React Query is a powerful library for managing server state in React applications. It provides hooks for fetching, caching, and updating asynchronous data without touching any global state. React Query's ability to work with initial data is one feature that makes it stand out.
Initial data in React Query refers to the default data a query starts with before any actual data fetching occurs. This feature is handy when displaying placeholder or cached data while the fresh data is fetched from the server. React Query can render components with this data immediately by providing initial data, improving user experiences by reducing perceived loading times.
To provide initial data to a React Query hook, you can use the initialData option. This option accepts a function that returns the initial data you want to use for your query. Here is a basic example of how to use initialData:
1import { useQuery } from 'react-query'; 2import axios from 'import axios'; 3 4const fetchTodo = async () => { 5 const { data } = await axios.get('/todos/1'); 6 return data; 7}; 8 9function TodoComponent() { 10 const { data } = useQuery('todo', fetchTodo, { 11 initialData: () => { 12 return { 13 id: 1, 14 title: 'Placeholder Title', 15 }; 16 }, 17 }); 18 19 return <div>{data.title}</div>; 20} 21
In the above code, before the fetchTodo function is called to fetch the real data, React Query will use the object returned by the initialData function as the data for the query.
Using initial data can significantly enhance the performance of React applications by allowing components to render immediately with the initial data, thus improving user experiences. This is especially beneficial when the same data is being fetched multiple times, as it can be cached and reused as initial data for subsequent queries.
React Query's sophisticated caching mechanism allows developers to manage cached data effectively. The cache is not just a simple storage; it is smart enough to handle stale data, background updates, and even provides timestamps like dataUpdatedAt to track when the data was fetched.
Here's an example of how React Query manages caching:
1import { QueryClient, QueryClientProvider } from 'react-query'; 2 3const queryClient = new QueryClient(); 4 5function App() { 6 return ( 7 <QueryClientProvider client={queryClient}> 8 <TodoComponent /> 9 </QueryClientProvider> 10 ); 11} 12
In this example, QueryClientProvider wraps the TodoComponent, allowing it to use the caching features provided by React Query.
React Query can also be considered a state management solution but primarily focuses on the server state. It allows you to manage your queries' loading, error, and data states without creating a complex state management infrastructure.
When fetching data, handling loading states and errors gracefully is essential. React Query provides built-in states for these scenarios. Here's an example of how to handle loading and error states:
1function TodoComponent() { 2 const { data, error, isLoading } = useQuery('todo', fetchTodo, { 3 initialData: () => { 4 return { 5 id: 1, 6 title: 'Placeholder Title', 7 }; 8 }, 9 }); 10 11 if (isLoading) { 12 return <span>Loading...</span>; 13 } 14 15 if (error) { 16 return <span>An error occurred: {error.message}</span>; 17 } 18 19 return <div>{data.title}</div>; 20} 21
In the above component, isLoading and error render different UIs based on the query's status.
The QueryClient can set initial query data for more advanced scenarios. This allows you to provide initial data for a query before the query is even created. Here's how you can set initial query data with QueryClient:
1const queryClient = new QueryClient(); 2 3queryClient.setQueryData('todo', { 4 id: 1, 5 title: 'Persisted Todo Title', 6}); 7
Using setQueryData, you can pre-populate the cache with initial data that can be used immediately by any component that uses the todo query key.
In the context of React Query, initial data refers to the data provided to a query as a starting point. This data is immediately available to the query upon initialization and can be used to render the component while the actual data fetching process is underway. Initial data is beneficial for improving the perceived performance of an application by displaying something to the user right away, rather than showing a loading spinner.
On the other hand, placeholder data is a broader concept that can refer to any temporary data used to fill the space of content before the real data is available. While React Query does not have a specific concept called "placeholder data," the term is often used in UI/UX design to refer to generic loading states or skeleton screens that do not necessarily contain meaningful data.
In React Query, isLoading is a true boolean when a query fetches data for the first time and has no data yet. It indicates that the query is in a "loading" state and typically triggers loading UIs like spinners or placeholders.
isFetching, however, is true any time the query is fetching, including background refetching. This means that isFetching can be true even if data is already displayed on the screen, as React Query might be updating the data in the background. This distinction is essential for creating nuanced user experiences where you can show a loading indicator on initial fetch but not necessarily during background updates.
To access cached data from React Query, you can use the useQuery hook with the appropriate query key. If the data for that query key is already in the cache and is not marked as stale, React Query will return the cached data.
Here's an example:
1const { data } = useQuery('todos', fetchTodos); 2
In this example, if the todos query has been fetched previously and the data is still fresh in the cache, data will contain the cached data.
Additionally, you can directly access the cache using the QueryClient instance:
1const queryClient = new QueryClient(); 2const cachedData = queryClient.getQueryData('todos'); 3
This method lets you retrieve the cached data outside of a React component.
staleTime is an option in React Query that determines how long a piece of fetched data should be considered fresh. During this time, React Query will not automatically refetch the data when a component mounts or the window is refocused. Once the stale time has passed, the data is considered stale, and React Query will attempt to refetch it the next time it is accessed.
Here's how to set staleTime:
1const { data } = useQuery('todos', fetchTodos, { staleTime: 5000 }); 2
In this example, the fetched todos data will remain fresh for 5 seconds before becoming stale.
React Query's initial data feature is a powerful tool for improving React applications' performance and user experience. By pre-populating queries with initial data, you can render components immediately, manage cached data effectively, and handle loading and error states gracefully. Whether you are fetching, updating, or caching data,