useMemo and useCallback: when and how to use memoization

Memoization in React

Memoization caches a result and reuses it if inputs haven't changed. React provides two hooks for this: useMemo for values and useCallback for functions.

useMemo

import { useMemo, useState } from 'react';

function ProductList({ products, filterText }) {
  // Only recalculates when products or filterText changes
  const filtered = useMemo(
    () => products.filter(p => p.name.includes(filterText)),
    [products, filterText]
  );

  return <ul>{filtered.map(p => <li key={p.id}>{p.name}</li>)}</ul>;
}

useCallback

import { useCallback, useState } from 'react';

function Parent() {
  const [count, setCount] = useState(0);

  // Stable function reference across renders
  const handleDelete = useCallback((id) => {
    console.log('Delete', id);
  }, []); // no dependencies - function never changes

  return <Child onDelete={handleDelete} />;
}

Both hooks are optimization tools, not something to reach for by default. Every memoization has a cost: memory and comparison overhead. Apply them when profiling shows a real performance problem, or when passing callbacks to memoized child components. Premature memoization adds complexity without benefit.