React

library for managing view
component based
- helps split the app into small pieces
used to create SPA

Client

Server

Database

HTTP

browser

request

html page

Client

Server

Database

HTTP

React in browser, mobile app...

API

request

data

Single page application

Web server

html, js

HTML Page

import React & ReactDOM

<html>
<head>
  <script src="https://unpkg.com/react@16/umd/react.development.js" crossorigin></script>
  <script src="https://unpkg.com/react-dom@16/umd/react-dom.development.js" crossorigin></script>
</head>
<body>
  <div id="app">
  </div>
</body>
<script>
  ... your code
</script>
</html>

Building blocks

Elements

React app is composed of React elements
- type
  - string ➡️ HTML element
  - function / class ➡️ component
- properties
  - = props = data passed down
  - used for attributes
- ...children
  - content
  - text / another element (single or array)

const myDiv = React.createElement('div', null, 'Hello World');

Elements

const label = React.createElement('a', {
  href: 'https://google.com'
}, 'Go to Google.com');

<a href="https://google.com">Go to Google.com</a>

children

props

type

Render

display elements
ReactDOM for HTML

<div id="app"></div>

<script>
  const myDiv = React.createElement('h1', null, 'Hello World');
  ReactDOM.render(myDiv, document.getElementById('app'));
</script>

Create a table

table with 2 columns, 3 rows
text in every cell: Hello
attribute: border="1"

Hello	Hello
Hello	Hello
Hello	Hello

Components

Component

reusable unit
just a function
input
- properties (data)
- executed only when props are changed
output
- React element

const myNameElement = React.createElement(NameComponent, {
  name: 'Martin Nuc'
});
ReactDOM.render(myNameElement, document.getElementById('app'));

function NameComponent(props) {
  return React.createElement('h1', null, props.name);
}

Component tree

split big problems to smaller ones

Component tree

Component

Component tree

Stateful components (smart)
- used to fetch data
- data manipulation
Stateless components (dumb)
- only display data
pass data down, emit events up

Component tree

Component

User info

ArticleList

Article

Today Weather

Article

I am smart 💡

Dynamic table

create a component which displays a table
receives number of columns and rows as parameter

Hello	Hello	Hello	Hello
Hello	Hello	Hello	Hello
Hello	Hello	Hello	Hello

node.js

What is node.js?

environment for running Javascript
doesn't need browser
using V8

Install node

https://nodejs.org/en/

Hello world

create file index.js with Javascript code
execute file using: node index.js

console.log('Hello world');

Let's get serious...

create new folder (mkdir my-app; cd my-app)
initialize node package:
- ```
npm init
```
it creates package.json file which contains
- basic package info (author, licence, repository url, etc.)
- dependencies
- scripts

Wait, what was that

npm

??

Node package manager

when you want to use a code written by someone else
packaging system
large repository (>550k packages)
https://www.npmjs.com

{
  "name": "my-package",
  "version": "1.0.0",
  "description": "This is just description of my awesome package",
  "main": "index.js",
  "scripts": {
    "dev": "nodemon --exec npm run start",
    "start": "tsc && node dist/index.js",
    "test": "mocha --opts mocha.opts"
  },
  "author": "Martin Nuc",
  "license": "ISC",
  "dependencies": {
    "@types/chai": "4.0.4",
    "@types/mocha": "2.2.43",
    "@types/node": "8.0.28",
    "@types/sinon": "2.3.4",
    "chai": "4.1.2",
    "mocha": "3.5.3",
    "nodemon": "1.12.1",
    "sinon": "3.2.1",
    "ts-node": "3.3.0",
    "typescript": "2.5.2"
  }
}

package.json

yarn

alternative app to manage dependencies
made by facebook
similar to npm

npm install

installs dependencies based on package.json
downloads into node_modules folder

yarn

Dependencies and GIT

we don't commit dependencies
put node_modules folder in .gitignore
npm install to install dependencies

Scripts


"scripts": {
  "dev": "nodemon --exec npm run start",
  "start": "node index.js",
  "test": "mocha --opts mocha.opts"
},

used to execute commands
npm dependencies executables resolution (from node_modules/.bin/*)

npm run <name>

Shortcut for start and test scripts only. For others you have to use npm run

Runs any script from npm.

npm start

npm test

👉

yarn <name>

Runs any script from npm.

👉

Create your script

create script called start for running your app
use yarn to execute it

npx

part of npm
downloads package and executes it
executes apps in current project

create-react-app

use to create new React project
setups JSX, build process, dev server...
command:
- npx create-react-app hello-world

Structure

public
- index.html
- assets
src
- index.js -> ReactDOM.render()
- App.js
  - root component (JSX)
  - we use PascalCase for component filenames

JSX

What is JSX?

syntactic sugar around createElement
almost like HTML
transpiled to Javascript
example in App.js:

import React from 'react';

function App() {
  return (
    <div className="App">
      Hello
    </div>
  );
}

function App() {
  return React.createElement('div', { className: 'App' }, 'Hello');
}

Q: Why className?

import React from 'react';

function App() {
  return (
    <div className="App">
      Hello
    </div>
  );
}

function App() {
  return React.createElement('div', { className: 'App'}, 'Hello');
}

Q: What happens now?

import React from 'react';

function App() {
  return (
    <div>
      Yes
    </div>
    <div>
      No
    </div>
  );
}

function App() {
  return ????????
}

Solution: React Fragment

like empty element
when you want to return multiple elements - wrap them in fragment

import React from 'react';

function App() {
  return (
    <>
      <div>
        Yes
      </div>
      <div>
        No
      </div>
    </>
  );
}

Before we continue

start project
remove everything in the root component
notice the browser reloads

Print a variable

function App() {
  let something = 'hello';
  return <div>{something}</div>;
}

Print an array

function Array() {
  let array = [1,2,3];
  return <div>
    {array.map((item, index) => <span key={index}>{item}</span>)}
  </div>;
}

Stateless component

everything to display is received via props
just a function
- input: props (=properties)
- output: React element
easy to test

function NameComponent(props) {
  return React.createElement('h1', null, props.name);
}

function NameComponent(props) {
  return <h1>{props.name}</h1>;
}

Use stateless component

pass data down

function App() {
  return <NameComponent name="Martin" />
}

Dynamic table

create a component which displays a table using JSX
receives number of columns and rows as parameter

Hello	Hello	Hello	Hello
Hello	Hello	Hello	Hello
Hello	Hello	Hello	Hello

Children props

you might pass HTML as body of element:

<Table columns={5} rows={2}>
  <h1>Hello</h1>
</Table>

Table component receives react element via children prop:

function Table(props) {
  return (
    <table>
      <tr>
        <td>
          {props.children}
        </td>
      </tr>
    </table>
  )
}

Stateful component

Stateful components

they hold state
use classes
state may change

import React from 'react';

export class Hello extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      name: 'Martin'
    };
  }
  
  render() {
    const name = this.state.name;
    return <div>{name}</div>;
  }
}

How to change state?

using setState
only schedules change
triggers rerender
component must be already mounted

this.setState({
  name: 'Renamed'
});

Handeling events

passing function as prop
events are always camelCase
- eg. onClick

class Hello extends React.Component {
  handleClick() {
    this.setState({ name: 'ooooo' })
  }

  render() {
    const { name } = this.state;
    return <div>
      {name}
      <button onClick={this.handleClick}>
        change
      </button>
    </div>
  }
}

Will it work?

what is the value of this?

class Hello extends React.Component {
  handleClick() {
    this.setState({ name: 'ooooo' })
  }

  render() {
    const { name } = this.state;
    return <div>
      {name}
      <button onClick={this.handleClick}>
        change
      </button>
    </div>
  }
}

Solution 1

using arrow function
creates a function on every render

class Hello extends React.Component {
  handleClick() {
    this.setState({ name: 'ooooo' })
  }

  render() {
    const { name } = this.state;
    return <div>
      {name}
      <button onClick={() => this.handleClick()}>
        change
      </button>
    </div>
  }
}

Solution 2

binding handler to the class

class Hello extends React.Component {
  
  constuctor(props) {
    super(props);
    this.handleClick = this.handleClick.bind(this);
  }
  
  handleClick() {
    this.setState({ name: 'ooooo' })
  }

  render() {
    const { name } = this.state;
    return <div>
      {name}
      <button onClick={() => this.handleClick()}>
        change
      </button>
    </div>
  }
}

Solution 3

ES2018 syntax (experimental)
https://babeljs.io/docs/en/babel-plugin-proposal-class-properties

class Hello extends React.Component {
  
  handleClick = () => {
    this.setState({ name: 'ooooo' })
  }

  render() {
    const { name } = this.state;
    return <div>
      {name}
      <button onClick={this.handleClick}>
        change
      </button>
    </div>
  }
}

Create counter

create button with counter as text
start from 0
everytime you click the button the counter is increased

Conditions

use if statement
use ternary operator

render() {
  const random = Math.random();
  if (random < 0.5) {
    return <span>lower</span>
  } else {
    return <span>higher</span>
  }
}

render() {
  const random = Math.random();
  return <span>
    {random < 0.5 ? 'lower' : 'higher'}
  </span>
}

Component lifecycle

Lifecycle

constructor
mounted
- componentDidMount()
- state available
render, render, render...
unmounted
- componentWillUnmount()
- clean up

Caution !

beware of setting state after component has been unmounted

solutions:

clean after yourself
create flag whether component has been unmounted

Create automatic counter

create component which increases counter every second
in parent component create button which shows/hides this component

Creating own event

component emits event up

class ChildComponent extends React.Component {
  handleClick = () => {
    this.props.doWork();
  }

  render() {
    return <button onClick={this.handleClick}>emit event</button>;
  }
}

<ChildComponent doWork={() => console.log('triggered')} />

parent component:

child component:

Hide counter

when counter reaches 5 it hides itself
emit event to the parent to hide component

Controlled input

use component state as the only source of truth

class Component extends React.Component {
  constructor() {
    this.state = {
      value: ''
    };
  }
  
  handleChange = (event) => {
    this.setState({
      value: event.target.value
    });
  }

  render() {
    return <>
      {this.state.value}
      <input value={this.state.value} onChange={this.handleChange}>
    </>
  }
}

Create input

input of type number
how much the counter will increment

3rd party libs

Dependencies

npm install <name>
yarn add <name>
download library to node_modules
example:
- axios - used to make HTTP requests
- yarn add axios
- import in code:

import axios from 'axios';

Let's make http request

open API request in browser to see structure of response
- https://api.chucknorris.io
display joke in the component
create a button to load another joke

GET https://api.chucknorris.io/jokes/random

Component tree

Component

User info

JokeFetcher

Joke

I am smart 💡

data down

Conditional styling

classnames library
yarn add classnames
key = class to be applied
value = condition

import cn from 'classnames';

...

<div class={cn({ invalid: this.state.invalid })}>
   
</div>

Component composition

Composition

separate logic from view
methods
- HOC = Higher order component
- render or children props
counter example
- component for holding counter state
- component for displaying counter

Children props

component accepts a function as children prop
passes it's state to the children prop
parent component uses and displays these data

Children props

export class CounterState extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      counter: 0
    }
  }

  inc = () => {
    this.setState({counter: this.state.counter + 1});
  }

  dec = () => {
    this.setState({counter: this.state.counter - 1});
  }

  render() {
    return this.props.children({
      counter: this.state.counter,
      increment: this.inc,
      decrement: this.dec
    });
  }
}

Children props in parent

render() {
  return <CounterState>
    {({counter, increment}) =>
      <>
        <div>Current value: {counter}</div>
        <button onClick={increment}>inc</button>
      </>
    }
  </CounterState>
}

Fetch joke component

encapsulate fetching joke logic
use children props to display joke

Enriching component

component may wrap another component
passes all props down to the underlaying element
example: ValidatedInput

Example: ValidatedInput

displays red border when invalid
usage:

<ValidatedInput type="number"
                name="age"
                validator={value => value < 10} />

validator fn passed via props

Example: ValidatedInput

displays red border when invalid

export class ValidatedInput extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      value: '',
      invalid: true
    }
  }

  handleChange = (event) => {
    const value = event.target.value;
    this.setState({value});
    this.setState({
      invalid: !this.props.validator(value)
    });
  }

  render() {
    return <div class={cn({ invalid: this.state.invalid })}>
      <input {...this.props} value={this.state.value} onChange={this.handleChange} />
    </div>
  }
}

validator fn passed via props

Formik

library for forms
supports schema validation
- uses library yup
docs

const schema = yup.object().shape({
  email: yup.string().required().email(),
  age: yup.number().required().positive().integer()
})

const initialValues = {
  email: '',
  age: 0
}

export function MyForm() {
  return (
    <Formik
      initialValues={initialValues}
      validationSchema={schema}
      onSubmit={values => console.log(values)}
      render={() => (
        <Form>
          <Field type="email" name="email" />
          <ErrorMessage name="email" component="div" />
          
          <Field type="number" className="error" name="age" />
          <ErrorMessage name="age" className="error" component="div"/>
          
          <button type="submit">
            Submit
          </button>
        </Form>
      )}
    />
  );
}

Registration form

create form with fields:
- email
- name
- age (>18)

Routing

React router

used to create multiple pages
install react-router-dom
docs

Define pages and links

import { BrowserRouter as Router, Route, Link } from "react-router-dom";

function App() {
  return (
    <Router>
      <Link to="/">Home</Link>
      <Link to="/categories">About</Link>
      <Link to="/categories/animals">Joke about animals</Link>
      <Link to="/categories/history">Joke about history</Link>
      
      <Route exact path="/" component={Home} />
      <Route path="/categories" component={JokeCategories} />
      <Route path="/categories/:category" component={Joke} />
    </Router>
  );
}

everything must be inside of <Router>

Reading parameters

function Joke({match}) {
  return (
    {match.params.category}
  );
}

via props

Try routing

Create routes
- / -> categories list
- /categories/:category -> joke from category
load joke based on category
- https://api.chucknorris.io

React hooks

way how to use state in functional component
no need to use classses
since React 16.8 (Feb 2019)

useState

hook for storing data

import React, { useState } from 'react';

function Counter() {
  const [counter, setCounter] = useState(0);
	
  function handleIncrement() {
    setCounter(counter + 1);
  }

  return <button onClick={handleIncrement}>{counter}</button>
}

initial value

Try useState

create functional component
have button to toggle between two texts:
- awesome
- cool

useEffect

hook for side effects
second argument say when it runs
- empty - on every render
- [ ] - only at the begining (=componentDidMount)
- [ variable ] - only when variable changes
should return cleanup function (=componentWillUnmount)

useEffect

track mouse position

export const MyMouse = () => {
  const [mousePosition, setMousePosition] = useState({x: 0, y: 0});
  const onMouseMove = event => {
    setMousePosition({
      x: event.clientX,
      y: event.clientY
    });
  };
  useEffect(() => {
    window.addEventListener('mousemove', onMouseMove);
    return () => {
      window.removeEventListener('mousemove', onMouseMove);
    };
  }, []);
  const {x, y} = mousePosition;
  return (
    <div>My mouse x position is {x} and y position is {y}</div>
  );
};