Tieto React

🙋‍♂️

Frontend engineer

9 years FE experience
AngularJS, Angular, Ember, React
currently Staff engineer at Productboard

❤️ 📸

❤️ 🛞

Slides

https://slides.com/nuc

Schedule

9-12 morning
- recap of the previous day
- a break: 10:15 - 10:30
lunch?
13-17 afternoon
- a break: 14:30 - 14:45

Course

exercises build on top of each other!
discussions welcomed

Setup IDE

Install

Google Chrome
nodejs
- https://nodejs.org/en/
Visual Studio Code
- https://code.visualstudio.com
Quokka plugin
- VS Code ➡ extensions ➡ search for quokka

Javascript & Typescript

Let's talk about React! 💪

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

Vite

tool for scaffolding react app
replaced create-react-app

npm create vite@latest

Project name: … (react-skoleni)
Select a framework: › React
Select a variant: › JavaScript

Important parts

package.json

describes the package
dependecies list
npm scripts

index.html

HTML
notice div id="root"

/public folder

contains assets

main.jsx

renders React into HTML element

ReactDOM.createRoot(document.getElementById('root')).render(
  <React.StrictMode>
    <App />
  </React.StrictMode>,
)

➡️ Start the React app

npm run dev

http://localhost:5173/

Create React app

CRA

tool for scaffolding react app

npx create-react-app my-app --template typescript

Important parts

package.json

describes the package
dependecies list
npm scripts

tsconfig.json

settings for typescript compiler
"target" = build for version of JS

/public folder

contains assets
index.html

index.tsx

renders React into HTML element

const root = ReactDOM.createRoot(
  document.getElementById('root') as HTMLElement
);

root.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>
);

App.tsx

the main component

➡️ Start the React app

JSX

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

What is JSX?

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

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

remove everything in the body of App.tsx component
notice the browser reloads

function App() {
  return <h1>Hello</h1>;
}

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>;
}

Components

Component

reusable unit
just a function
input
- ="props"
output
- React element

type Props = {
  name: string;
};

function NameComponent(props: Props) {
  return <h1>Hi, my name is {props.name}!</h1>;
}

ReactDOM.render(
  <NameComponent name="Martin" />,
  document.getElementById('root')
);

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 💡

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 <h1>{props.name}</h1>;
}

How to use a component?

pass data down via props

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

<Table columns={4} rows={3} />

Event handling

React unifies API of events (link)

<button type="button" onClick={() => console.log('Hello')}>
  Hello world
</button>

State

useState

hook for storing data
instead of declaring variable

import React, { useState } from 'react';

function Counter() {
  const [name, setName] = useState('nobody');
	
  function handleGiveName(name: string) {
    setName(name);
  }

  return <div>
    My name is {name}.
    <button onClick={() => handleGiveName('Martin')}>
      Give me name
    </button>
  </div>
}

initial value

➡️ Create counter

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

Component lifecycle

mounted
updated
- triggered by change of state
- triggered by change of props
- ➡️ render
unmounted

Class components

rarely used nowadays
uses a class instead of a function
this.props
this.setState() to change state
life cycle hooks
- componentDidMount
- componentWillUnmount

Counter example

import React from 'react';

type State = {
  counter: number;
}

export class MyComponent extends React.Component<{}, State> {
  state = {
    counter: 0
  };

  increment() {
    this.setState({ counter: this.state.counter + 1 });
  }

  render() {
    const { counter } = this.state;
    return <div>
      Counter: {counter}
      <button type="button" onClick={() => this.increment()}>Increment</button>
    </div>
  }
}

➡️ Rewrite class component as a functional component

type Props = {
  pregeneratedCount: number
}

type State = {
  generatedNumbers: number[];
}

export class NumberGeneratorClass extends React.Component<Props, State> {
  constructor(props: Props) {
    super(props);
    const generatedNumbers = [...Array(props.pregeneratedCount)].map(() => Math.random());
    this.state = {
      generatedNumbers
    };
  }

  generateNew() {
    this.setState({ generatedNumbers: [...this.state.generatedNumbers, Math.random()] });
  }

  render() {
    const { generatedNumbers } = this.state;
    return <div>
      {generatedNumbers.map((num, index) => <div key={index}>{num}</div>)}
      <button type="button" onClick={() => this.generateNew()}>Generate new</button>
    </div>
  }
}

Important things to notice

setter needs a new reference
the initial set is generated on every render

Conditions

use if statement
use ternary operator

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

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

function MyComponent() {
  const condition = true;
  
  return <>{condition && <span>It's true</span></>
}

Styling app

Import CSS

global CSS
can use preprocessors (SCSS, SASS)

import './App.css';

function Component() {
  return <div className="red">Hello</div>
}

.red {
  color: red;
}

App.css

App.tsx

CSS modules

scoped CSS
can use preprocessors (SCSS, SASS)
css file must be named .module

import styles from './Component.module.css';

function Component() {
  return <div className={styles.red}>Hello</div>
}

.red {
  color: red;
}

App.module.css

App.tsx

Conditional styling without CSS modules

classnames library
npm i classnames @types/classnames
key = class to be applied
value = condition

import cn from 'classnames';

function ValidationState() {
  const [invalid, setInvalid] = useState(false);
  
  return <div className={cn({ red: invalid })}>
    Status
  </div>
}

Conditional styling with CSS modules

dynamic keys

import cn from 'classnames';
import styles from './ValidationState.module.css';

function ValidationState() {
  const [invalid, setInvalid] = useState(false);
  
  return <div className={cn({ [styles.red]: invalid })}>
    Status
  </div>
}

useEffect

hook for side effects
- = synchronization with external system
second argument say when it runs
- empty - on every render
- [ ] - only at the begining (=on mount)
- [ variable ] - when a variable changes
should return cleanup function

useEffect example

tracks mouse position

export const MyMouse = () => {
  const [mousePosition, setMousePosition] = useState({x: 0, y: 0});
  
  useEffect(() => {
    const onMouseMove = event => {
      setMousePosition({
        x: event.clientX,
        y: event.clientY
      });
    };
    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>
  );
};

Try useEffect

show text with mouse position
show text with last mouse click coordinates
when the mouse position is
- on the left to the last click - change text to green color
- on the right to the last click - change text to red color

➡️ Create automatic counter

create a component which increases the counter every second
in the parent component create a button which shows/hides automatic counter component

Data down, events up

Creating own event

component emits event up

type Props = {
  onTrigger: () => void;
};

function ChildComponent(props: Props) {
  return <button onClick={props.onTrigger}>emit event</button>;
}

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

parent component:

child component:

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>
  )
}

➡️ Create a dropdown

What is dropdown?
button which opens a menu when clicked
3 components
- smart + button + dropdown
Improvement: use children to pass dropdown content

www.google.com

www.instagram.com

www.facebook.com

Toggle button

emit click

DropdownComponent

shows

Controlled input

use component state as the only source of truth

function Component() {
  const [name, setName] = useState('nobody');
  const [inputName, setInputName] = useState(name);

  function handleGiveName() {
    setName(inputName);
  }

  return <>
    My name is {name}.

    <input
      value={inputName} 
      onChange={(e) => setInputName(e.target.value)} />

    <button onClick={() => handleGiveName()}>Save</button>
  </>
}

➡️ Create input

input of type number
how much the counter will increment

Strict mode

checks for common mistakes
only in dev mode
runs effects twice
renders components twice

API request

Axios library

used to make HTTP requests
supports promises
docs: https://axios-http.com/docs/example
install: npm install axios

Axios POST usage

import axios from 'axios';

const payload = { name: 'Martin' };
const response = await axios.post('/api/users', payload);
console.log(response);

must be used in useEffect

import axios from 'axios';

const payload = { name: 'Martin' };
axios.post('/api/users', payload)
  .then(response => console.log(response));

➡️ 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

Custom hooks

separate logic from view
no render
named use*
hooks to component lifecycle
clear API

useMouseMove

const useMouseMove = () => {
  const [mousePosition, setMousePosition] = useState({ x: 0, y: 0 });

  useEffect(() => {
    const onMouseMove = (event: MouseEvent) => {
      setMousePosition({
        x: event.clientX,
        y: event.clientY,
      });
    };
    window.addEventListener('mousemove', onMouseMove);
    return () => {
      window.removeEventListener('mousemove', onMouseMove);
    };
  }, []);

  return { x: mousePosition.x, y: mousePosition.y };
};

mouse position example
- no input
- outputs x, y of mouse

Fetch joke hook

encapsulate fetching joke logic into a custom hook
think about API first
Improvement: remember already fetched jokes, load next adds into the list

Debugging

Main tools

console.log
React dev tools
Chrome debugger

debugger;

Chrome dev tools

Network
Source
Performance
Application
React dev tools
- Components
- Profiler

Logging

Sentry.io
TrackJS

React Context

Context

"global" state for subtree of components
avoids passing props deep
Provider + Consumer

type ContextValue = boolean;

const MyContext = React.createContext<ContextValue>(false);

function App() {
  return <MyContext.Provider value={true}>
    <Component />
  </MyContext.Provider>;
}

function Component() {
  const value = useContext(MyContext);
  
  return <div>{value}</div>;
}

Context

pass object with value + setter

type ContextValue = {
  value: number;
  setValue: (value: ContextValue['value']) => void
};

const MyContext = React.createContext<ContextValue>({} as unknown as ContextValue);

function App() {
  const [value, setValue) = useState(0);
  
  return <MyContext.Provider value={{ value, setValue}}>
    <Component />
  </MyContext.Provider>;
}

function Component() {
  const {value, setValue} = useContext(MyContext);
  
  return <>
    <div>{value}</div>
    <button onClick={() => setValue(value + 1)}>Click</button>
  </>;
}

➡️ Dark & Light theme 🌗

style any component to support dark & light theme
- different background + text color
create a button to switch the theme

useRef

manipulate with DOM elements
object with mutable current property

function Component() {
  const inputRef = useRef(null);
  
  function handleClick() {
    inputRef.current.focus();
  }
  
  return <div>
    <input ref={inputRef} />
    <button onClick={handleClick}>Focus the input</button>
  </div>
}

Render props

pass function as children
composition pattern

function Counter({children}) {
  const [counter, setCounter] = useState(0);
  
  function increment() {
    setCounter(counter + 1);
  }
  
  return <>{children({counter, increment})}</>
}
  
function MyComponent() {
  return <div>
    <Counter>
      {({counter, increment}) => <>
        <div>Counter value: {counter}</div>
        <button onClick={increment}>INC</button>
      </>}
    </Counter>
  </div>
}