Pietro Passarelli

How to prevent unnecessary re-renders in React - draft

Some notes on how to how to prevent unnecessary re-renders in React

Some notes on how to how to prevent unnecessary re-renders in React


Originally from @bbc/react-transcript-editor, 'How to prevent unnecessary re-renders - Draft' notes for issue #159 'Unecessary re-renders' and PR #160 'remove unnecessary re-renders to improve performance'.

These notes are more a collection of links and resources that where used to troubleshoot the react performance issue.

This is the before. You can see that there are all sorts of areas that light up like a christmas tree 🎄, even tho not currently involved in the present operation.

And then after you can see it's significantly less, for instance you can see the media controls are no longer updating while the media is playing.

This was visualised using the Chrome dev tool, highlight updates feature.

Also notes this does not considers hooks yet.

Start here

React docs - Thinking in React

React JS cheat sheet

SOLID: Part 1 - The Single Responsibility Principle

if want to go deeper in 'the single responsibility principle' read this

Another wording for the Single Responsibility Principle is:

Gather together the things that change for the same reasons. Separate those things that change for different reasons.

States and Props

State is reserved only for interactivity, that is, data that changes over time. Since this is a static version of the app, you don’t need it.
from react docs

Let’s go through each one and figure out which one is state. Simply ask three questions about each piece of data:

  1. Is it passed in from a parent via props? If so, it probably isn’t state.
  2. Does it remain unchanged over time? If so, it probably isn’t state.
  3. Can you compute it based on any other state or props in your component? If so, it isn’t state.
    React docs - thinking in react

Remember: React is all about one-way data flow down the component hierarchy. It may not be immediately clear which component should own what state. This is often the most challenging part for newcomers to understand, so follow these steps to figure it out:

For each piece of state in your application:

  • Identify every component that renders something based on that state.
    -Find a common owner component (a single component above all the components that need the state in the hierarchy).
  • Either the common owner or another component higher up in the hierarchy should own the state.
  • If you can’t find a component where it makes sense to own the state, create a new component simply for holding the state and add it somewhere in the hierarchy above the common owner component.

React docs - thinking in react

updating state

updating state is async

There is one crucial case where it makes sense to use a function over an object: when you update the state depending on the previous state or props. If you don't use a function, the local state management can cause bugs. The React setState() method is asynchronous. React batches setState() calls and executes them eventually. Sometimes, the previous state or props changes between before we can rely on it in our setState() call.
React book

With the function approach, the function in setState() is a callback that operates on the state and props at the time of executing the callback function. Even though setState() is asynchronous, with a function it takes the state and props at the time when it is executed.

React book

Component Lifecycle

react lifecycle methods diagram

good to read along side this chapter Chapter 3 boook -the-road-to-learn-react - Getting Real with APIs


A React Component works hard. As the user manipulates the state of the application, it may re-render 5, 10, 100 times. Sometimes, that’s a good thing. But if you don’t understand what is causing the re-renders, and whether they are necessary, your app could suffer serious slowdown.
How to Benchmark React Components: The Quick and Dirty Guide

A re-render can only be triggered if a component’s state has changed. The state can change from a props change, or from a direct setState change. The component gets the updated state and React decides if it should re-render the component. Unfortunately, by default React is incredibly simplistic and basically re-renders everything all the time.
Component changed? Re-render. Parent changed? Re-render. Section of props that doesn't actually impact the view changed? Re-render.


Re-render vs re-paint

calling a re-render doesn't mean that React is repainting the DOM.
it means that react it's calling it's diffing algorith to check changes against the virtual dom. it might do that, and still determine that no re-paint is needed. the issue is that calling unecessary re-renders (those when it ends up determine that no change is neeeded) to often, can lead to slower performance.

Now, I might be painting an alarming picture of unnecessary work that has been going on right under our noses. One thing to keep in mind is that a render method being called is not the same thing as the DOM ultimately getting updated. There are a few additional steps React takes where the DOM is diffed (aka the previous version compared with the new/current version) to truly see if any changes need to be represented. All of these few additional steps is work, and for more complex apps with a lot of components, there will be many MANY instances of a few additional steps that will start to add up. Some of this is additional work done by React's internals. Some of this additional work is just important things we do in our render methods, for we often have a lot of code there to help generate the appropriate JSX. Rarely is our render method returning a static piece of JSX with no evaluation or calculation happening, so minimizing unnecessary render calls is a good thing!


By default, shouldComponentUpdate returns true. That’s what causes the “update everything all the time” we saw above. However, you can overwrite shouldComponentUpdate to give it more “smarts” if you need the performance boost. Instead of letting React re-render all the time, you can tell React when you don’t want to trigger a re-render.


Returning false does not prevent child components from re-rendering when their state changes.
We do not recommend doing deep equality checks or using JSON.stringify() in shouldComponentUpdate(). It is very inefficient and will harm performance.
source - react docs

This applies to the children’s state but not their props. So if a child component is internally managing some aspect of its state (with a setState of its own), that will still be updated. But if the parent component returns false from shouldComponentUpdate it will not pass the updated props along to its children, and so the children will not re-render, even if their props had updated.


This method only exists as a performance optimization. Do not rely on it to “prevent” a rendering, as this can lead to bugs. Consider using the built-in PureComponent instead of writing shouldComponentUpdate() by hand. PureComponent performs a shallow comparison of props and state, and reduces the chance that you’ll skip a necessary update.
React docs

How to Benchmark React Components: The Quick and Dirty Guide

React Pure components


import React, { PureComponent } from "react";

class MenuButton extends PureComponent {

React pure components

Based on the concept of purity in functional programming paradigms, a function is said to be pure if:

  • its return value is only determined by its input values.
  • its return value is always the same for the same input values.
    A React component can be considered pure if it renders the same output for the same state and props. For class components like this, React provides the PureComponent base class. Class components that extend the React.PureComponent class are treated as pure components.

Pure components have some performance improvements and render optimizations since React implements the shouldComponentUpdate() method for them with a shallow comparison for props and state.

in a normal component, before it updates, shoulComponentUpdate will fire.
You can add code there to check if you want the component to update. eg if you return false, it won’t update.

there’s a force update, but if you are using that, you might need to check the logic.

Pure component → expec to work similar to pure function

  • no side effects
  • no notification shouldComponentUpdate

They are an optimization pattern, faster then ordinary component. (coz don’t fire shouldComponentUpdate decide for itself if needs to update)

pure components should include components that “act as pure components”.

used for presentation/read only.

Useful for refactoring, and speeding up for performance. eg if you want it to refresh less often.

from React BBC academy course.

If your React component’s render() function renders the same result given the same props and state, you can use React.PureComponent for a performance boost in some cases.
React - Pure Component


use React.StrictMode to identify components with unsafe lifecycles

StrictMode is a tool for highlighting potential problems in an application. Like Fragment, StrictMode does not render any visible UI. It activates additional checks and warnings for its descendants.

React docs - Strict Mode

Strict mode warnings

Error Handling

error, store it in your local state, and show a message to the user.

React 16 - error handling

why not use try catch

Lastly, componentDidCatch(error, info) was introduced in React 16 as a way to catch errors in components. For instance, displaying the sample list in your application works fine, but there could be a time when a list in the local state is set to null by accident (e.g. when fetching the list from an external API, but the request failed and you set the local state of the list to null). It becomes impossible to filter and map the list, because it is null and not an empty list. The component would be broken, and the whole application would fail. Using componentDidCatch(), you can catch the


Error boundaries

A JavaScript error in a part of the UI shouldn’t break the whole app. To solve this problem for React users, React 16 introduces a new concept of an “error boundary”.

Error boundaries are React components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed. Error boundaries catch errors during rendering, in lifecycle methods, and in constructors of the whole tree below them.

A class component becomes an error boundary if it defines a new lifecycle method called componentDidCatch(error, info)
React docs

Where to Place Error Boundaries
The granularity of error boundaries is up to you. You may wrap top-level route components to display a “Something went wrong” message to the user, just like server-side frameworks often handle crashes. You may also wrap individual widgets in an error boundary to protect them from crashing the rest of the application.
react docs


in js - try and catch, finally - to manage errors

in react, do only in event handling.

error boundaries, allows to handle errors in constructor and render.

componentDidCatch is the error handler catch errors in any components, underneath this one in the component hierarchy.
error boundary for anything below it.

does not trap errors in itself, in it’s own component.

generally added to the container.

react defaults was, if somethings goes wrong, render nothing.
with error boundaries, can narrow down when error occoured

From react BBC course

Error Handling API requests

Error Handling - with fetch API


The initial value of that isLoading property is false. We don't load anything before the App component is mounted. When the request is made, the loading state is set to true. The request will succeed eventually, and you can set the loading state to false.

Book - loadding...


React.Suspense let you specify the loading indicator in case some components in the tree below it are not yet ready to render. Today, lazy loading components is the only use case supported by <React.Suspense>:

React docs

React events

events cheat sheet


Tests - book chapter


react docs


Actions you can take to improve performance


shouldComponentUpdate(nextProps, nextState){
return false;


use PureComponent

import React, { PureComponent } from "react";

class MenuButton extends PureComponent {


use why-did-you-update lib to identify unecessary re-renders

npm install --save-dev why-did-you-update
import React from 'react';

if (process.env.NODE_ENV !== 'production') {
const {whyDidYouUpdate} = require('why-did-you-update');

Common Fixing Scenarios

React DevTools tools profiler

React docs - Introducing the React Profiler

add a console.log in render method

add a console.log in render method of component, to see count of how often is called, before and after tweaks


use React.StrictMode as described above to identify other issues in your code.

consider refactoring

consider refactoring the logic of the app and components to break them into smaller more manageable units.



You Probably Don't Need Derived State