What's the use of key prop in Suspense component in React?

question react front-end
Ram Patra Published on February 1, 2024

In React, the key prop is used to uniquely identify and track a set of elements during their life cycle. When used with the Suspense component, the key prop helps React keep track of suspended components and their associated data dependencies.

Here’s an explanation of how the key prop is used with the Suspense component:

  1. Uniquely Identifying Suspended Components:

    • When you have multiple instances of a component that can potentially suspend rendering (e.g., fetching data with React.lazy or useEffect with asynchronous operations), React needs a way to distinguish between them.
  2. Preventing Component Mismatch:

    • The key prop ensures that React doesn’t mistake one suspended component for another. Without the key prop, React might incorrectly reuse a suspended component’s state, leading to incorrect rendering or data fetching behavior.
  3. Re-rendering Suspended Components:

    • When a suspended component is re-rendered (due to a state change or other reasons), React uses the key prop to determine whether the component is the same as the previous one. If the key remains the same, React knows it can reuse the suspended state.

Here’s an example illustrating the use of key with Suspense:

import { Suspense } from 'react';
import { fetchData } from './api';

function MyComponent({ postId }) {
  const data = fetchData(postId);

  return <div>{data}</div>;

function App() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <MyComponent key={postId} postId={1} />

In this example, the key prop is used with the MyComponent inside the Suspense component. If the postId changes and triggers a re-render, the key helps React identify that it’s a different instance of MyComponent and should not reuse the previous suspended state.

It’s important to note that while key is a helpful tool, it should be used judiciously. The value of key should be stable across renders and unique among siblings, but it doesn’t necessarily need to be globally unique in the entire application.

Ram Patra Published on February 1, 2024
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