Understanding the Differences Between useCallback and useMemo
Tags: react, hooks, webdev, javascript
Published: 2023-07-26 19:34:07

React has revolutionized the way we build interfaces with its innovative and dynamic approach. 

Ever since version 16.8 rolled out, Hooks have become a game-changer, enabling developers to work with state and other React features without the need for classes. 

Among these Hooks, two stand out for their significance: `useCallback` and `useMemo`. 

In this article, we'll take a deep dive into these hooks, understand their differences, and learn **when**, **why**, and **how** they should (or should _not_) be used.

## A Brief Introduction to useCallback and useMemo

The `useCallback` hook returns a memoized version of the callback function that only changes if one of the dependencies has changed. It's useful when passing callbacks to optimized child components that rely on reference equality to prevent unnecessary renders.

The `useMemo` hook returns a memoized value. Like `useCallback`, it only re-calculates the memoized value when one of the dependencies has changed. It's great for expensive calculations.

## When and How to Use useCallback and useMemo

Now that we have a brief understanding, let's jump into some practical examples.

### An Intricate Game Scenario - Using useCallback

Imagine we're creating an intricate game application where a player can increase their level by collecting gems. Every time they level up, they get a celebratory message. Here's a simplified version of that.

```jsx
import React, { useState, useCallback } from 'react';

function Game() {
  const [level, setLevel] = useState(1);

  const levelUp = () => {
    setLevel(level + 1);
  };

  return <Board level={level} onLevelUp={levelUp} />;
}

function Board({ level, onLevelUp }) {
  // A heavy computation function that renders the game board
  renderBoard(level);

  return (
    <div>
      <h2>Current Level: {level}</h2>
      <button onClick={onLevelUp}>Collect a gem</button>
    </div>
  );
}
```

Now, the issue here is that the `levelUp` function is created each time `Game` renders. Thus, `Board` re-renders every time, even when there are no level changes. This might slow down our app, especially with complex board rendering. Here's where `useCallback` shines:

```jsx
import React, { useState, useCallback } from 'react';

function Game() {
  const [level, setLevel] = useState(1);

  const levelUp = useCallback(() => {
    setLevel(level + 1);
  }, [level]);

  return <Board level={level} onLevelUp={levelUp} />;
}
```

With this change, a memoized `levelUp` function is passed to `Board` unless the `level` changes. The expensive board rendering process only happens when necessary, improving performance.

### An E-Commerce Filter - Using useMemo

Suppose you're building an e-commerce app with a product listing page. There's a filter feature that allows users to search for products by their names. Here's a basic setup:

```jsx
import React, { useState } from 'react';

function ProductList({ products }) {
  const [filter, setFilter] = useState('');

  const filteredProducts = products.filter(product =>
    product.name.toLowerCase().includes(filter.toLowerCase())
  );

  return (
    <>
      <input
        type="text"
        value={filter}
        onChange={e => setFilter(e.target.value)}
      />
      <ul>
        {filteredProducts.map(product => (
          <li key={product.id}>{product.name}</li>
        ))}
      </ul>
    </>
  );
}
```

The issue is, the `filter` function runs each time `ProductList` renders. If you have thousands of products, this could slow down your app significantly. The `useMemo` hook can solve this problem:

```jsx
import React, { useState, useMemo } from 'react';

function ProductList({ products }) {
  const [filter, setFilter] = useState('');

  const filteredProducts = useMemo(() => 
    products.filter(product =>
      product.name.toLowerCase().includes(filter.toLowerCase())
    ), [products, filter]);

  return (
    // ...
  );
}
```

Now, the expensive filter function only runs when `products` or `filter` changes, leading to much better performance for your product list.

## When Not to Use useCallback and useMemo

While `useCallback` and `useMemo` can provide performance boosts in specific scenarios, they should not be used everywhere. Here's why.

### Overuse of useCallback

Using `useCallback` unnecessarily can lead to more harm than good. It creates an overhead of maintaining the memoized version of the function, which can be more expensive than the function invocation itself. Let's consider an example:

```jsx
import React, { useState, useCallback } from 'react';

function Greeting() {
  const [name, setName] = useState('');

  const updateName = useCallback((event) => {
    setName(event.target.value);
  }, []);

  return (
    <input
      type="text"
      value={name}
      onChange={updateName}
    />
  );
}
```

In this example, `useCallback` is not needed, because the `updateName` function is not computationally expensive or passed as a prop causing unnecessary re-renders. Removing `useCallback` from this code simplifies it without any performance downside.

### Misuse of useMemo

`useMemo` can also be overused or misused. If the calculation isn't computationally expensive, then `useMemo` might bring more overhead than benefits. For example:

```jsx
import React, { useMemo } from 'react';

function TotalPrice({ quantity, price }) {
  const totalPrice = useMemo(() => quantity * price, [quantity, price]);

  return (
    <h2>Total Price: {totalPrice}</h2>
  );
}
```

In this case, `useMemo` is unnecessary since the multiplication operation isn't expensive. It would be better to simply calculate `totalPrice` without memoization:

```jsx
import React from 'react';

function TotalPrice({ quantity, price }) {
  const totalPrice = quantity * price;

  return (
    <h2>Total Price: {totalPrice}</h2>
  );
}
```

## Conclusion

`useCallback` and `useMemo` are powerful tools in the React developer's toolkit. They are designed to help optimize React apps by preventing unnecessary renders and computations. 

However, like many powerful tools, they can lead to issues when used inappropriately or excessively. By understanding the use cases for these hooks and using them judiciously, we can create more performant and maintainable React applications.