React 45 Minute Intro

R 45 M
Maarten Mulders (@mthmulders)#reactin45mins

“React is a library for declaratively
building user interfaces using
JavaScript and (optionally) XML.

R
No two-way data binding
No templating language
Just user interface (no routing, no HTTP client)
Plain JavaScript (or add JSX, TypeScript, ...)
Virtual DOM vs. actual DOM

M J S

C
SEK
class Amount {
   constructor(currency, value) {
       this.currency = currency;
       this.value = value;
   }
   getCurrency() {
       return this.currency;
   }
}
const text = new Amount('SEK', 1500).getCurrency();
document.getElementById('app').innerText = text;
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F
true
function isEven(number) {
return number % 2 == 0;
}
const text = isEven(42);
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A F
true
const isEven = (number) => {
return number % 2 == 0;
}
const isEven2 = (number) => number % 2 == 0
const text = isEven(42);
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O D
Jane Doe
const person = { name : 'Jane Doe', age: 42, occupancy: 'JavaScript dev' };
const { name, age } = person;
const text = name;
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A D
1
const numbers = [ 1, 2 ];
const [ first, second ] = numbers;
const text = first;
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O S N
{"name":"Jane Doe","age":42}
const name = 'Jane Doe';
const age = 42;
const person = { name, age };
const text = JSON.stringify(person)
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S I
SEK 1500
class Amount {
   constructor(currency, value) {
       this.currency = currency;
       this.value = value;
   }
   toString() {
       return `this.currency{this.currency} {this.value}`;
   }
}
const text = new Amount('SEK', 1500).toString();
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B JSX

W JSX
A syntax extension to JavaScript
real XML, not a string of characters
allows embedded expressions
supports attributes
Can be nested
Automatic XSS prevention
Needs to be transpiled to JavaScript
e.g. React.createElement(...)

E
Elements can be regular DOM elements... (for now, but not for long)
Hej
const element = <div>Hej</div>
ReactDOM.render(element, document.getElementById('app'));
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A
Elements can have attributes...
Hej
const element = <div id='example'>Hej</div>
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... but they can have different names than HTML attributes:
Hej
const element = <div className='redtext'>Hej</div>
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... and they can behave differently:
Hej
const style = { color: 'red', fontWeight: 'bold' };
const element = <div style={ style }>Hej</div>
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S R N
Values must have a single root node
x
y
const element = <><div>x</div><div>y</div></>
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C
Function that takes props (think: arguments) and returns a React
element.
Hej Jfokus!
const Greeter = (props) => <div>Hej { props.name }!</div>
ReactDOM.render(<Greeter name='Jfokus' />, document.getElementById('app'));
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C
Alternatively, using object decomposition:
Hej Jfokus!
const Greeter = ({ name }) => <div>Hej { name }!</div>
ReactDOM.render(<Greeter name='Jfokus' />, document.getElementById('app'));
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A JSX

E JSX
The answer to the ultimate question of life, universe and everything:
42
const answerToQuestionOfLife = 40 + 2;
const askQuestionOfLife = () => answerToQuestionOfLife;
const Example = () => <div>
   <div>
       The answer to the ultimate question of life,
       universe and everything:
   </div>
   <div><strong>{ askQuestionOfLife() }</strong></div>
</div>;
ReactDOM.render(<Example />, document.getElementById('app'));
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C JSX
Clapping hands...
const ClapHands = () => <span>Clapping hands...</span>;
const DryTears = () => <span>Drying tears...</span>;
const ShowEmotion = ({ isHappy }) => isHappy ? <ClapHands /> : <DryTears />;
ReactDOM.render(<ShowEmotion isHappy={ true } />, document.getElementById('app'));
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C JSX (2)
ASML
HEIA
PHIA
const Ticker = ({ symbol }) => <div>{ symbol }</div>;
const TickerList = ({ symbols }) => symbols.map(
(symbol) => <Ticker symbol={ symbol } />
);
const symbols = ['ASML', 'HEIA', 'PHIA'];
ReactDOM.render(<TickerList symbols={ symbols } />, document.getElementById('app'));
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R M

A
So far, we've written components and wired them together.
Babel or tsc transpiles them to React.createElement(...)
invocations:
<Greeter name={ 'Jfokus' }
/** transpiles into
React.createElement(Greeter, { name: 'Jfokus' }, null)

A
The React.createElement invocations form a tree of components.
React maintains a virtual DOM based on your component tree.
The virtual DOM is compared to the actual DOM.
Only necessary changes are executed.

R
React syncs the virtual and the actual DOM based on two assumptions:
1. If two elements are of different type, the (sub) tree will be different.
2. The key prop identiﬁes child elements over re-renders.

1 E
Hej, Jfokus
const SwedishGreeter = ({ name }) => <div>Hej, { name } </div>
const DutchGreeter = ({ name }) => <div>Hallo, { name } </div>;
const EnglishGreeter = ({ name }) => <div>Hello, { name } </div>;
const App = ({ lang, name }) => {
switch(lang) {
   case 'se': return <SwedishGreeter name={ name } />
   case 'nl': return <DutchGreeter name={ name } />
   case 'en':
   default  : return <EnglishGreeter name={ name } />
}
};
ReactDOM.render(<App name='Jfokus' lang='se' />, document.getElementById('app'));
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2 T KEY
ASML
HEIA
PHIA
const Ticker = ({ symbol }) => <div>{ symbol }</div>;
const TickerList = ({ symbols }) => symbols.map(
(symbol) => <Ticker key={ symbol } symbol={ symbol } />
);
const symbols = ['ASML', 'HEIA', 'PHIA'];
ReactDOM.render(<TickerList symbols={ symbols } />,
document.getElementById('app'));
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Y E
1. Keeping state
2. Reacting to events
3. Fetching data over HTTP
4. Storing data

L S C
Counter: 0
const Counter = () => {
const [ counter, setCounter ] = React.useState(0);
return <div>Counter: { counter }</div>
}
ReactDOM.render(<Counter />, document.getElementById('app'));
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R E
Similar to DOM event handling, but
1. event names are different: onClick vs onclick.
2. event handlers are always functions, never strings.
3. event handlers are bound to a component, should not live globally.
4. event handlers receive an synthetic event - browser-agnostic!

C C
Counter: 0
+ ‑
const Counter = () => {
   const [ counter, setCounter ] = React.useState(0);
   const increate = () => setCounter(counter + 1);
   const decreate = () => setCounter(counter 1);
   return <div>Counter: { counter }<br />
               <button onClick={ increate }>   +   </button> &nbsp;
               <button onClick={ decreate }>      </button>
          </div>
}
ReactDOM.render(<Counter />, document.getElementById('app'));
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C C
Greet!
const Greeter = () => {
   const [ name, setName ] = React.useState('');
   const updateName = (e) => setName(e.target.value);
   const callback = () => alert(`Hej ${name}!`);
   return <div><input type='text' onChange={ updateName } ></input><br />
               <button onClick={ callback }>Greet { name }!</button></div>
}
ReactDOM.render(<Greeter />, document.getElementById('app'));
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F HTTP
What we need:
1. A bit of JavaScript to fetch some data
2. A component to show the fetched data

1 A API
const checkStatus = (response) {
if (response.status 200 response.status < 300) {
return Promise.resolve(response);
} else {
return Promise.reject(`HTTP error ${response.status}: ${response.statusText}`);
}
};
const parseJson = (response) response.json();
const url = 'https: hqd7fvgovgs2jyoosjgryaylcy.apigateway.eu frankfurt-1.oci.customer oci.com'
+ '/v1/joke';
const getJoke = () {
return fetch(url)
.then(checkStatus)
.then(parseJson)
.then(response response.joke);
};

2 A
My sweater had a lot of static electricity. The store gave me a new one,
free of charge.
const RandomJoke = () => {
   const [ { joke, loading }, setState ] = React.useState({ loading: true });
   const fetchRandomJoke = async () => {
       // Does the actual API call to Oracle Cloud function,
       // see code on previous slide.
       const joke = await getJoke();
       setState({ loading: false, joke });
   }
   React.useEffect(() => {
       fetchRandomJoke()
   }, [ ]);
   if (loading) return <div>Loading...</div>
   return <div>{ joke }</div>;
};
ReactDOM.render(<RandomJoke />, document.getElementById('app'));
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S
Local Storage & Session Storage
Part of the
Stores and retrieves string values
Serialise objects with JSON.stringify()
Deserialise with JSON.parse()
Persistent
during browser session with sessionStorage
over browser shutdowns with localStorage
Web Storage API

D A
1. Debugging
2. Testing
3. Building

D
Install the React Developer Tools for your browser of choice.

I C T

D

T
Use Jest (testing platform & library) and Enzyme, a testing utility for
React
Render a React component in a unit test
Make assertions about its output and behaviour

T C
import { shallow } from 'enzyme';
describe('<HelloMessage ', () {
it('should render text', () {
const wrapper = shallow(<HelloMessage name='React' );
expect(wrapper.text()).toBe('Hello React');
});
});

T B C
import { shallow } from 'enzyme';
const dummy = create a mock/stub of some API using framework of choice
describe('<AwesomeButton ', () {
it('should invoke action on click', () {
const wrapper = mount(<AwesomeButton action={ dummy } );
wrapper.f nd('a').simulate('click');
expect(dummy).toHaveBeenCalled();
});
});

D S
tl;dr: use (CRA)
Uses Webpack, Babel, ESLint and a dozen of other tools
Tested to work together
Live-reloading of changed code
Source maps for easy debugging
Have an ready-to-go app in one command
Create React App
npx create react app my next killer app
or
npm i g create react app
create react app my next killer app

U CRA
npm run start to start developing
npm run build to create a production build
npm run test to run unit tests

G B

T R (C ) H
1. Name must start with use!
2. Only use in React function components
not in classes
not outside React components

C H
What's the fastest growing city on earth? The capital of Ireland, it's
Dublin every day.
const useRandomJoke = () => {
   const [ { joke, loading }, setState ] = React.useState({ loading: true });
   const fetchRandomJoke = async () => {
       const joke = await getJoke();
       setState({ loading: false, joke });
   }
   React.useEffect(() => { fetchRandomJoke() }, [ ]);
   return { loading, joke };
};
const JokeDisplay = ({  }) => {
   const { joke, loading } = useRandomJoke()
   if (loading) return <div>Loading...</div>;
   return <div>{ joke }</div>;
}
ReactDOM.render(<JokeDisplay />, document.getElementById('app'));
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T
Use Create React App
Think in (small) components
Think declaratively
Please rate this talk using the Jfokus app!
U
Create React App:
Dad Jokes API:
https://bit.ly/c-r-a
https://bit.ly/dad-joke-api

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