Let’s Talk about Re-renders

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React is a fantastic tool to implement complicated applications fast, we all know it. But are they going to be fast when implemented fast? Let’s talk about re-renders and their danger in react: how easy it is to make a mistake, why some small mistakes can have a huge downstream effect, and how to avoid and prevent them.

This is a deep-dive type of talk, that focuses on why React components re-render, what kind of performance impact it can have, and what to do about it

Nadia Makarevich
Nadia Makarevich
23 min
17 Jun, 2022

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Video Summary and Transcription

This Talk discusses React performance and how re-renders can affect it. It highlights common mistakes and misconceptions, such as the overuse of useMemo and useCallback hooks. The importance of React.memo in preventing unnecessary child component re-renders is emphasized. Creating components in render functions is identified as a major performance killer, and the benefits of moving state down and wrapping state around children are explained. The Talk also covers optimizing component rendering through memoization and provides a recap of the key points.

Available in Español

1. Introduction to React Performance and Re-renders

Short description:

Hi everyone. My name is Nadia. I am a front-end architect, coder, and writer. I have worked in Atlassian and now I am a founding engineer in a startup called PIN. Today, I want to share my knowledge about React performance and how re-renders affect it. A re-render is when a component updates its data. There are three ways to trigger a re-render: state or props change, context value change, and parent component re-render. Unnecessary re-renders can slow down the app and should be avoided.

Hi everyone. My name is Nadia. So, first a little bit of introduction. I am a front-end architect. I am a coder. I am a writer. I used to work in Atlassian for a few years. So I worked on Jira front-end, and now I am a founding engineer in a small startup that is called PIN in Australia.

So, the topic of React performance and especially how re-renders affect React performance is something like a passion of mine. I find it fascinating that just one tiny change in the right place can either completely destroy or vastly improve performance of a huge application. So, all the knowledge about this topic is what I want to share with you today.

But first, what exactly is a re-render and why do we want to talk about re-renders in the context of performance. Generally speaking, we have two major stages of a React lifecycle that we need to care about. The first one is initial render when an app first mounted and appears on the screen, and then re-render. A re-render is second and all of them, all the consecutive renders of an app that is already on the screen. And from a code perspective, we have at least three ways to trigger re-render of a component. First is the most known one is when a state or props change, this is when a component will be re-rendered. Second one if we use a context, then when a value changes, every component that uses this value will also re-render. And the third and most under appreciated one is when a parent component re-renders or if we look from the top, that means that when a component re-renders itself, it will re-render every single child that it has. If we want to visualize the last one, because it's the most important one, it will look something like this. We have a tree of components, the top one will re-render, and then this re-render will trigger a re-render of all the children there, and then all the children underneath, so it will be a whole chain of re-renders that triggers from the top.

And generally speaking, a re-render itself is not something that we would want to fight with, because it's an essential part of a React lifecycle, it's when React updates all the data that has changed. What we want to avoid at all cost is unnecessary re-renders. And by unnecessary re-render, I mean something like this. So imagine we have an input component somewhere at the bottom, we type something there, and then this component naturally will re-render itself. That is good, and that is expected. What we don't want is when we type in this tiny input component, to re-render the entire app. This, depending on the size of the app, can be extremely slow. And in today's world, users will expect all the interactions on the page to be really, really fast. So unnecessary re-renders is a performance killer.

2. React Performance Mistakes and Useless Hooks

Short description:

And to demonstrate how bad performance can be, I implemented an app that renders a list of components. Mistakes in the code made the app unbearably slow. One common mistake is the myth of useMemo and useCallback, which leads to an app full of useMemo and useCallback hooks. However, memoizing everything can make the app incomprehensible and not debuggable. Additionally, wrapping onClick in useCallback may be useless because child components can still re-render when the parent component re-renders.

And to demonstrate to you how bad those can be, I even implemented a little bit of an app. So this is an app that renders a list of components, and has a little bit of interactivity. So take a look. On the right, Performance tab, I click everywhere, and everything is instantaneous. On the left, exactly the same app, but I made a couple mistakes there, and look how unbearably slow all of this is. Just a few tiny mistakes in the right places, and I just destroyed this app. And it's just a list of components.

So common mistakes that lead to performance like that, and also useful performance tips and tricks to avoid re-renders of the entire app, is what I want to share with you today. Let's start with mistakes. The very first one is one of my favorite, is what I call the myth of useMemo and useCallback. So as probably most of you know, React uses referential equality when it compares props or dependencies in all the various hooks. And referential equality is basically this. We have two arrays or two objects. If we want to compare them, if we do it like this, the result will be false, because we're comparing them by the reference, not by the actual value.

And from React's perspective, it sounds like this. We have a component, it renders a child component. I pass a value to this child component that is an array. If I do it like this, during a rerender, this value will become a completely different value. So if React compares those props, React will think that value of the prop has changed. And if I write a memo and use callback hooks, hooks that allow you to memoize this value, and basically to preserve reference to this value between rerenders. So if I extract this array into useMemo hook, then when a rerender happens, React will think that value in a child component will be exactly the same. And the fact that one of the most important reasons why a component rerenders is a state or prop change, in combination with how those hooks work, lead to the widespread belief that if we memoize all the props on a component, that will prevent this component from re-rendering. And this results in something that I call a useMemo or useCallback hook hell, because memoizing absolutely everything leads to your app becoming, having useMemo, you wrap in useCallback, and then another useCallback, it's just useMemo and useCallbacks everywhere, and the app becomes incomprehensible and completely not readable and not debuggable. So I think those become really horrible.

But the worst part of all of this is that it's actually sometimes, it's actually useless, because we're forgetting one key component in all of this construction. So if we take a look, for example, at this code, we see a component, it has a child component and then onClick, Prop, and we want to prevent child component from re-rendering by wrapping onClick in a useCallback hook. But what exactly can trigger child components to re-render? We prevented Prop from changes. The only thing that is left when a parent component re-renders. So we will trigger a state, for example, a child component will re-render, and React will not actually check whether Prop has changed or not at this stage, because React's natural way of dealing with components is components re-render, and then re-render every single child. Wrapping onClick here in useCallback is just completely useless, we are doing nothing here.

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