I was preparing a blog post on a subject related to React re-renders when I stumbled upon this little React gem of knowledge I think you'll really appreciate:

Kent C. Dodds 🌌 @kentcdodds

If you give React the same element you gave it on the last render, it wont bother re-rendering that element.

4:53 PM (UTC) · June 24th, 2019

1 89

After reading this blog post, Brooks Lybrand implemented this trick and this was the result:

Brooks Lybrand @BrooksLybrand

A little before and after optimization on a react component. I didn't use any memoization to accomplish this, yet I was able to go from a 13.4ms to a 3.6ms render. I also didn't do anything besides move code into an extra component, which ended up cutting out 27 lines of code.

10:00 PM (UTC) · July 12th, 2019

5 145

Excited? Let's break it down with a simple contrived example and then talk about what practical application this has for you in your day-to-day apps.

An example

// play with this on codesandbox: https://codesandbox.io/s/react-codesandbox-g9mt5

import * as React from 'react'
import ReactDOM from 'react-dom'

function Logger(props) {
	console.log(`${props.label} rendered`)
	return null // what is returned here is irrelevant...
}

function Counter() {
	const [count, setCount] = React.useState(0)
	const increment = () => setCount((c) => c + 1)
	return (
		<div>
			<button onClick={increment}>The count is {count}</button>
			<Logger label="counter" />
		</div>
	)
}

ReactDOM.render(<Counter />, document.getElementById('root'))

When that's run, "counter rendered" will be logged to the console initially, and each time the count is incremented, "counter rendered" will be logged to the console. This happens because when the button is clicked, state changes and React needs to get the new React elements to render based on that state change. When it gets those new elements, it renders and commits them to the DOM.

Here's where things get interesting. Consider the fact that <Logger label="counter" /> never changes between renders. It's static, and therefore could be extracted. Let's try that just for fun (I'm not recommending you do this, wait for later in the blog post for practical recommendations).

// play with this on codesandbox: https://codesandbox.io/s/react-codesandbox-o9e9f

import * as React from 'react'
import ReactDOM from 'react-dom'

function Logger(props) {
	console.log(`${props.label} rendered`)
	return null // what is returned here is irrelevant...
}

function Counter(props) {
	const [count, setCount] = React.useState(0)
	const increment = () => setCount((c) => c + 1)
	return (
		<div>
			<button onClick={increment}>The count is {count}</button>
			{props.logger}
		</div>
	)
}

ReactDOM.render(
	<Counter logger={<Logger label="counter" />} />,
	document.getElementById('root'),
)

Did you notice the change? Yeah! We get the initial log, but then we don't get new logs when we click the button anymore! WHAAAAT!?

What's going on?

So what's causing this difference? Well, it has to do with React elements. Why don't you take a quick break and read my blog post "What is JSX?" to get a quick refresher on React elements and their relationship to JSX.

When React calls the counter function, it gets back something that looks a bit like this:

// some things removed for clarity
const counterElement = {
	type: 'div',
	props: {
		children: [
			{
				type: 'button',
				props: {
					onClick: increment, // this is the click handler function
					children: 'The count is 0',
				},
			},
			{
				type: Logger, // this is our logger component function
				props: {
					label: 'counter',
				},
			},
		],
	},
}

These are called UI descriptor objects. They describe the UI that React should create in the DOM (or via native components for react native). Let's click the button and take a look at the changes:

const counterElement = {
	type: 'div',
	props: {
		children: [
			{
				type: 'button',
				props: {
					onClick: increment,
					children: 'The count is 1',
				},
			},
			{
				type: Logger,
				props: {
					label: 'counter',
				},
			},
		],
	},
}

As far as we can tell, the only changes are the onClick and children props of the button element. However, the entire thing is completely new! Since the dawn of time using React, you've been creating these objects brand new on every render. (Luckily, even mobile browsers are pretty fast at this, so that has never been a significant performance problem).

It may actually be easier to investigate at the parts of this tree of React elements is the same between renders, so here are the things that did NOT change between those two renders:

const counterElement = {
	type: 'div',
	props: {
		children: [
			{
				type: 'button',
				props: {
					onClick: increment,
					children: 'The count is 1',
				},
			},
			{
				type: Logger,
				props: {
					label: 'counter',
				},
			},
		],
	},
}

All the element types are the same (this is typical), and the label prop for the Logger element is unchanged. However the props object itself changes every render, even though the properties of that object are the same as the previous props object.

Ok, here's the kicker right here. Because the Logger props object has changed, React needs to re-run the Logger function to make sure that it doesn't get any new JSX based on the new props object (in addition to effects that may need to be run based on the props change). But what if we could prevent the props from changing between renders? If the props don't change, then React knows that our effects shouldn't need to be re-run and our JSX shouldn't change (because React relies on the fact that our render methods should be idempotent). This is exactly what React is coded to do right here and it's been that way since React first started!

Ok, but the problem is that react creates a brand new props object every time we create a React element, so how do we ensure that the props object doesn't change between renders? Hopefully now you've got it and you understand why the second example above wasn't re-rendering the Logger. If we create the JSX element once and re-use that same one, then we'll get the same JSX every time!

Let's bring it back together

Here's the second example again (so you don't have to scroll back up):

// play with this on codesandbox: https://codesandbox.io/s/react-codesandbox-o9e9f

import * as React from 'react'
import ReactDOM from 'react-dom'

function Logger(props) {
	console.log(`${props.label} rendered`)
	return null // what is returned here is irrelevant...
}

function Counter(props) {
	const [count, setCount] = React.useState(0)
	const increment = () => setCount((c) => c + 1)
	return (
		<div>
			<button onClick={increment}>The count is {count}</button>
			{props.logger}
		</div>
	)
}

ReactDOM.render(
	<Counter logger={<Logger label="counter" />} />,
	document.getElementById('root'),
)

So let's checkout the things that are the same between renders:

const counterElement = {
	type: 'div',
	props: {
		children: [
			{
				type: 'button',
				props: {
					onClick: increment,
					children: 'The count is 1',
				},
			},
			{
				type: Logger,
				props: {
					label: 'counter',
				},
			},
		],
	},
}

Because the Logger element is completely unchanged (and therefore the props are unchanged as well), React can automatically provide this optimization for us and not bother re-rendering the Logger element because it shouldn't need to be re-rendered anyway. This is basically like React.memo except instead of checking each of the props individually, React is checking the props object holistically.

So what does this mean for me?

In summary, if you're experiencing performance issues, try this:

1. "Lift" the expensive component to a parent where it will be rendered less often.
2. Then pass the expensive component down as a prop.

You may find doing so solves your performance problem without needing to spread React.memo all over you codebase like a giant intrusive band-aid 🤕😉

Demo

Creating a practical demo of a slow app in React is tricky because it kinda requires building a full app, but I do have a contrived example app that has a before/after that you can check out and play with:

And this isn't just something that's useful at the top-level of your app either. This could be applied to your app anywhere it makes sense. What I like about this is that "It's both natural for composition and acts as an optimization opportunity." (that was Dan). So I do this naturally and get the perf benefits for free. And that's what I've always loved about React. React is written so that idiomatic React apps are fast by default, and then React provides optimization helpers for you to use as escape hatches.

Good luck!