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 which use the callback function” is the reasoning of his teammates.

This statement is far from the truth. Moreover, such usage of useCallback() makes the component slower, harming the performance.

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 the hook solves: functions equality check.

Let’s define 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.

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

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

2. The purpose of useCallback()

Different function instances sharing the same code are often created inside React components.

When the React component body creates a function (e.g. a callback or event handler), this function is re-created on every rendering:

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.

However, there are cases when you need to keep one instance of a function:

1. A component wrapped inside React.memo() (or shouldComponentUpdate) accepts a callback prop
2. When the function is used as a dependency to other hooks, e.g. useEffect(..., [callback])

That’s the case when useCallback(callbackFun, deps) helps you: giving the same dependency values deps, the hook returns the same 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 will have always the same object of the callback function 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';

function MyBigList({ items, handleClick }) {
  const map = (item, index) => (
    <div onClick={() => handleClick(index)}>{item}</div>;
  );
  return <div>{items.map(map)}</div>;
}

export const MyBigList = React.memo(MyBigList);

MyBigList renders a list of items. Knowing the list could be big, probably a few hundreds of items. To preserve the list re-rendering, you wrap it into React.memo.

The parent component of MyBigList needs to provide the list of items and the handler function when an item is clicked.

import React, { useCallback } from 'react';

import useSearch from './fetch-items';

function MyParent({ term }) {
  const handleClick = useCallback((item) => {
    console.log('You clicked ', item);
  }, [term]);

  const items = useSearch(term);

  return (
    <MyBigList
      items={items}
      handleClick={handleClick}
    />
  );
}

handleClick callback is memoizied by useCallback(). As long as term variable stays the same, useCallback() returns the same function object.

Even if for some reason MyParent component re-renders, handleClick stays the same and doesn’t break the memoization of MyBigList.

4. A bad use case

Let’s look back at the example in the introduction to the article:

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 memoize handleClick?

No, because calling useCallback() requires lots of things working. Every time MyComponent is rendered the useCallback() hook is called. Iternally React makes sure to return the same object function. Even so, the inline function is still created on every render, useCallback() just skips it.

Even having useCallback() returning the same function instance, it 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 make sure to keep the deps of useCallback() in sync with what you’re using inside the memoized callback.

Simply accept that on each re-render 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

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

When thinking about performance tweaks, I always recall the statement:

Profile before optimizing

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

Either way, even if your situation fits, profile and then decide to apply useCallback().

What use cases of useCallback() do you know?