Table of Contents

Preface
Introduction to React
How much JavaScript you need to know to use
React?
Why should you learn React?
How to install React
React Components
Introduction to JSX
Using JSX to compose UI
The difference between JSX and HTML
Embedding JavaScript in JSX
Managing state in React
Component Props in React
Data flow in a React application
Handling user events in React
Lifecycle events in a React component
Where to go from here

1
 Introduction to React
The goal of this handbook is to provide a starter guide
to learning React.

At the end of the book, you'll have a basic

understanding of:

What is React and why it's so popular

How to install React
The concepts of React: Components
The concepts of React: State
The concepts of React: Props
Handling user events in React
Lifecycle events in a React component

Those topics will be the base upon which you will work
on in other more advanced React courses.

This book is especially oriented at JavaScript

programmers new to React.

React is a JavaScript library that aims to simplify

development of visual interfaces.

Developed at Facebook and released to the world in

2013, it drives some of the most widely used apps,
powering Facebook and Instagram among countless
other applications.

Its primary goal is to make it easy to reason about an

interface and its state at any point in time, by dividing
the UI into a collection of components.

3
 You will find some initial difficulties learning React, but
once it "clicks", I guarantee it's going to be one of the
best experiences you will have, because React makes
many things easier than ever, and its ecosystem is
filled with great libraries and tools.

React in itself has a very small API, and you basically

need to understand 4 concepts to get started:

Components
JSX
State
Props

We'll explore all of these in this book, and we'll leave

the more advanced concepts to other learning
resources.

4
 How much JavaScript
you need to know to use
React?
Before jumping straight into React, you should have a
good understanding of some core JavaScript
concepts.

You don't have to be an expert, but I think you need a

good overview of:

Variables
Arrow functions
Work with objects and arrays using Rest and
Spread
Object and array destructuring
Template literals
Classes
Callbacks
Promises
Async/Await
ES Modules

If those terms sounds unfamiliar, I provided you some

links to find out more about those subjects.

5
 Why should you learn
React?
I highly recommend any Web developer to have at
least a basic understanding of React.

That's because of a few reasons.

1. React is very popular. As a developer, it's quite

likely that you're going to work on a React project
in the future. Perhaps an existing project, or
maybe your team will want you to work on a
brand new app based on React.
2. A lot of tooling today is built using React at the
core. Popular frameworks and tools like Next.js,
Gatsby and many others use React under the
hood.
3. As a frontend engineer, React is probably going to
come up in a job interview.

Those are all good reasons, but one of the reasons I

want you to learn React is that it's great.

It promotes several good development practices,

including code reusability and components-driven
development. It is fast, it is lightweight and the way it
makes you think about the data flow in your
application perfectly suits a lot of common scenarios.

6
 How to install React
There are a few different ways to install React.

To start with, I highly recommend one approach, and

that's using the officially recommended tool called
create–react–app .

create–react–app is a command line application,

aimed at getting you up to speed with React in no
time.

You start by using npx , which is an easy way to

download and execute Node.js commands without
installing them.

See my npx guide here:

https://flaviocopes.com/npx/

npx comes with npm (since version 5.2) and if you

don't have npm installed already, do it now from
https://nodejs.org (npm is installed with Node).

If you are unsure which version of npm you have, run

npm –v to check if you need to update.

Tip: check out my OSX terminal tutorial at

https://flaviocopes.com/macos-terminal/ if you're
unfamiliar with using the terminal, applies to Mac
and Linux.

When you run npx create–react–app <app–name> ,

npx is going to download the most recent create–

react–app release, run it, and then remove it from your

system. This is great because you will never have an

7
 outdated version on your system, and every time you
run it, you're getting the latest and greatest code
available.

Let's start then:

npx create–react–app todolist

This is when it finished running:

8
 create–react–app created a files structure in the
folder you told ( todolist in this case), and initialized
a Git repository.

It also added a few commands in the package.json

file:

so you can immediately start the app by going into the

newly created application folder and run npm start .

9
 By default this command launches the app on your
local port 3000, and it opens your browser showing
you the welcome screen:

Now you're ready to work on this application!

10
 React Components
You just saw how to create your first React application.

This application comes with a series of files that do

various things, mostly related to configuration, but
there's one file that stands out: App.js .

App.js is the first React Component you meet.

Its code is this:

import React from 'react'

import logo from './logo.svg'
import './App.css'

function App() {
return (
<div className="App">
<header className="App–header">
<img src={logo} className="App–logo" alt="l
<p>
Edit <code>src/App.js</code> and save to
</p>
<a
className="App–link"
href="https://reactjs.org"
target="_blank"
rel="noopener noreferrer"

>
Learn React
</a>
</header>
</div>
)
}

export default App

11
 An application built using React, or one of the other
popular frontend frameworks like Vue and Svelte for
example, is built using dozens of components.

But let's start by analyzing this first component. I'm

going to simplify this component code like this:

import React from 'react'

import logo from './logo.svg'
import './App.css'

function App() {
return /* something */
}

export default App

You can see a few things here. We import some

things, and we export a function called App .

The things we import in this case are a JavaScript

library (the react npm package), an SVG image, and
a CSS file.

create–react–app is set up in a way that allows

us to import images and CSS to use in our
JavaScript, but this is not something you need to
care now. What you need to care about is the
concept of a component

App is a function that in the original example returns

something that at first sight looks quite strange.

It looks like HTML but it has some JavaScript

embedded into it.

12
 That is JSX, a special language we use to build a
component's output. We'll talk more about JSX in the
next section.

In addition to defining some JSX to return, a

component has several other characteristics.

A component can have its own state, which means it

encapsulates some variables that other components
can't access unless this component exposes this state
to the rest of the application.

A component can also receive data from other

components. In this case we talk about props.

Don't worry, we're going to look in details at all those

terms (JSX, State and Props) soon.

13
 Introduction to JSX
We can't talk about React without first explaining JSX.

You met your first React component, the App

component defined in the default application built by

create–react–app .

Its code was this:

import React from 'react'

import logo from './logo.svg'
import './App.css'

function App() {
return (
<div className="App">
<header className="App–header">
<img src={logo} className="App–logo" alt="l
<p>
Edit <code>src/App.js</code> and save to
</p>
<a
className="App–link"
href="https://reactjs.org"
target="_blank"
rel="noopener noreferrer"
>
Learn React
</a>
</header>

</div>
)
}

export default App

14
 We previously ignored everything that was inside the
return statement, and in this section we're going to
talk about it.

We call JSX everything inside wrapped inside the

parentheses returned by the component:

<div className="App">
<header className="App–header">
<img src={logo} className="App–logo" alt="logo"
<p>
Edit <code>src/App.js</code> and save to relo
</p>
<a
className="App–link"
href="https://reactjs.org"
target="_blank"
rel="noopener noreferrer"
>
Learn React
</a>
</header>
</div>

This looks like HTML, but it's not really HTML. It's a
little different.

And it's a bit strange to have this code inside a

JavaScript file. This does not look like JavaScript at
all!

Under the hood, React will process the JSX and it will
transform it into JavaScript that the browser will be
able to interpret.

So we're writing JSX, but in the end there's a

translation step that makes it digestible to a JavaScript
interpreter.

15
 React gives us this interface for one reason: it's
easier to build UI interfaces using JSX.

Once you'll get more familiar with it, of course.

In the next section we'll talk about how JSX lets you
easily compose a UI, then we'll look at the differences
with "normal HTML" that you need to know.

16
 Using JSX to compose
UI
As introduced in the last section, one of the main
benefits of JSX is to make it very easy to build a UI.

In particular, in a React component you can import

other React components, and you can embed them
and display them.

A React component is usually created in its own file,

because that's how we can easily reuse it (by
importing it) in other components.

But a React component can also be created in the

same file of another component, if you plan to only
use it in that component. There's no "rule" here, you
can do what feels best to you.

I generally use separate files when the number of lines

in a file grows too much.

To keep things simple let's create a component in the

same App.js file.

We're going to create a WelcomeMessage component:

function WelcomeMessage() {
return <p>Welcome!</p>
}

See? It's a simple function that returns a line of JSX

that represents a p HTML element.

We're going to add it to the App.js file.

17
 Now inside the App component JSX we can add
<WelcomeMessage /> to show this component in the
user interface:

import React from 'react'

import logo from './logo.svg'
import './App.css'

function WelcomeMessage() {
return <p>Welcome!</p>
}

function App() {
return (
<div className="App">
<header className="App–header">
<img src={logo} className="App–logo" alt="l
<p>
Edit <code>src/App.js</code> and save to
</p>
<WelcomeMessage />
<a
className="App–link"
href="https://reactjs.org"
target="_blank"
rel="noopener noreferrer"
>
Learn React
</a>
</header>
</div>
)
}

export default App

And here's the result. Can you see the "Welcome!"

message in the screen?

18
 We say WelcomeMessage is a child component of
App, and App is its parent componnet.

We're adding the <WelcomeMessage /> component like

if it was part of the HTML language.

That's the beauty of React components and JSX: we

can compose an application interface and use it like
we're writing HTML.

With some differences, as we'll see in the next section.

19
 The difference between
JSX and HTML
JSX kind of looks like HTML, but it's not.

In this section I want to introduce you some of the

most important things you need to keep in mind when
using JSX.

One of the differences might be quite obvious if you

looked at the App component JSX: there's a strange
attribute called className .

In HTML we use the class attribute. It's probably the

most widely used attribute, for various reasons. One of
those reasons is CSS. The class attribute allows us
to style HTML elements easily, and CSS frameworks
like Tailwind put this attribute to the center of the CSS
user interface design process.

But there's a problem. We are writing this UI code in a

JavaScript file, and class in the JavaScript
programming language is a reserved word. This
means we can't use this reserved word as we want. It
serves a specific purpose (defining JavaScript
classes) and the React creators had to choose a
different name for it.

That's how we ended up with className instead of

class .

You need to remember this especially when you're

copy/pasting some existing HTML.

20
 React will try its best to make sure things don't break,
but it will raise you a lot of warnings in the Developer
Tools:

This is not the only HTML feature that suffers from this
problem, but it's the most common one.

Another big difference between JSX and HTML is that

HTML is very relaxed, we can say. Even if you have
an error in the syntax, or you close the wrong tag, or
you have a mismatch, the browser will try its best to
interpret the HTML without breaking.

It's one of the core features of the Web. It is very

forgiving.

JSX is not forgiving. If you forget to close a tag, you

will have a clear error message:

21
 React usually gives very good and informative
error messages that point you in the right direction
to fix the problem.

Another big difference between JSX and HTML is that

in JSX we can embed JavaScript.

Let's talk about this in the next section.

22
 Embedding JavaScript in
JSX
One of the best features of React is that we can easily
embed JavaScript into JSX.

Other frontend frameworks, for example Angular and

Vue, have their own specific ways to print JavaScript
values in the template, or perform things like loops.

React is not adding new things. Instead, it lets us use

JavaScript in the JSX, by using curly brackets.

The first example of this that I will show you comes

directly from the App component we studied so far.

We import the logo SVG file using

import logo from './logo.svg'

and then in the JSX we assign this SVG file to the

src attribute of an img tag:

<img src={logo} class="App–logo" alt="logo" />

Let's do another example. Suppose the App

component has a variable called message :

function App() {
const message = 'Hello!'
//...
}

23
 We can print this value in the JSX by adding
{message} anywhere in the JSX.

Inside the curly brackets { } we can add any

JavaScript statement, but just one statement for every
curly bracket block.

And the statement must return something.

For example this is a common statement you will find

in JSX. We have a ternary operator where we define a
condition ( message === 'Hello!' ), and we print one
value if the condition is true, or another value (the
content of message in this case) if the condition is
false:

{
message === 'Hello!' ? 'The message was "Hello!"'
}

24
 Managing state in React
Every React component can have its own state.

What do we mean by state? The state is the set of

data that is managed by the component.

Think about a form, for example. Each individual input

element of the form is responsible for managing its
state: what is written inside it.

A button is responsible for knowing if it's being clicked,

or not. If it's on focus.

A link is responsible for knowing if the mouse is

hovering it.

In React, or in any other components-based

framework/library, all our applications are based and
make heavy use of components state.

We manage state using the useState utility provided

by React. It's technically a hook (you don't need to
know the details of hooks right now, but that's what it
is).

You import useState from React in this way:

import React, { useState } from 'react'

Calling useState() , you will get back a new state

variable, an a function that we can call to alter its
value.

25
 useState() accepts the initial value of the state item
and returns an array containing the state variable, and
the function you call to alter the state.

Example:

const [count, setCount] = useState(θ)

This is important. We can't just alter the value of a

state variable directly. We must call its modifier
function. Otherwise the React component will not
update its UI to reflect the changes of the data. Calling
the modifier is the way we can tell React that the
component state has changed.

The syntax is a bit weird, right? Since useState()

returns an array we use array destructuring to access

each individual item, like this: const [count, setCount]
= useState(θ)

Here's a practical example:

import { useState } from 'react'

const Counter = () => {

const [count, setCount] = useState(θ)

return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>C
</div>
)
}

ReactDOM.render(<Counter />, document.getElementByI

26
 You can add as many useState() calls you want, to
create as many state variables as you want:

const [count, setCount] = useState(θ)

const [anotherCounter, setAnotherCounter] = useStat

27
 Component Props in
React
We call props the initial values passed to a
component.

We previously created a WelcomeMessage component

function WelcomeMessage() {
return <p>Welcome!</p>
}

and we used it like this:

<WelcomeMessage />

This component does not have any initial value. It

does not have props.

Props can be passed as attributes to the component in

the JSX:

<WelcomeMessage myprop={'somevalue'} />

and inside the component we receive the props as

argument:

function WelcomeMessage(props) {
return <p>Welcome!</p>
}

28
 It's common to use object destructuring to get the
props by name:

function WelcomeMessage({ myprop }) {

return <p>Welcome!</p>
}

Now that we have the prop, we can use it inside the

component, for example we can print its value in the
JSX:

function WelcomeMessage({ myprop }) {

return <p>{myprop}</p>
}

Curly brackets here have various meanings. In the

case of the function argument, curly brackets are used
as part of the object destructuring syntax.

Then we use them to define the function code block,

and finally in the JSX to print the JavaScript value.

Passing props to components is a great way to pass

values around in your application.

A component either holds data (has state) or receives

data through its props.

We can also send functions as props, so a child

component can call a function in the parent
component.

A special prop is called children . That contains the

value of anything that is passed between the opening
and closing tags of the component, for example:

29
 <WelcomeMessage> Here is some message </WelcomeMess

In this case, inside WelcomeMessage we could access

the value Here is some message by using the
children prop:

function WelcomeMessage({ children }) {

return <p>{children}</p>
}

30
 Data flow in a React
application
In a React application, data typically flows from a
parent component to a child component, using props
as we saw in the previous section:

<WelcomeMessage myprop={'somevalue'} />

If you pass a function to the child component, you can

however change the state of the parent component
from a child component:

const [count, setCount] = useState(θ)

<Counter setCount={setCount} />

Inside the Counter component we can now grab the

setCount prop and call it to update the count state
in the parent component, when something happens:

function Counter({ setCount }) {

//...

setCount(1)

//...
}

You need to know that there are more advanced ways

to manage data, which include the Context API and
libraries like Redux, but those introduce more

31
 complexity and 90% of the times using those 2 ways I
just explained are the perfect solution.

32
 Handling user events in
React
React provides an easy way to manage events fired
from DOM events like clicks, form events and more.

Let's talk about click events, which are pretty simple to

digest.

You can use the onClick attribute on any JSX

element:

<button
onClick={(event) => {
/* handle the event */
}}
>
Click here
</button>

When the element is clicked, the function passed to

the onClick attribute is fired.

You can define this function outside of the JSX:

const handleClickEvent = (event) => {

/* handle the event */
}

function App() {
return <button onClick={handleClickEvent}>Click h
}

When the click event is fired on the button, React

calls the event handler function.

33
 React supports a vast amount of types of events, like
onKeyUp , onFocus , onChange , onMouseDown ,
onSubmit and many more.

34
 Lifecycle events in a
React component
So far we've seen how to manage state with the
useState hook.

There's another hook I want to introduce in this book:

useEffect .

The useEffect hook allows components to have

access to the lifecycle events of a component.

When you call the hook, you pass it a function. The

function will be run by React when the component is
first rendered, and on every subsequent re-
render/update.

React first updates the DOM, then calls any function

passed to useEffect() .

All without blocking the UI rendering even on blocking

code.

Here is an example:

35
 const { useEffect, useState } = React

const CounterWithNameAndSideEffect = () => {

const [count, setCount] = useState(θ)

useEffect(() => {
console.log(`You clicked ${count} times`)
})

return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>C
</div>
)
}

Since the useEffect() function is run on every

subsequent re-render/update of the component, we
can tell React to skip it, for performance purposes, by
adding a second parameter which is an array that
contains a list of state variables to watch for. React will
only re-run the side effect if one of the items in this
array changes.

useEffect(() => {
console.log(`Hi ${name} you clicked ${count} time
}, [name, count])

Similarly, you can tell React to only execute the side

effect once (at mount time), by passing an empty
array:

useEffect(() => {
console.log(`Component mounted`)
}, [])

36
 You migth find yourself using this option a lot.

useEffect() is great for adding logs, accessing 3rd

party APIs and much more.

37
 Where to go from here
Mastering the topics explained in this book is a great
step towards your goal of learning React.

I want to give you some pointers now, because it's

easy to get lost in the sea of tutorials and courses
about React.

What should you learn next?

Learn more theory about the Virtual DOM, writing

declarative code, unidirectional data flow, immutability,
composition.

Start building some simple React applications. For

example build a simple counter or a interact with a
public API.

Learn how to perform conditional rendering, how to

perform loops in JSX, how to use the React Developer
Tools.

Learn how to apply CSS in a React application, with

plain CSS or Styled Components.

Learn how to manage state using the Context API,

useContext and Redux.

Learn how to interact with forms.

Learn how to use React Router.

Learn how to test React applications.

Learn an application framework built on top of React,

like Gatsby or Next.js.

38