optimize the Performance of React Apps

This article contains some tips to improve the Performance of your React App.

memo and PureComponent

Consider this small example here :

import React from "react";

const myApp = (props) => {
            <FirstComponent propFirst ={props.first}>
            <SecondComponent propFirst ={props.second}>

const FirstComponent = (props) => {
const SecondComponent = (props) => {

Do you think that <SecondComponent> will re-render if only props.first changes ? The Answer is yes, This is because MyApp will be re-rendered and <SecondComponent> is in there . So even if its own props didnt change, its parent component causes it to rerender. Here we can improve the Performance by using React.memo and React.PureComponent . The Idea here is to simply compare old and new props before re-rendering .

When using memo you just need to wrap your component with a memo() function . Now it will only rerender when second changes value regardless of how many times its parent rerenders. Here is te above Example should look like :

const SecondComponent = memo((props) => {
  return <p> {props.second}</p>;

the PureComponent is essentially equivalent to memo, but for class based components

class SecondComponent extends PureComponent {
  render() {
    return <p>{this.props.second}</p>;

you might wonder why React Componets dont automatically include these internal safeguards against excessive rerendering . Thats because those helpers compare old and new props,for example if the props are very large or we are passing components as props , the comparaison of old/new props can be very expensive .

With React Hooks we can use useMemo Hook as a similar way to prevent unnecessary work.

Don't use anonymous Functions

Functions that are used in the main body of a component are usually event handlers or callbacks, In many cases we tempt to use anoynmous Functions like onClick={() => console.log('test')}. As Anonymous Functions aren't assigned an identifier , they aren't persistent when the component gets renderd again . This causes Javascript to allocate a new Memory each time the Component rerenders instead of allocating a Memory only once.

As an optimization Concept we have two Variations :

1- naming and placing handler outside the component

const eventHandler = () => console.log("test");
function FirstComponent() {
  return <button onClick={eventHandler}>Click Me</button>;

2- using the useCallback Hook

function FirstComponent() {
  const eventHandler = useCallback(() => console.log("test"), []);
  return <button onClick={eventHandler}>Click Me</button>;

Don't use Object Literals

This is similar to the "avoiding the use of anonymous Functions", for the simple reason is that the Object Literals don't have a persistent memory space, so the component will need to allocate a new location in memory whenever the component rerenders.

function firstComponent() {
    <SecondComponent props={{ name: "mustermann" }} />

function secondComponent(props) {

each time the <firstComponent/> is re-rendered , a new Object Literal has to be created. That means that the <secondComponent /> will receive new props too and will rerender. This Issue can be fixed by giving the object a name (outside of the Component Body) .

React.lazy and React.Suspense

Code Splitting is a way to make React apps fast, By Code Splitting we mean that our javascript client source (react code) is splitted(broken) into chunks , and we load this chunks only in lazy way. without code splitting a single chunk could be very large.

How Code Splitting is done in Reactjs ?

If you are already using react create app , you can simply use react.lazy and react.suspense , when not then you have to configure your webpack configuration . Here is an example with react.lazy and react.suspense

import React, { lazy, suspense } from "react";
import Header from "./header";
import Footer from "./footer";
const Posts = React.lazy(() => import("./posts"));
function home() {
  return (
      <Header />
      <Suspense fallback={<div> Loading</div>}>
        <Posts />
      <Footer />

The Fallback Prop will be shown to the user while the second bundle chunk is loaded (in our Case Posts)

Maher Gallaoui

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