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Luke Brannagan
Luke Brannagan

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useCallback and useMemo: Understanding Performance Hooks.


Before we dive into the hooks, it will be beneficial to understand what memoization is. In computing, memoization or memoisation is an optimization technique used primarily to speed up computer programs. It involves storing the results of expensive function calls and returning the cached result when the same inputs occur again.

To put it simply, when we refer to a function as "expensive," it means that the function takes a long time to complete. This could be due to various reasons such as complex computations or waiting for a response from an API.

If you'd like to explore memoization further, I recommend reading this blog post for more information.


Now that we understand memoization, you might have an idea of what useMemo can be used for. useMemo, short for memoization, is used for storing the results of expensive function calls and returning the cached result when the same inputs occur again (sound familiar? 👀).

Let's break it down with an example. Consider the following expensive function:

const getPhrase = (phraseType) => {
  let i = 0;
  // Added to simulate an expensive function
  while (i < 1000000000) i++;

  if (phraseType === "greeting") {
    return "hello";
  } else {
    return "goodbye";
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In this example, I've added a while loop to simulate an expensive function. It could be replaced with any other computationally intensive task, such as making a call to an API that returns a large amount of data.

Now, let's apply this to a React component.

You can find the code here.

When you choose "Preview" in the top-right corner, you can see the component in action.

Try clicking the "Increment" button rapidly. Notice that it's slow to update the counter value. This slowness is caused by the expensive function being called on every re-render of the component. We can address this performance issue using useMemo.

Check out the updated code here.

Now, if you use the "Increment" button, it responds instantly.

Let's analyze what we have learned from memoization and apply it to the useMemo hook.

Here's the key change between the slow and fast examples:

const greeting = useMemo(() => getPhrase(type), [type]);
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As we know from memoization, it's an optimization technique that stores the results of expensive function calls and returns the cached result when the same inputs occur again.

useMemo accepts two arguments. The first argument is a function that you want to apply memoization to, and the second argument is a dependency array. The function itself is self-explanatory: in our case, we want to apply useMemo to getPhrase since it's an expensive function.

The dependency array might sound more complex than it actually is. If the values provided in the dependency array change, our expensive function will be executed again. However, if they don't change, React will use the cached result instead of re-running the function to obtain a new result (which explains the performance increase in the above example).

Consider the following example where we have added the counter to the dependency array. Now, as expected, our component becomes slow again because the counter variable changes, causing the component to re-render.

Check out the example here.

That's all for useMemo. By now, you should have a strong understanding of memoization and knowledge of how and where to use the useMemo hook to achieve performance gains.


Now that you understand useMemo, let's move on to useCallback. useCallback is similar to useMemo, as it also accepts two arguments: a function and a dependency array.

useCallback(() => someFunction(), [someDependency]);
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However, instead of caching the return value of the function, useCallback caches the function instance itself, preserving the same instance until one of the dependencies changes.

Let's break it down with an example. Below is a React component with two click handlers and a counter that tracks the number of new functions created in the component.

Check out the code here.

Each click of the "Increment" button creates two new functions per render. This example demonstrates the behavior without useCallback. Keep in mind that this is just to showcase what useCallback does; it may not necessarily be the best approach in this specific case.

Now, check out the updated code here

after applying useCallback. Click the buttons, and you'll notice that no extra functions are created per render.

In my opinion, the primary use case for useCallback is when you're using a function as a dependency for another hook. Although there are other use cases, I find them to be rare. Take the following code as an example:

function MyApp() {
  const memoizedFoo = useCallback(function foo() {
    // do something with state or props data
  }, [/* related state/props */]);

  useEffect(() => {
    // do something with memoizedFoo
    // maybe fetch data from an API and pass it to memoizedFoo
  }, [memoizedFoo]);

  return <>...</>;
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You can find an example here (thanks 🥰).

Without useCallback, this useEffect would execute on every re-render and ignore the dependency of memoizedFoo. As shown in the previous examples, functions are recreated on each re-render, which is NOT desirable. However, useCallback prevents this by using the memorized instance of the memoizedFoo function.

It's worth mentioning that it's not always the best approach to use useCallback for all your functions, although it might be tempting to do so. Kent C Dodds wrote an excellent blog post about when to use and when not to use useMemo and useCallback. You can check it out here.

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