JAVASCRIPT
Track Previous Values of Props or State in React
Get access to the previous value of any prop or state in a React component, useful for comparing current and past states or triggering effects based on changes.
import { useRef, useEffect } from 'react';
function usePrevious(value) {
const ref = useRef();
useEffect(() => {
ref.current = value;
});
return ref.current;
}
// Example Usage:
// function Counter() {
// const [count, setCount] = useState(0);
// const prevCount = usePrevious(count);
// return (
// <div>
// <p>Current: {count}</p>
// <p>Previous: {prevCount}</p>
// <button onClick={() => setCount(count + 1)}>Increment</button>
// </div>
// );
// }
How it works: The `usePrevious` hook provides a simple way to access the value of a prop or state from the previous render cycle. It uses a `useRef` to store the value. In the `useEffect` hook, which runs *after* every render, the `ref.current` is updated to the current `value`. Before this update, `ref.current` holds the value from the *previous* render, allowing you to compare it with the current `value` in your component's logic.