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. Moreover, such usage of useCallback() 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.

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

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.

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

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:

import React from 'react';

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 one 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])

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

import React, { 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 React from 'react';
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 React, { useCallback } from 'react';

export default 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 React, { useCallback } from 'react';

function MyComponent() {
  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.

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).

This doesn’t bring any benefits because the optimization costs more than not having the optimization.

Don’t forget about the increased code complexity. You have to keep the deps of useCallback(..., deps) in sync with what you’re using inside the memoized callback.

Simply accept that on each re-rendering new functions are created:

import React, { 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

Any optimization adds complexity. Any optimization added too early is a risk because the optimized code may change many times.

These considerations apply to useCallback() hook too. Its appropriate use case is to memoize the callback functions that are supplied to memoized heavy child components.

Either way:

• profile
• 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()?

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