Your Guide to React.useCallback()

A reader of my blog reached me on Facebook with an interesting question. He said his teammates, no matter the situation, were wrapping every callback function inside useCallback():

import React, { useCallback } from 'react';

function MyComponent() {
  const handleClick = useCallback(() => {    // handle the click event  }, []);
  return <MyChild onClick={handleClick} />;
}

“Every callback function should be memoized to prevent useless re-rendering of child components that use the callback function” is the reasoning of his teammates.

This reasoning is far from the truth. Such usage of useCallback() without profiling makes the component slower.

In this post, I’m going to explain how to use correctly useCallback().

1. Understanding functions equality check

Before diving into useCallback() usage, let’s distinguish the problem useCallback() solves — the functions equality check.

Functions in JavaScript are first-class citizens, meaning that a function is a regular object. The function object can be returned by other functions, be compared, etc.: anything you can do with an object.

Let’s write a function factory() that returns functions that sum numbers:

function factory() {
  return (a, b) => a + b;
}

const sum1 = factory();
const sum2 = factory();

sum1(1, 2); // => 3
sum2(1, 2); // => 3

sum1 === sum2; // => falsesum1 === sum1; // => true

sum1 and sum2 are functions that sum two numbers. They’ve been created by the factory() function.

The functions sum1 and sum2 share the same code source but they are different function objects. Comparing them sum1 === sum2 evaluates to false.

That’s just how JavaScript objects works. An object (including a function object) equals only to itself.

2. The purpose of useCallback()

Different function objects sharing the same code are often created inside React components:

function MyComponent() {
  // handleClick is re-created on each render
  const handleClick = () => {    console.log('Clicked!');  };
  // ...
}

handleClick is a different function object on every rendering of MyComponent.

Because inline functions are cheap, the re-creation of functions on each rendering is not a problem. A few inline functions per component are acceptable.

But in some cases you need to maintain a single function instance between renderings:

1. A functional component wrapped inside React.memo() accepts a function object prop
2. When the function object is a dependency to other hooks, e.g. useEffect(..., [callback])
3. When the function has some internal state, e.g. when the function is debounced or throttled.

That’s when useCallback(callbackFun, deps) is helpful: given the same dependency values deps, the hook returns the same function instance between renderings (aka memoization):

import { useCallback } from 'react';

function MyComponent() {
  // handleClick is the same function object
  const handleClick = useCallback(() => {    console.log('Clicked!');  }, []);
  // ...
}

handleClick variable has always the same callback function object between renderings of MyComponent.

3. A good use case

Imagine you have a component that renders a big list of items:

import useSearch from './fetch-items';

function MyBigList({ term, onItemClick }) {
  const items = useSearch(term);

  const map = item => <div onClick={onItemClick}>{item}</div>;

  return <div>{items.map(map)}</div>;
}

export default React.memo(MyBigList);

The list could be big, maybe hundreds of items. To prevent useless list re-renderings, you wrap it into React.memo().

The parent component of MyBigList provides a handler function to know when an item is clicked:

import { useCallback } from 'react';

export function MyParent({ term }) {
  const onItemClick = useCallback(event => {
    console.log('You clicked ', event.currentTarget);
  }, [term]);

  return (
    <MyBigList
      term={term}
      onItemClick={onItemClick}    />
  );
}

onItemClick callback is memoized by useCallback(). As long as term is the same, useCallback() returns the same function object.

When MyParent component re-renders, onItemClick function object remains the same and doesn’t break the memoization of MyBigList.

That was a good use case of useCallback().

4. A bad use case

Let’s look at another example:

import { useCallback } from 'react';

function MyComponent() {
  // Contrived use of `useCallback()`
  const handleClick = useCallback(() => {    // handle the click event  }, []);
  return <MyChild onClick={handleClick} />;
}

function MyChild ({ onClick }) {
  return <button onClick={onClick}>I am a child</button>;
}

Does it make sense to apply useCallback()? Most likely not because <MyChild> component is light and its re-rendering doesn’t create performance issues.

Don’t forget that useCallback() hook is called every time MyComponent renders. Even useCallback() returning the same function object, still, the inline function is re-created on every re-rendering (useCallback() just skips it).

By using useCallback() you also increased code complexity. You have to keep the deps of useCallback(..., deps) in sync with what you’re using inside the memoized callback.

In conclusion, the optimization costs more than not having the optimization.

Simply accept that rendering creates new function objects:

import { useCallback } from 'react';

function MyComponent() {
  const handleClick = () => {    // handle the click event  };
  return <MyChild onClick={handleClick} />;
}

function MyChild ({ onClick }) {
  return <button onClick={onClick}>I am a child</button>;
}

5. Summary

When thinking about performance tweaks, recall the statement:

Profile before optimizing

When deciding to use an optimization technique, including memoization and particularly useCallback(), do:

• First — profile
• Then quantify the increased performance (e.g. 150ms vs 50ms render speed increase)

Then ask yourself: does the increased performance, compared to increased complexity, worth using useCallback()?

To enable the memoization of the entire component output I recommend checking my post Use React.memo() wisely.

Do you know use cases that are worth using useCallback()? Please share your experience in a comment below.