Most React performance tutorials end with the same bad advice: wrap everything in and and call it a day. If you have fol

Most React performance tutorials end with the same bad advice: wrap everything in useMemo and useCallback and call it a day. If you have followed that advice, you have probably made your application slower. These hooks are not free. Each one allocates memory, compares dependencies, and stores cached values. Used without purpose, they become overhead instead of optimization.

Let’s strip this down to what actually matters.

What Each Hook Really Does

useMemo remembers a value. You hand it a function that does heavy work, and it returns the result. On the next render, if your dependencies have not changed, React skips the calculation and hands back the old result.

useCallback remembers a function. It does not run the function for you. It simply returns the same function instance between renders as long as its dependencies stay the same.

That is the entire difference. One caches a computed value. One caches a reference. Mixing these up leads to code that looks optimized but behaves identically to unwrapped code while costing extra memory.

Why Function Identity Breaks Your Tree

When a component re-renders, React executes the entire function body again. Every variable is recreated. Every inline function gets a brand new address in memory.

In JavaScript, two functions that contain the exact same logic are not equal. () => {} === () => {} evaluates to false. The same rule applies to objects and arrays. If your parent component defines handleSubmit and passes it to a child, that child receives a new prop on every single render. Even if the child is wrapped in React.memo, it cannot tell that the new function does the same thing as the old one. The reference changed, so the child re-renders.

This is the root problem useCallback was built to solve. It is not about speed. It is about stability.

When useMemo Earns Its Keep

You need useMemo when you are performing work that is objectively expensive and you can see a measurable lag.

Think about filtering a massive dataset. If you have a table with tens of thousands of rows and a search input, you might write something like this inside your component:

const visibleRows = rows.filter(r => r.name.includes(query));

Without useMemo, that loop runs on every render. If the user clicks a button that toggles a sidebar, the parent re-renders, and your filter runs again even though rows and query never changed. On a large dataset, that stutter is visible.

useMemo fixes this by pinning the result:

const visibleRows = useMemo(() => {
  return rows.filter(r => r.name.includes(query));
}, [rows, query]);

Now React only re-runs that filter when the dependencies actually change.

The same logic applies to complex mathematical calculations, transforming API responses into chart-friendly formats, or deriving state that would otherwise be recalculated constantly.

There is a second, less obvious use case. If you create an object or array locally and include it in a useEffect dependency array, you can accidentally trigger that effect on every render. Inline objects and arrays get new identities each time, so the effect sees a changed dependency and fires again. Memoizing that object with useMemo keeps the reference stable and lets your effect run only when the underlying data actually changes.

When useCallback Becomes Necessary

useCallback matters most when you are passing handlers into child components that are optimized with React.memo.

Imagine a parent component that holds a counter. It also renders an expensive child list:

function Parent() {
  const [count, setCount] = useState(0);
  
  const handleItemClick = (id) => {
    console.log(id);
  };
  
  return (
    <div>
      <button onClick={() => setCount(c + 1)}>{count}</button>
      <ExpensiveList onItemClick={handleItemClick} />
    </div>
  );
}

Every time count changes, Parent re-renders. A fresh handleItemClick is created. Because ExpensiveList receives a new prop reference, it re-renders too. If ExpensiveList is wrapped in React.memo, that memoization is completely wasted because the function prop changed.

useCallback preserves the reference:

const handleItemClick = useCallback((id) => {
  console.log(id);
}, []);

Now ExpensiveList only re-renders when it truly needs to.

Another critical situation involves useEffect. If an effect subscribes to a function defined inside your component, and that function changes identity every render, the effect will teardown and resubscribe repeatedly. Memoizing the function keeps the effect stable.

The Dependency Array Trap and Stale Closures

Both hooks rely on dependency arrays, and this is where most bugs hide.

Jika Anda melewatkan sebuah variabel dari array dependensi, fungsi atau nilai yang telah dimemoisasi akan mengunci (close over) versi lama dari variabel tersebut. Ini adalah stale closure. UI mungkin menampilkan data terbaru, tetapi callback Anda masih melihat state dari tiga render sebelumnya. Solusinya sederhana tetapi mudah terlewatkan saat code review: sertakan setiap nilai yang digunakan di dalam hook yang dapat berubah.

Jalankan aturan ESLint react-hooks/exhaustive-deps. Aturan ini akan menangkap kelalaian yang nyata. Namun, jangan menganggapnya sebagai robot. Pahami mengapa setiap dependensi itu penting.

Pajak Tersembunyi dari Optimasi Berlebihan

Pemula sering kali membentengi setiap fungsi dan setiap nilai dengan hook ini karena terasa aman. Kebiasaan tersebut justru akan menjadi bumerang.

React harus menyimpan nilai yang di-cache di dalam memori. Pada setiap render, ia harus melakukan iterasi melalui array dependensi Anda dan membandingkan setiap item menggunakan Object.is. Perbandingan tersebut memang murah, tetapi tidak gratis. Jika Anda membungkus event handler sederhana seperti onClick={() => setOpen(true)} di dalam useCallback, Anda membayar biaya memori dan CPU untuk menghindari pembuatan fungsi yang sebenarnya instan untuk dialokasikan.

Hook juga menambah kebisingan (noise). Kode yang dibungkus dengan useMemo dan useCallback lebih sulit dibaca dan lebih sulit dipelihara. Setiap array dependensi adalah potensi stale closure yang siap menjebak Anda.

Aturan praktis yang sebenarnya tidaklah glamor tetapi efektif: tulis kode biasa terlebih dahulu. Optimalkan hanya ketika Anda memiliki bukti adanya masalah. Gunakan React DevTools Profiler untuk mengidentifikasi komponen mana yang berat dan render mana yang boros. Jika sebuah render memakan waktu kurang dari beberapa milidetik, tidak ada pengguna yang akan menyadarinya, dan memoisasi Anda tidak menyelesaikan apa pun.

Intinya

useMemo adalah untuk nilai yang berat (expensive values). useCallback adalah untuk referensi fungsi yang stabil. Tidak ada satu pun hook yang membuat komponen Anda merender lebih cepat dengan sendirinya; mereka mencegah pekerjaan downstream yang tidak perlu. Mulailah tanpa keduanya, ukur dengan alat bantu (tooling) yang sebenarnya, dan tambahkan mereka tepat di tempat profiler menunjukkan adanya hambatan (bottleneck). Kode bersih yang sesekali melakukan rerender hampir selalu lebih baik daripada kode yang terlalu rumit (over-engineered) yang melakukan memoisasi pada segalanya.