Learning React Hands-on

Going hand on (tutorial 2)

In this tutorial we will discuss the following React concepts:

You should be able to discuss the following topics after watching this video:

  • onClick as an arrow function, bind, non-bind

  • setState as a function and via state destruct

  • Checking for prev value in componentDidUpdate

  • Defining state examples, in constructor, as a property

  • Updating state in component did-update consequences

  • Keeping previous property in state

This tutorial builds on the previous one. If you’ve been wanting to work with ScandiPWA you’ll know that it’s meant to be extended. This is done by creating new files in your project that will override the defaults.

Run yarn start to start the development set-up and let’s start by overriding the index.js file. Create a new index.js file in your src folder. The application should compile automatically after you’ve saved any changes.

This is what our src/index.js should contain:

import { Component } from 'react';
import ReactDOM from 'react-dom';
class Button extends Component {
render () {
return (
<button>Click me!</button>
);
}
}
ReactDOM.render(
// this is the component's template
<Button />,
document.getElementById('root')
);

The component should be passed as a render template and after this we should tell React where we expect this element to render.

If you go to localhost:3000 in your browser you’ll see that the element has rendered.

Let’s make it a clickable button:

import { Component } from 'react';
import ReactDOM from 'react-dom';
class Button extends Component {
onButtonClick() {
// writes logs in console
console.log('you clicked me!');
}
render () {
return (
// adds event listener
<button onClick={ this.onButtonClick }>Click me!</button>
);
}
}
ReactDOM.render(
<Button />,
document.getElementById('root')
);

In React you need to provide a listener for the element when it’s initially rendered, with the listener in this case being <button onClick={ this.onButtonClick }>. You can read more about handling events in React here.

Component state

Let’s set the default state and change it dynamically. The following code should start counting the clicks from zero:

import { Component } from 'react';
import ReactDOM from 'react-dom';
class Button extends Component {
state = {
// sets the default state
clickCount: 0
};
onButtonClick = () => {
// imports the default state
const { clickCount } = this.state;
// updates the new state
this.setState({ clickCount: clickCount + 1 })
console.log('you clicked me!');
};
render () {
return (
<button onClick={ this.onButtonClick }>Click me!</button>
);
}
}
ReactDOM.render(
<Button />,
document.getElementById('root')
);

We need to transform the simple method onButtonClick() to a function property in order to not get a TypeError: Cannot read property ‘state’ of undefined. This can be done by adding an arrow function to onButtonClick = () =>. You can read about passing fuctions to components in React here.

Components update tracking logic

Let’s see how we can update the component with a simple tracking feature:

import { Component } from 'react';
import ReactDOM from 'react-dom';
class Button extends Component {
state = {
clickCount: 0
};
onButtonClick = () => {
const { clickCount } = this.state;
this.setState({ clickCount: clickCount + 1 });
};
render () {
// imports the latest click count from state
const { clickCount } = this.state;
return (
<div>
<span>
You clicked me
<b>{ clickCount }</b>
</span>
<button onClick={ this.onButtonClick }>Click me!</button>
</div>
);
}
}
ReactDOM.render(
<Button />,
document.getElementById('root')
);

Note

JSX expects one root element

Note:

JSX expects one root element

Since JSX expects only one root element to be present, we encase the span and button tags with div.

An alternative way to set the state would be:

onButtonClick = () => {
// const { clickCount } = this.state;
// this.setState({ clickCount: clickCount + 1 });
this.setState(({ clickCount }) => ({ clickCount: clickCount + 1 }));
};

const { clickCount } = this.state; lets us further on just use { clickCount } to refer to this.state.clickCount. These are called state hooks and you can read more about them here.

import { Component } from 'react';
import ReactDOM from 'react-dom';
class Button extends Component {
// proper state declaration
// state = {
// clickCount: 0
// };
// alternative state declaration
constructor (props){
super(props);
this.state = {
clickCount: 0
};
}
onButtonClick = () => {
const { clickCount } = this.state;
this.setState({ clickCount: clickCount + 1 });
};
render () {
// imports the latest click count from state
const { clickCount } = this.state;
return (
<div>
<span>
You clicked me
<b>{ clickCount }</b>
</span>
<button onClick={ this.onButtonClick }>Click me!</button>
</div>
);
}
}
ReactDOM.render(
<Button />,
document.getElementById('root')
);

An alternative way to set the default state would be the following, which is very similar to PHP:

class Button extends Component {
constructor (props){
// reference to a parent class that we extend
super(props);
this.state = {
clickCount: 0
};
}

Let’s consider the component’s life cycle. We’ve created a state, but how can we trace what the component is/was doing?

First, let’s name our division <div id="abc">, add some console logs and find out when they’ll get triggered:

import { Component } from 'react';
import ReactDOM from 'react-dom';
class Button extends Component {
constructor (props){
super(props);
this.state = {
clickCount: 0
};
// make any request here, besides DOM manipulation
console.log('constructor', document.getElementById('abc'));
}
componentDidMount() {
// implement any DOM manipulation
console.log('mount', document.getElementById('abc'));
}
onButtonClick = () => {
const { clickCount } = this.state;
this.setState({ clickCount: clickCount + 1 });
};
render () {
const { clickCount } = this.state;
return (
<div id="abc">
<span>
You clicked me
<b>{ clickCount }</b>
</span>
<button onClick={ this.onButtonClick }>Click me!</button>
</div>
);
}
}
ReactDOM.render(
<Button />,
document.getElementById('root')
);

When we try this out in our browser we can see in the Console that the constructor returns null and mount returns <div id="abc"></div>.

This shows us that the component’s constructor is called before it is mounted. You can read more about the constructor here and componentDidMount here.

If you want to add a CSS variable or have access to a DOM node, you can do it from here:

componentDidMount() {
// implement any DOM manipulation
console.log('mount', document.getElementById('abc'));
}

Next, we have the componentDidUpdate() method which allows us to see if something was updated in the component:

componentDidMount() {
// implement any DOM manipulation
console.log('mount', document.getElementById('abc'));
}
componentDidUpdate() {
// triggered by state & props change
console.log('update');
}

After adding the update log you should be able to see update in the Console any time the button had been clicked.

Let’s add a new class. If ESlint is showing you a ‘max classes per file’ error, disable it for the sake of this tutorial. Note that when developing an actual project, try to stick to the one component per file rule.

You can disable ESlint rule warnings for the entire file by adding the /* eslint-disable */ at the top of it.

/* eslint-disable max-classes-per-file, @scandipwa/scandipwa-guidelines/only-one-class */
import { Component } from 'react';
import ReactDOM from 'react-dom';
class Button extends Component {
constructor (props){
super(props);
this.state = {
clickCount: 0
};
console.log('constructor', document.getElementById('abc'));
}
componentDidMount() {
console.log('mount', document.getElementById('abc'));
}
componentDidUpdate() {
// triggered by state & props change
console.log('update');
}
onButtonClick = () => {
const { clickCount } = this.state;
this.setState({ clickCount: clickCount + 1 });
};
render () {
const { clickCount } = this.state;
return (
<div id="abc">
<span>
You clicked me
<b>{ clickCount }</b>
</span>
<button onClick={ this.onButtonClick }>Click me!</button>
</div>
);
}
}
// the new class
class Wrapper extends Component {
// defines the state
state = {
clickCount: 0
};
onButtonClick = () => {
const { clickCount } = this.state;
this.setState({ clickCount: clickCount + 1 });
};
render() {
return(
<div>
{/* calls the button component */}
<Button />
{/* button with event listener */}
<button onClick={ this.onButtonClick }>Update wrapper</button>
</div>
);
}
}
ReactDOM.render(
// renders the wrapper component instead
<Wrapper />,
document.getElementById('root')
);

Let’s go to our browser and try out the changes. So, if you click on the 1st button, the update will get triggered in Console, but if you click the 2nd button, the update will also get triggered. Why is that? This happens because any updates in the top component will update the bottom component in a very inefficient way.

ShouldUpdate method

Instead we should put shouldComponentUpdate() in Button:

class Button extends Component {
constructor (props){
super(props);
this.state = {
clickCount: 0
};
console.log('constructor', document.getElementById('abc'));
}
componentDidMount() {
console.log('mount', document.getElementById('abc'));
}
shouldComponentUpdate(nextProps, nextState) {
const { clickCount: nextClickCount } = nextState;
const { clickCount } = this.state;
// updates only if state's click count changes
if (clickCount !== nextClickCount) {
return true;
}
return false;
}
componentDidUpdate() {
// triggered by state & props change
console.log('update');
}
onButtonClick = () => {
const { clickCount } = this.state;
this.setState({ clickCount: clickCount + 1 });
};
render () {
const { clickCount } = this.state;
return (
<div id="abc">
<span>
You clicked me
<b>{ clickCount }</b>
</span>
<button onClick={ this.onButtonClick }>Click me!</button>
</div>
);
}
}

Going back to the browser we can see that the clicks in Button will trigger updates, but clicks in Wrapper will not. The shouldComponentUpdate() method might get tedious if you’re working with multiple components. This is where PureComponents come in.

PureComponents VS traditional Components

PureComponent performs a shallow comparison of the props and state any time props or state changes. PureComponent essentially is Component with built in shouldComponentUpdate method.Copy

/* eslint-disable max-classes-per-file, @scandipwa/scandipwa-guidelines/only-one-class */
import { Component, PureComponent } from 'react';
import ReactDOM from 'react-dom';
class Button extends PureComponent {
constructor (props){
super(props);
this.state = {
clickCount: 0
};
console.log('constructor', document.getElementById('abc'));
}
componentDidMount() {
console.log('mount', document.getElementById('abc'));
}
componentDidUpdate() {
// triggered by state & props change
console.log('update');
}
onButtonClick = () => {
const { clickCount } = this.state;
this.setState({ clickCount: clickCount + 1 });
};
render () {
const { clickCount } = this.state;
return (
<div id="abc">
<span>
You clicked me
<b>{ clickCount }</b>
</span>
<button onClick={ this.onButtonClick }>Click me!</button>
</div>
);
}
}
class Wrapper extends Component {
// defines the state
state = {
clickCount: 0
};
onButtonClick = () => {
const { clickCount } = this.state;
this.setState({ clickCount: clickCount + 1 });
};
render() {
return(
<div>
<Button />
<button onClick={ this.onButtonClick }>Update wrapper</button>
</div>
);
}
}
ReactDOM.render(
// renders the wrapper component instead
<Wrapper />,
document.getElementById('root')
);

This should act the same way as the previous Component with shouldComponentUpdate method. This is why it is preferred to use PureComponent by default.

Note:

shouldComponentUpdate only works if you’re extending Component, not PureComponent

Let’s change up the wrapper class and add the wrapperCount= { clickCount } property to Button.

class Wrapper extends Component {
// defines the state
state = {
clickCount: 0
};
onButtonClick = () => {
const { clickCount } = this.state;
this.setState({ clickCount: clickCount + 1 });
};
render() {
const { clickCount } = this.state;
return(
<div>
{/* wrapperCount prop passed to Button component */}
<Button wrapperCount= { clickCount } />
<button onClick={ this.onButtonClick }>Update wrapper</button>
</div>
);
}
}

Now, when trying this out in the browser, the update should also get triggered when clicking the ‘Update wrapper’ button.

Let’s go back to the Button class. In order to change up the componentDidUpdate method we need to either set the required prop or set the default value.

/* eslint-disable max-classes-per-file, @scandipwa/scandipwa-guidelines/only-one-class */
import { Component, PureComponent } from 'react';
import ReactDOM from 'react-dom';
import PropTypes from 'prop-types';
class Button extends PureComponent {
// use either propTypes or defaultProps
static propTypes = {
// sets the required prop
wrapperCount: PropTypes.number
};
static defaultProps = {
// sets the default value
wrapperCount: 0
};
constructor (props){
super(props);
this.state = {
clickCount: 0
};
console.log('constructor', document.getElementById('abc'));
}
componentDidMount() {
console.log('mount', document.getElementById('abc'));
}
componentDidUpdate(prevProps) {
// triggered by state & props change
const { wrapperCount } = this.props;
const { wrapperCount: prevWrapperCount } = prevProps;
if (wrapperCount !== prevWrapperCount) {
// console.log('update');
this.setState({ clickCount: wrapperCount })
}
}
onButtonClick = () => {
const { clickCount } = this.state;
this.setState({ clickCount: clickCount + 1 });
};
...

This is where ESLint would notify you that you shouldn’t use setState in componentDidUpdate. This is because setting the state here could lead to an infinite loop if the checkpoint before doing so would not be specific enough.

Read more about componentDidUpdate and infinite loops here.

Handling side-effects in getDerivedStateFromProps

We can bypass the componentDidUpdate issue by using getDerivedStateFromProps. Note that this method can’t access any values from the component.

class Button extends PureComponent {
// use either propTypes or defaultProps
static propTypes = {
// sets the required prop
wrapperCount: PropTypes.number
};
static defaultProps = {
// sets the default value
wrapperCount: 0
};
constructor (props){
super(props);
this.state = {
clickCount: 0,
// set a new default
prevWrapperCount: 0
};
static getDerivedStateFromProps(props, state) {
// no access to current value is present
// no access to `this`
const { wrapperCount } = props;
const { prevWrapperCount } = state;
// you need to keep previous value in state
// if wrapper count is not equal to previous value
if (wrapperCount !== prevWrapperCount) {
return {
// update click count to wrapper count
clickCount: wrapperCount,
// update previous value
prevWrapperCount: wrapperCount
};
}
return null;
}
componentDidMount() {
console.log('mount', document.getElementById('abc'));
}
...

You should put getDerivedStateFromProps before componentDidMount. This should work as previously with componentDidUpdate, except there is no possibility to enter an infinite loop.