inherently coupled with other UI logic: how events are handled, how the state changes over time, and how the data is prepared for display. Instead of artificially separating technologies by putting markup and logic in separate files, React separates concerns with loosely coupled units called “components” that contain both. React doesn’t require using JSX, but most people find it helpful as a visual aid when working with UI inside the JavaScript code. It also allows React to show more useful error and warning messages.
a programming concept where an ideal, or “virtual”, representation of a UI is kept in memory and synced with the “real” DOM by a library such as ReactDOM. This process is called reconciliation. This approach enables the declarative API of React: You tell React what state you want the UI to be in, and it makes sure the DOM matches that state. This abstracts out the attribute manipulation, event handling, and manual DOM updating that you would otherwise have to use to build your app. Since “virtual DOM” is more of a pattern than a specific technology, people sometimes say it to mean different things. In React world, the term “virtual DOM” is usually associated with React elements since they are the objects representing the user interface. React, however, also uses internal objects called “fibers” to hold additional information about the component tree. They may also be considered a part of “virtual DOM” implementation in React
h1>Hello! This is a function component clock.</ h1> < h2>It is TIME.</h2> </ div> Change <div className ="clock"> <h1>Hello! This is a function component clock.</ h1> <h2>It is NEW TIME .</h2> </div> <div className ="clock"> <h1>Hello! This is a function component clock.</ h1> <h2>It is TIME.</h2> </div> reconciliation <div className ="clock"> <h1>Hello! This is a function component clock.</ h1> <h2>It is NEW TIME .</h2> </div> Re-Render Only Changes
[status, setStatus] = useState ('idle'); const [value, setValue] = useState (null); const [error, setError] = useState (null); // The execute function wraps asyncFunction and // handles setting state for pending, value, and error. // useCallback ensures the below useEffect is not called // on every render, but only if asyncFunction changes. const execute = useCallback (() => { setStatus ('pending' ); setValue (null); setError (null); return asyncFunction () . then(response => { setValue (response); setStatus ('success' ); }) . catch(error => { setError (error); setStatus ('error' ); }); }, [asyncFunction]); // Call execute if we want to fire it right away. // Otherwise execute can be called later, such as // in an onClick handler. useEffect (() => { if (immediate) { execute (); } }, [execute, immediate]); return { execute, status, value, error }; }; Using async Values