React’s useCallback is a powerful hook that helps optimize your components by memoizing functions. But have you ever wondered how it works behind the scenes?

🧠 What Does useCallback Do?

useCallback returns a memoized version of a callback function that only changes if its dependencies change. This is especially useful when passing functions to child components, preventing unnecessary re-renders.

const memoizedCallback = useCallback(() => {
  console.log('Callback invoked!');
}, [dependency]);

In this example, the function remains the same unless dependency changes.

💡 Creating a Polyfill for useCallback

import { useRef } from 'react';

const useCallbackPolyfill = (callback, dependencies) => {
  const ref = useRef({ callback, dependencies });

  if (
    !ref.current.dependencies ||
    !areDepsEqual(ref.current.dependencies, dependencies)
  ) {
    ref.current.callback = callback;
    ref.current.dependencies = dependencies;
  }

  return ref.current.callback;
};

const areDepsEqual = (oldDeps, newDeps) => {
  return (
    oldDeps.length === newDeps.length &&
    oldDeps.every((dep, i) => dep === newDeps[i])
  );
};

🛠️ Breaking It Down:

1. Ref Storage: We use useRef it to persist the function and its dependencies across renders.

2. Dependency Check: We compare the old and new dependencies on every render.

3. Memoization: If dependencies haven’t changed, we return the previous function; otherwise, we update the function and dependencies.

🚀 Wrapping Up

By building a polyfill, you get a deeper understanding of how React optimizes re-renders with hooks like useCallback. Try experimenting with this polyfill in a small project to see the impact firsthand!

Would you like me to write more polyfills or explore another React hook? Let me know in the comments! 🚀