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():
jsx
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 and increases code complexity.
In this post, I'm going to explain how to use correctly useCallback().
1. Understanding functions equality check
Before diving into useCallback() use, 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
javascript
function factory() {
return (a, b) => a + b;
}
const sumFunc1 = factory();
const sumFunc2 = factory();
console.log(sumFunc1(1, 2)); // => 3
console.log(sumFunc2(1, 2)); // => 3
console.log(sumFunc1 === sumFunc2); // => false
console.log(sumFunc1 === sumFunc1); // => true
3 that returns functions that sum numbers:javascript
function factory() {
return (a, b) => a + b;
}
const sumFunc1 = factory();
const sumFunc2 = factory();
console.log(sumFunc1(1, 2)); // => 3
console.log(sumFunc2(1, 2)); // => 3
console.log(sumFunc1 === sumFunc2); // => false
console.log(sumFunc1 === sumFunc1); // => true
Try the demo.
javascript
function factory() {
return (a, b) => a + b;
}
const sumFunc1 = factory();
const sumFunc2 = factory();
console.log(sumFunc1(1, 2)); // => 3
console.log(sumFunc2(1, 2)); // => 3
console.log(sumFunc1 === sumFunc2); // => false
console.log(sumFunc1 === sumFunc1); // => true
4 andjavascript
function factory() {
return (a, b) => a + b;
}
const sumFunc1 = factory();
const sumFunc2 = factory();
console.log(sumFunc1(1, 2)); // => 3
console.log(sumFunc2(1, 2)); // => 3
console.log(sumFunc1 === sumFunc2); // => false
console.log(sumFunc1 === sumFunc1); // => true
5 are functions that sum two numbers. They've been created by thejavascript
function factory() {
return (a, b) => a + b;
}
const sumFunc1 = factory();
const sumFunc2 = factory();
console.log(sumFunc1(1, 2)); // => 3
console.log(sumFunc2(1, 2)); // => 3
console.log(sumFunc1 === sumFunc2); // => false
console.log(sumFunc1 === sumFunc1); // => true
3 function.The functions
javascript
function factory() {
return (a, b) => a + b;
}
const sumFunc1 = factory();
const sumFunc2 = factory();
console.log(sumFunc1(1, 2)); // => 3
console.log(sumFunc2(1, 2)); // => 3
console.log(sumFunc1 === sumFunc2); // => false
console.log(sumFunc1 === sumFunc1); // => true
4 andjavascript
function factory() {
return (a, b) => a + b;
}
const sumFunc1 = factory();
const sumFunc2 = factory();
console.log(sumFunc1(1, 2)); // => 3
console.log(sumFunc2(1, 2)); // => 3
console.log(sumFunc1 === sumFunc2); // => false
console.log(sumFunc1 === sumFunc1); // => true
5 share the same code source, but they are different function objects. Comparing themjavascript
function factory() {
return (a, b) => a + b;
}
const sumFunc1 = factory();
const sumFunc2 = factory();
console.log(sumFunc1(1, 2)); // => 3
console.log(sumFunc2(1, 2)); // => 3
console.log(sumFunc1 === sumFunc2); // => false
console.log(sumFunc1 === sumFunc1); // => true
9 evaluates tojsx
function MyComponent() {
// handleClick is re-created on each render
const handleClick = () => {
console.log('Clicked!');
};
// ...
}
0.That's just how JavaScript objects work. An object (including a function object) only to itself.
2. The purpose of useCallback()
Different function objects sharing the same code are often created inside React components:
jsx
function MyComponent() {
// handleClick is re-created on each render
const handleClick = () => {
console.log('Clicked!');
};
// ...
}
jsx
function MyComponent() {
// handleClick is re-created on each render
const handleClick = () => {
console.log('Clicked!');
};
// ...
}
1 is a different function object on every rendering ofjsx
function MyComponent() {
// handleClick is re-created on each render
const handleClick = () => {
console.log('Clicked!');
};
// ...
}
2.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:
- A functional component wrapped inside
- When the function object is a dependency to other hooks, e.g.
jsx
function MyComponent() {
// handleClick is re-created on each render
const handleClick = () => {
console.log('Clicked!');
};
// ...
}
3 - When the function has some internal state, e.g. when the .
That's when
jsx
function MyComponent() {
// handleClick is re-created on each render
const handleClick = () => {
console.log('Clicked!');
};
// ...
}
4 is helpful: given the same dependency valuesjsx
function MyComponent() {
// handleClick is re-created on each render
const handleClick = () => {
console.log('Clicked!');
};
// ...
}
5, the hook returns the same function instance between renderings (aka memoization):jsx
import { useCallback } from 'react';
function MyComponent() {
// handleClick is the same function object
const handleClick = useCallback(() => {
console.log('Clicked!');
}, []);
// ...
}
jsx
function MyComponent() {
// handleClick is re-created on each render
const handleClick = () => {
console.log('Clicked!');
};
// ...
}
1 variable has always the same callback function object between renderings ofjsx
function MyComponent() {
// handleClick is re-created on each render
const handleClick = () => {
console.log('Clicked!');
};
// ...
}
2.3. A good use case
You have a component
jsx
function MyComponent() {
// handleClick is re-created on each render
const handleClick = () => {
console.log('Clicked!');
};
// ...
}
8 that renders a big list of items:jsx
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
jsx
function MyComponent() {
// handleClick is re-created on each render
const handleClick = () => {
console.log('Clicked!');
};
// ...
}
9.The parent component of
jsx
import { useCallback } from 'react';
function MyComponent() {
// handleClick is the same function object
const handleClick = useCallback(() => {
console.log('Clicked!');
}, []);
// ...
}
0 provides a handler function to know when an item is clicked:jsx
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}
/>
);
}
jsx
import { useCallback } from 'react';
function MyComponent() {
// handleClick is the same function object
const handleClick = useCallback(() => {
console.log('Clicked!');
}, []);
// ...
}
1 callback is memoized by useCallback(). As long asjsx
import { useCallback } from 'react';
function MyComponent() {
// handleClick is the same function object
const handleClick = useCallback(() => {
console.log('Clicked!');
}, []);
// ...
}
3 is the same, useCallback() returns the same function object.When
jsx
import { useCallback } from 'react';
function MyComponent() {
// handleClick is the same function object
const handleClick = useCallback(() => {
console.log('Clicked!');
}, []);
// ...
}
5 component re-renders,jsx
import { useCallback } from 'react';
function MyComponent() {
// handleClick is the same function object
const handleClick = useCallback(() => {
console.log('Clicked!');
}, []);
// ...
}
1 function object remains the same and doesn't break the memoization ofjsx
import { useCallback } from 'react';
function MyComponent() {
// handleClick is the same function object
const handleClick = useCallback(() => {
console.log('Clicked!');
}, []);
// ...
}
0.That was a good use case of useCallback().
4. A bad use case
Let's look at another example:
jsx
import { useCallback } from 'react';
function MyComponent() {
// Contrived use of `useCallback()`
const handleClick = useCallback(() => {
console.log('You clicked');
}, []);
return <MyChild onClick={handleClick} />;
}
function MyChild ({ onClick }) {
return <button onClick={onClick}>I am a child</button>;
}
The first problem is that useCallback() hook is called every time
jsx
function MyComponent() {
// handleClick is re-created on each render
const handleClick = () => {
console.log('Clicked!');
};
// ...
}
2 renders. This already reduces the render performance.The second problem is that using useCallback() increases the complexity of the code. You have to keep the
jsx
function MyComponent() {
// handleClick is re-created on each render
const handleClick = () => {
console.log('Clicked!');
};
// ...
}
5 ofjsx
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);
3 in sync with what you're using inside the memoized callback.Does it worth using useCallback()? Most likely not because
jsx
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);
5 component is light, and its re-rendering doesn't create performance issues. The optimization costs more than not having the optimization.Simply accept that rendering creates new function objects:
jsx
import { useCallback } from 'react';
function MyComponent() {
const handleClick = () => {
console.log('You clicked');
};
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, such as memoization with useCallback(), do:
- Profile
- Quantify the increased performance (e.g.
jsx
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);
7 vsjsx
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);
8 render speed increase) - 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.
Like the post? Please share!
Suggest Improvement
Quality posts into your inbox
I regularly publish posts containing:
- Important JavaScript concepts explained in simple words
- Overview of new JavaScript features
- How to use TypeScript and typing
- Software design and good coding practices
Subscribe to my newsletter to get them right into your inbox.
SubscribeJoin 6946 other subscribers.
About Dmitri Pavlutin
Tech writer and coach. My daily routine consists of (but not limited to) drinking coffee, coding, writing, coaching, overcoming boredom 😉.