Suspense boundary, the lifecycle and loading behavior of the lazy-loaded component involve specific timing. Here's how it works in relation to useEffect:

Lifecycle of Lazy-Loaded Component

1. Initial Render:

   • When the lazy-loaded component is first rendered, React starts fetching the component's code asynchronously. During this time, the fallback UI specified in the Suspense component is displayed.

2. Code Loading:

   • Once the component's code has been fetched and evaluated, React renders the component with the Suspense fallback replaced by the actual component.

3. useEffect Execution:

   • After the component is rendered, the useEffect hook inside the lazy-loaded component will run. The execution of useEffect happens after the component has been mounted in the DOM, which means it occurs after the component's code has been fully loaded and rendered.

Summary of Timing

• Before useEffect Runs: The component’s code is fetched and loaded asynchronously when using React.lazy(). During this loading phase, the fallback UI is displayed.
• After useEffect Runs: Once the lazy-loaded component has been successfully fetched and rendered, useEffect hooks within that component are executed. This means useEffect will run only after the component has been fully loaded and mounted in the DOM.

Example

Here’s a simple example to illustrate:

// LazyLoadedComponent.js
import React, { useEffect } from 'react';

const LazyLoadedComponent = () => {
  useEffect(() => {
    console.log('useEffect runs after component mounts');
  }, []);

  return <div>Lazy Loaded Component</div>;
};

export default LazyLoadedComponent;

// App.js
import React, { Suspense, lazy } from 'react';

const LazyLoadedComponent = lazy(() => import('./LazyLoadedComponent'));

function App() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <LazyLoadedComponent />
    </Suspense>
  );
}

export default App;

Execution Order

1. Initial Render: When App renders, React starts loading LazyLoadedComponent and shows "Loading..." as the fallback.
2. Loading: React asynchronously fetches and evaluates LazyLoadedComponent.
3. Rendering: Once LazyLoadedComponent is fully loaded, React replaces the fallback with LazyLoadedComponent.
4. useEffect Execution: After the component is rendered and mounted, the useEffect inside LazyLoadedComponent runs.