JAVASCRIPT
React usePrevious Hook to Track Previous State or Prop Values
Create a usePrevious custom hook in React to store and retrieve the previous value of any state or prop, useful for comparing current and past data.
import { useRef, useEffect } from 'react';
function usePrevious(value) {
const ref = useRef();
useEffect(() => {
ref.current = value;
}, [value]); // Only re-run if value changes
return ref.current;
}
// Usage example:
// function Counter() {
// const [count, setCount] = useState(0);
// const prevCount = usePrevious(count);
//
// return (
// <div>
// <p>Current: {count}</p>
// <p>Previous: {prevCount !== undefined ? prevCount : 'N/A'}</p>
// <button onClick={() => setCount(count + 1)}>Increment</button>
// </div>
// );
// }
How it works: The `usePrevious` hook allows you to easily get the previous value of a prop or state. It uses a `useRef` to store the value from the *last* render cycle. Inside `useEffect`, the current `value` is assigned to `ref.current` *after* the render, ensuring that `ref.current` always holds the value from the previous render when accessed.